Guide to second hand medium format digital backsThis guide is mainly for those that need a medium format digital back (MFDB) for a technical camera, but much information is also useful to those that aim for a medium format SLR camera.
I have written this guide with the assumption that the reader has not used a digital back before but rather 35mm DSLRs.
Disclaimer: this text is based on my own research when I invested in my own medium format system. I hope everything is correct but I cannot give guarantees. Most backs I have only studied theoretically, not actually tried them myself. I suggest you use this text as one of many inputs for your own research.
What a DSLR user should know about MFDBsIf you are used to modern 35mm digital DSLRs there are a number of things to know about MFDBs so you know what you are getting in to. Most MFDBs have CCD sensors while (modern) 35mm DSLRs have CMOS sensors, this property is what makes MFDBs less flexible in some aspects.
In 2014 a number of CMOS digital backs were introduced, all based on a 44x33mm sensor by Sony, and just as with the smaller formats it's likely that CMOS will replace CCD in the coming years. However, in the second hand market CCDs will be dominating for some years still.
Why not CMOS?Not many years ago CCD had vastly superior image quality, but today the best CMOS sensors are just as good. The CMOS sensors also provide advantages such as good image quality at high ISOs, faster image readout and possibility to have a high quality live view.
If MFDBs were sold in the same volumes as 35mm digital and Sony/Canon/Nikon had medium format systems most MFDBs would probably have CMOS sensors since many years, but the current market is too small to support the huge investments an exclusive MF sized CMOS sensor would require. It has also been very difficult to manufacture large CMOS sensors with photographic qualities. However, Sony recently released a 44x33mm CMOS sensor based on its smaller format technology, and this sensor is now used in several different digital backs. Things are changing, but how quickly CMOS will be dominating is hard to predict.
Meanwhile all MFDBs we will consider in the second hand market will have CCD sensors.
No live view?!If you are a still life, architecture or landscape photographer (which you probably are if you are interested in technical cameras) you probably love the DSLR live view, and use it for framing and critical focusing, even in poor light conditions.
The bad news is that not even the current CCD-based MFDB models have a live view comparable to DSLRs, and older models often don't have it at all. Some have a "live preview" or "live video" (rarely called "live view") mode when tethered to a computer, but those update very slowly (about one frame per second) often have no color and are barely usable for focusing (many lack 100% zoom too, making it no easier to see fine focus than a viewfinder). So when you use an MFDB you will still need to use traditional composition and focusing methods, like using the ground glass.
This actually means that technical cameras are considerably more expensive than they needed to be if MFDBs had a good live view. A sliding back with a ground glass needs very high precision to avoid focus shifts and thus becomes expensive. If live view existed a sliding back would not be necessary, just mount the back alone and focus using live view. Precisely distance-calibrated (and thus expensive) helical focusing mounts would not be needed either, although they still have some value (for hyperfocal focusing for example).
Poor ISO performanceCCD-based MFDBs are not good at high ISOs, the noise increases very rapidly and above say ISO200 performance is starting to get poor. To achieve the nice image quality MFDBs are capable of you should only shoot at base ISO, which typically is ISO50, but on some even as low as ISO25. This means that shutter speeds will get longer, which may be further elongated by using say f/11 (instead of f/8 on a DSLR) and having non-retrofocus wide angles with center filters.
Long exposure issuesCCDs have problems with thermal noise which means that the MFDBs rarely support long exposures. There are a few notable exceptions but most are limited to 32 seconds maximum exposure time. The Phase One P45+ is one of the exceptions and can do 1 hour exposures in cool weather.
Maximum exposure times (those 32 seconds) typically only gives good results if shot at base ISO, so it's better to underexpose at 32 seconds than increasing ISO. Just as with DSLRs most MFDBs will take a black-frame to subtract (so you need to wait additional 32 seconds before the back becomes responsive).
Background noise in these exposures is typically well-controlled, but there is a large amount of hot pixels, so your raw converter needs to have a good hot pixel removal tool. If so, the end result of these exposures is good, although noticably noisier. If you shoot in cold weather you typically get considerably less noise.
Some of the very old backs, the tethered-only ones may only work well up to 3-4 second exposures.
Moiré issuesMost MFDBs don't have an antialias filter. This means that images can be very sharp at the pixel level, but moiré can also occur. Photographing wowen fabrics (clothing) or brick walls on a distance or other regular structures that outresolve the sensor leads to ugly moiré patterns that can be difficult to deal with in post-processing.
When photographing nature those moiré pattern issues rarely occur since structures in nature are more irregular. However some sort of color aliasing will appear in almost every picture. Color aliasing is slight discolourings around fine structures. These are rarely a problem in practice though and can most often be successfully cleaned in post-processing if that would be necessary.
It should also be noted that the antialias filters you usually find in 35mm DSLRs are weak (to retain most resolution), so they are not immune to moiré issues either, although visible artifacts occur less often.
The higher pixel count your back has the less problems with moiré you will get, since resolution is then more often limited by other factors than the sensor. The 22 megapixel backs (9 um pixels) are known as "moiré machines" so it's something you need to relate to, the problem is noticably reduced already at the next step up (33 megapixels).
BloomingAnother issue of the CCD is blooming. When shooting directly into the sun so that pixels rapidly get overexposed there may be leaking to neighbouring pixels which forms a light-streak in the picture. However in less strong sunlight such as a sunset this rarely occurs, and if so it can usually be fixed by using a smaller aperture to get longer shutter speed (reducing saturation rate, and thus risk of blooming). This means that blooming is generally a non-issue in practical photography.
Centerfold and tiling issueSome digital backs have problem with maintaining the same amplification from multiple readout channels. This can be seen as vertical split of the image when brightness differs slightly of the two halves. In normal use this is not visible, but in some occasions on extreme contrast increases or together with lens color cast it may become visible. This can be dealt with in post-processing, and for technical cameras where you do a calibration shot for the LCC issue (described below) this will also automatically be taken care of.
Some backs can get calibration issues and get an unnaturally large centerfold, those then need to be sent to the manufacturer for recalibration (for a fee, typically around 500 euro and then includes general overview and cleaning).
If you hold up a medium format sensor against the light you can typically see tiles. These might become visible in extreme situations, typically when the sensor is pushed into crosstalk (discussed separately) and that is combined with a strong contrast increase.
You are more likely to have problems with centerfold and tiling if you do advanced black and white processing as that often increase stronger contrast increases than is normal in color processing.
Boot time, speed, fanA DSLR user is used to immediate readiness of the camera when turned on. MFDBs can have a considerable bootup time, say 4 - 10 seconds before you can take the first picture, and picture-to-picture time is usually more than one second. Many MFDBs have a cooling fan too, which may be temperature-controlled and only used when needed when attached to an SLR body, but instead always on when mounted on a technical camera due to lack of integration, which can be a bit noisy.
Outdoor useWith features like externally mounted batteries fans and open firewire plugs many digital backs may seem ill-suited for outdoor use, however while generally not as sturdy as pro-level DSLRs these are tougher than they look. Robust metal casing and sealed key electronics typically make them reliable for normal outdoor use.
However, there are exceptions. For myself living in an arctic climate the use in cold weather can be a problem. The manufacturer only takes responsibility that the back works in the range 0-40C typically, so if you get unreliable operation at -5C you may not get any help. I have myself experienced this with the Leaf Aptus series, by being patient and persuasive I finally got it resolved in the end though.
So if you will be using your back in cold weather you need to be extra observant. It's more likely that an MFDB will not work in a tough weather condition than any other type of camera, as the typical MFDB use is in a studio.
Reliability, bugsMass-market DSLR products are extremely well-tested before they are released. MFDBs are no mass-market products and do not receive as much testing before release. This means that they can be a bit buggy, especially early releases. Experienced MFDB users often have a higher tolerance of "funny things" happening than a DSLR user, so an MFDB that is in the MFDB user community known to be reliable and error-free may not be as reliable and error-free as a DSLR user would expect (ie zero issues).
Technical camera specific issuesTechnical cameras are 100% mechanical and have limited integration with the backs.
Limited EXIF infoDue to the 100% mechanical camera there will be no information in the file about f-number, focal length or shutter speed, and not any camera movements (shift/tilt), so if you are interested in storing that information you must do it manually.
Lens color cast (LCC)The most important issue related to technical cameras is "lens color cast", which actually is a sensor problem. Sensors respond best to light falling perpendicular onto it. Technical wide angle lenses can project the light at a very low angle far out in the image circle, which the sensor is less sensitive to and differently so depending on color channel. This leads to an uneven color cast over the picture, typically a magenta cast.
If the sensor has microlenses to increase base sensitivity and ISO performance this color cast can become even worse. SLR camera bodies have a mirror box which forces wide angles to be retrofocus designs, thus light falls in closer to perpendicular and thus there is little or no color cast issues with them, and (older) digital backs with microlenses are generally designed for those.
Today Rodenstock makes technical camera lenses with (weak) retrofocus designs that reduces color cast issues so that some microlens sensors can be used with them. These are more expensive than traditional designs though. Even with those there is some residual color cast left.
Due to the color cast one have to shoot an extra calibration shot with a milk-white plexi glass in front of the lens, which then is applied in post-processing to neautralize the color cast (and as a bonus any dust spots and vignetting). This is called flat-field correction, LCC correction or just LCC.
It's cumbersome to have a pre-shot library of these calibration shots since shift/tilt/aperture affects it, and that is not recorded in the file. This means that most don't have that and instead shoot a calibration shot for each picture they shoot. Fortunately, any still life workflow is always multi-step and fairly slow so that extra calibration shot is not contributing significant work.
Irrecoverable color cast = crosstalkIf the wide angle lens projects the light at a much lower angle than the sensor can handle the light may leak over to the next pixel. For example the light may pass the red filter for the red pixel, but then get registered in the neighboring green pixel. This is called crosstalk, and is seen as a color cast but the difference is that it cannot be cancelled by doing the normal LCC correction.
The effect seen after LCC correction is generally desaturated colors and in worse cases color shifts, and mazing artifacts at the pixel level.
The onset is gradual and a small amount of crosstalk is generally tolerable in practical photography. How much that is too much is subjective.
Unfortunately the trend has been that newer sensors with smaller pixels are more prone to crosstalk. At the time of writing the worst sensors in this regard are actually the newest CMOS sensors.
If you get a digital back with 39 megapixels or less crosstalk is rarely an issue regardless of lens (still avoid the early microlens backs though). With higher pixel count it becomes more important to make a lens choice that is compatible with the sensor.
Even when crosstalk from a color fidelity aspect is acceptable some sensors can provide other artifacts as side-effect, namely visible tiling and ripple from micro-lenses. A basic flat-field correction algorithm will not correct those artifacts, but there are some software that can do it.
Schneider Digitar wide angle lenses, especially the 35mm and 28mm will push some sensors into severe crosstalk which limits their usability. So if you intend to use that lens range rather than the more expensive retrofocus Rodenstock Digaron range you must be careful when choosing back.
SynchronizationTo synchronize the digital back with the mechanical Copal shutter it's connected with the flash sync cable. The flash sync is simply a circuit which is open while the shutter is closed, and is short circuit when the leaf shutter is open. It's a simple mechanical mechanism which means that the short circuit can be formed when the shutter is half-open and broken when half-closed. In other words the signal comes after light already has started to come in and stops before light has stopped coming. At the end it's simple for the digital back to adapt just by waiting a few milliseconds so readout does not happen too soon. However the late signal at the start cannot be predicted and some digital backs can get problems (= color casts on the final picture) if light fall in on the sensor before it's ready for exposure.
However not all mechanical shutters are the same, which means that timings can vary a bit, and that leads to that a digital back may get color costs with one shutter but not with another. I've tried a back that had problems when synching on a Mamiya RZ camera, but actually managed Copal shutters, except for short exposures (1/250 and faster). Expect some individual variation. Short exposure times means higher risk of problems.
Backs with Dalsa sensors can handle early light, so for some backs all that is required for synchronization is the flash sync cable and you don't need to worry about any variations in shutters. Most Leaf backs have such sensors. However many of the backs using a Kodak sensor may require a wakeup trigger before each shot, pressing a wakeup button and then pushing the cable release, at least for faster shutter speeds.
The company Kapture group makes a "one shot cable release" which sends a wakeup signal to the back a fraction of a second before the pin that triggers the shutter is extended (one must not push the long-throw trigger too quickly though). This way the wakeup procedure is hidden for the backs that require it, and if the back can detect when the exposure ends then it becomes just as smooth to work with as a Dalsa-based back. However, the one shot release cable is expensive and a bit clumsy so some prefer using a manual wakeup anyway, especially when you have the type that only needs wakeup for the fastest shutter speeds (which are rarely used with a technical camera since one typically shoots with small apertures).
One example of back that requires wakeup signal is the Phase One P45+. Hasselblad backs that use Kodak sensors can often do without wakeup signal, but probably varies a bit between shutters and shutter speeds, and possibly firmware versions (having the latest firmware is always recommended when it comes to digital backs).
Some backs, like the Hasselblad backs, allows for setting the shutter speed in the back. This can lead to the misconception that you need to set shutter speed in the back despite having a flash sync cable. This is actually only necessary when you don't have flash sync cable connected at all (such as a pinhole camera), or possibly for some shutters which breaks the circuit too early(?).
Don't forget that as a technical camera user you will probably need to shoot a calibration shot with the LCC card in front of the lens. Usually one hand-helds this card, and then you only have one hand free to trigger the exposure. If the back requires pressing an additional button for wakeup you may end up wanting to have three hands.
CCD manufacturersThere are with very few exceptions only two CCD manufacturers for MFDBs, Kodak (nowadays Truesense Imaging) and Dalsa. In the current models almost all use Dalsa sensors, but among the older in the second hand market there are many Kodak sensors. With the proper implementation Kodak sensors can do long exposures (Phase One P45+), which Dalsa sensors cannot (exception: the recent 60 megapixel sensor used in IQ260).
Of special interest to technical camera users is that the Kodak 50 megapixel sensor used in many Hasselblad backs is much less crosstalk-prone and can handle symmetrical wide angles without any severe crosstalk issues. Dalsa sensor with the same pixel size (6um) do not handle symmetrical very well, the older Dalsa sensors (33 and 22 megapixels) can be used with symmetric wides though.
Dalsa sensors are typically considered to have a more realistic color reproduction than Kodak sensors, and the later ones have also a little bit better dynamic range.
Chip sizesDigital medium format comes in many different sizes, and none is as large as the smallest 120 film format (645). The largest is very close though with 54x41mm, and is by manufacturers considered as "full-frame" 645. This largest size exists only in current models so prices are still very high. Leaf also has the odd size 56x36mm which is the same area but a wider format (and less expensive).
44x33mm - entry level format (both today and yesterday), most often microlens designs ill-suited for technical cameras
48x36mm/49x37mm - (twice fullframe 135) common in yesterday's high end backs, often good second hand option for technical camera use.
36x36mm - square format, typically used for Hasselblad V-mount, not available among current models.
36x24mm - the first digital backs where not larger than 135 fullframe.
Medium format digital image qualityMFDB image quality is often claimed to be inherently superior to 35mm digital. You may hear things like "superior dimensionality and clarity" and other subjective unmeasurable things. Maybe you as well will see these things, or you maybe you will not. If these differences are real or not are frequently discussed in heated internet forum debates.
Medium format CCDs produced very good image quality already back in 2003 - 2004, which means that old second hand digital backs still makes images which match modern quality standards, something that cannot be said about a CMOS-based DSLR from 2004. The CMOS development for 35mm digital has been more dramatic, and more diverse. While all MFDBs from at least 2004 and onwards produce excellent image quality at base ISO, for 35mm digital only a handful makes similar results, but those even exceed medium format sensors in some measurable aspects.
What can be said for sure is that both systems can today produce very high quality images.
Sensor dynamic rangeThe most commonly claimed medium format image quality advantage is dynamic range, that is the space between unacceptable shadow noise to clipping highlights. Often the 16 bit files (14 bit on current DSLRs) is used in market material to indicate this advantage. Problem is that 3 - 4 of those 16 bits are pure noise, and today some CMOS sensors actually exceeds the raw signal-to-noise ratio per pixel. It's also a fact that many of the MF raw formats have actually never stored more than 14 bits per pixel.
However while this is a recent development on the DSLR side (CMOS chips), MFDBs has had very good dynamic range since more than ten years (CCD chips). Also if you compare with current Canon sensors which are good at high ISO but have noise problems at base ISO, any MFDB, even old ones is clearly ahead.
Since MF sensors are larger the photon shot noise levels are somewhat lower in the same image area meaning that midtone noise can be lower, but a fullframe 135 sensor is already large enough to keep this at a very low level. It's also the case that a modern 135 sensor can have so much more effective pixels that they actually record more photons than an older MF sensor, despite being smaller.
This image sensor noise test compares a couple of DSLRs with a fairly old MFDB. What can be said is that both yesterday's and the current MFDBs have competitive dynamic range so it's nothing to worry about.
It can be argued that while MFDBs look somewhat noisy compared to the current best DSLRs, the noise is well-behaved and random similar to film grain and might actually contribute to a more analog "film-like" look which some may prefer.
Although I personally think that dynamic range is fine also with the older backs, I know there are other opinions out there. Ever since the 36 megapixel Nikon D800 was launched in 2012, DSLR users have become used to very clean shadows which can be pushed hard in post-processing. If dynamic range is a primary concern for you and you have become used to what the best DSLRs do in that regard, it's likely that you will be somewhat disappointed with any MF back using a CCD sensor, they are simply not as noise free.
Sensor resolutionThe undisputable medium format advantage is resolution, today high end medium format backs have 80 megapixels since a couple of years and DSLRs have 36 at the time of writing. If you actually need all that resolution is not for sure, but many landscape photographers always seems to want more of it.
Somewhere in the range 30 - 50 megapixels is where most think that the image quality matches or exceed 4x5" large format film, assuming appropriate lenses are used.
Most MFDBs lack antialiasing filter which makes the images a little sharper on the pixel level, but also prone to moiré and color aliasing. Since AA filters are rather weak in cameras that have them the actual difference in look after post-processing sharpening is quite small. It should also be said that high resolution 35mm DSLRs is often without AA filters, just like MFDBs.
LensesIn image quality discussions the lenses are often completely left out of the equation. However, lenses may actually be the reason you start with medium format.
If you are an amateur aiming for second hand gear you will probably not get a high resolution digital back, perhaps settle with a 5-7 year old 22 megapixel back. That sensor will render great images, but it will be hard to claim that it is better than what a 36 megapixel Nikon D800E can do. However, when lenses are added to the equation it may become different.
Portrait photographers often talk about a superior bokeh quality in some medium format lenses. The stronger "3D look" that some see is also probably a lens quality. Although highly subjective it is real for many.
When you get to technical cameras the difference is more obvious. To start with there is only a low amount of tilt/shift lenses available for DSLRs, and many of them are not that good optically, and most are inflexible in movements. If you want to work in a similar fashion like large format film, a DSLR is not a very good option.
With the "Large format digital" (actually medium format, but designed in the large format way) Rodenstock Digarons and Schneider Digitars lenses there is more total resolution over an image circle large enough for shifting. Good corner-to-corner performance can be achieved in a way that can be difficult for DSLR lenses, especially on the wide angles.
With new super-resolution lenses like the Zeiss Otus series DSLRs is starting to reduce this gap, but there is still a long way to go, and again if you need tilt/shift there are not many options.
Color renditionAn advantage that often comes up is superior color rendititon, especially of Dalsa-based digital backs. How much this depends on the chip and how much that depends on the color management of the raw converter is unclear, but the difference is real to many and for the professional portrait photographer this can be a deciding factor.
One theory is that since DSLRs need to work well at high ISOs the color filters are compromised, while MFDBs are focused at maximum performance at base ISO and let high ISO suffer instead. I do not know if this is true or not though. In scientific measurements the best modern DSLRs have better color accuracy than older backs. "Pleasing color" is not so much about accuracy though. When color rendition is discussed many rather talk about a "film-like look" in positive terms and may find that MFDBs are better at producing that quality.
This color rendition aspect is generally more important to portrait photographers as humans are most sensitive to skin colors. As medium format backs are traditionally used in the studio often for portrait and fashion the manufacturers have been very focused at making them render skin tones as good as possible.
ConclusionTo sum up -- don't have too high expectations on the image quality. It is indeed great, but now when the high resolution DSLRs can produce MF-like quality it's harder to get impressed.
Price/performance of MFDBs has never been good, and with the tough competition from high resolution DSLRs that exist today it may look even worse. So if you get into second hand medium format it's best if it's not for the image quality alone, and certainly not for the pricing.
Tips for the second hand buyer
Old backs are still going strong!Due to pricing of new MFDBs looking around in the second hand market may be the only feasible way for an amateur to get one. Resolution of the older backs can be a bit low with today's standards, but apart from that the image quality is still competitive - CCD-based imaging systems were great already back in 2003-2004.
The same CCD sensors are often reused in newer MFDB models. There may still be improvements in image quality though, because sampling electronics is off chip and IR filter may be different, and firmware has some effect too. Typically what can be significantly improved is noise performance in long exposures, but apart from that the older backs with the same sensors provide almost the same image quality. Instead most improvements is in display and user interface. Manufacturers and dealers may exaggerate the improvement of new models since they would like people to upgrade.
If you buy second hand the best price/performance in terms of image quality is typically found in a late release of an early model when a sensor was new.
Watch out for early releasesMFDBs is speciality equipment for professionals sold in low volumes, manufactured by relatively small companies. This makes them expensive but also has an effect on the quality and how they are sold and supported.
The typical customer is a professional photographer that has a close relationship with the dealer and may pay a yearly fee for a very supportive guarantee. The small MFDB manufacturers don't have the testing resources as say Canon or Nikon, but since the sales volumes are low and customers has tight support relationships with them (via dealers) it allows for releasing products that may be a bit experimental, both concerning firmware and hardware. Problems with first releases are thus frequent, but these are swiftly fixed in subsequent releases, and the customer may through the support agreement get a temporary replacement back while the own is at the manufacturer for fixing.
For an amateur on the second hand market it's of course a risk that early releases of MFDBs can be a bit experimental. There are examples of early releases that had hardware bugs that caused them to fail and become unusable as one attempted to upgrade firmware. Therefore it's safest to get a back that has later firmware versions already installed, this makes sure that the back is not an early buggy release.
It may be a better deal to get a back which has been a professional photographer's workhorse, maybe tens of thousands of pictures taken but also well-maintained with latest firmware and if there was buggy hardware it has been fixed, than a back which has been sitting on a shelf as a backup. There are no moving parts in the back (except a cooling fan in some backs) so there is no problem if a large number of pictures has been taken with it. More heavily used backs are also sold at lower prices.
Another aspect to be aware of is the compact flash interface of the backs. These may not work with the latest and greatest CF cards. Later firmwares may expand compatibility, but for the oldest backs you may still need to get older CF cards that may be discontinued today, so you must find them on the second hand market.
Don't get a too old backBefore 2004 all backs were tethered only, that is they require a computer attached to it to operate and thus mainly meant for studio use. I don't recommend those backs. The latest of those can produce great image quality, but they are generally overpriced compared to the first untethered backs, and compatibility with newer computers may be a problem.
I have myself not investigated this option thoroughly so I do not discuss it more here. I'd recommend to look no further back in time than to the 22 megapixel backs (2003-2004), or their 16 megapixel square versions. Older than those resolution becomes inadequately low, and some sensors are not larger than 35mm digital.
Backs are usually supported for about 7-10 years, but after that it can be the case that repair is no longer possible. If you get a back that no longer is supported by the manufacturer make sure you get a low price.
BatteriesBatteries age, and it's not unlikely that you get old batteries with your second hand back. Some backs can start to behave strangely when the batteries are old. The back may sometimes refuse to boot up, lock up at times and show various stability problems. So it can be worthwhile to replace the batteries before you assume that there is some problem with the back itself.
Outdoor handlingSome backs have high risk of failure in cold or wet weather. A passively cooled back rather than a fan-cooled back may be an advantage, but many of the fan-cooled have been proven to work well. Do check the reputation of your particular back, and/or ask the seller which conditions it can be expected to work.
From a dealer or an individual?Dealers sell second hand gear too. Unfortunately the lower end backs are often priced 60 - 100% higher than in the individual-to-individual market (smaller difference on more expensive products). Dealers usually provide some sort of guarantee though. If it's worth the price is up to you.
If you are used to buy second hand gear you probably prefer to take some risk in exchange for the considerably lower price. This suggestion not to buy from a dealer but rather from an individual may be seen as a bit controversial by some. However, while dealers can do a great job with supporting their customers the business model is designed for professional users and for amateurs it may simply cost too much.
Also make sure you get a dealer which has good pricing and understand price-sensitive amateur users, this can become important if you get unlucky and your back needs service. Some dealers that are only used to high profile professionals where price is not an issue can be great at providing service quickly but poor at finding the most cost efficient solution to solve a problem.
RepairingIf the back breaks it can be repaired if not too old. However, repairs are very expensive since they relate to the original purchase prices.
I have myself been unlucky and had to repair my digital back. Due to a bad choice of dealer the repair cost would be almost twice than the price I got when I aborted and went to a different dealer. My advice is to use a dealer that has good reputation and long-term experience, and say that you are a price-sensitive amateur where cost efficiency is more important than time. If you like me are in Europe it may be worthwhile to use a dealer which has prices in euros, since some dealers make a quite large hidden markup in currency exchange.
Even with a good dealer repairing is not cheap. Fixing a scratched glass should cost about 600 euro + VAT, general service check (changing internal battery for the clock, aligning sensor, cleaning etc) is about 500 euro + VAT, and a big repair which includes replacing most electronics except sensor can be about 2000 euros + VAT. Replacing the sensor costs even more, but is a very unlikely type of error. Taking into account that failure can happen and having a repair budget is advisable.
In short, if your back ends up needing repair a good deal can suddenly become a pretty bad one, and if you bought a cheap back it can possibly be so bad that you will rather buy another. This is a risk you will have to deal with when buying second hand from private individuals. Buying from dealers gives you some sort of guarantee, but often that can cost more than a second hand back plus (basic) repair, and the guarantee typically only covers six months. If you intend to use your back in tough conditions (cold/hot weather) make sure that happens within those six months.
One way to view this expensive repair issue is that backs worth less than say 3000 euro on the second hand market are so uneconomical to repair (depending on fault) that if it fails you can consider it to be the end-of-life of it and plan to get a another one instead of repairing. It's always worth asking a dealer what repair will cost though. A thing like replacing a scratched sensor glass is usually worth it.
In Europe dealers are often small companies, sometimes only one-man companies, and therefore service may be suffering. Turn-around times for repair or service can be very slow, it happens that it takes several months to get an issue fixed, and you may need to nag quite a bit to make things happen. Professionals almost always have back-up backs or a costly guarantee that provides backup back during repair to cover this.
Special considerations for technical camerasWhen you are going to use the back with a technical camera there are some additional things to consider. Sensor size should be suitable for the lenses so you get useful field of views and suitable margins for shifting. The sensor should also not have too strong color cast or crosstalk problems with the wide angles you intend to use.
I do not recommend the 44x33mm format, that sensor size is mainly used in backs aimed at SLR type of cameras and often have microlenses that cause them to get too strong color casts with technical wide angles. There are backs that do work well in that size but there are fewer options, and when upgrading in a few years you would probably get a larger size and may then need to change lenses too.
The most practical sensor size I think is 48x36mm (sometimes closer to 49x37mm). The Rodenstock Digaron-W and Schneider Digitar lenses with 90mm image circle gives a generous but not overkill margin for shift. There are many alternatives on the second hand market, handling wide angles well too. Unfortunately this sensor format is not so common in current models though.
Instead the larger 54x41mm (almost fullframe 645) is today the most popular format for technical cameras, but it is actually a bit less flexible, less relative movements within the 90mm image circle, and as these are small pixel Dalsa sensors they have worse color casts and crosstalk issues so they need the more expensive retrofocus lens designs for wide angles. There are still a handful of years before this sensor size is affordable on the second hand market though.
Recommended second hand entry level models 2014Most of the suggested models are 22 megapixels 48x36mm. 22 megapixels is certainly not cutting edge 2014 when 35mm digital reached 36 megapixels with the Nikon D800 already in 2012. However, the actual resolution is not too bad, you can print about 27x20" (68x50 cm) in 200 ppi, or 15x11" (38x28 cm) in 360 ppi, so it is high enough resolution for most fine art and photo book applications. Sure it would be nice with more but 22 megapixels is an ok starting point.
Most technical cameras have adapter plates for all the mounts, Hasselblad V-mount, H-mount, Mamiya etc, but a plate usually costs around 500 euro, so if you already have an adapter plate there is some money to save to get a back that fits.
With 22 megapixels there is a huge 9 um pixel size which means that for a technical camera the low cost (and less sharp) Rodenstock Apo-Sironar digital lenses can be used with good results.
If you go for something as recent as a Dalsa 6 um sensor to your technical camera you may end up having to use retrofocus Rodenstock Digarons for the widest angles, due to crosstalk issues. These are considerably more expensive lenses so if you're building a budget system it's recommended to get a back that can handle the Schneider Digitar wide angles.
Phase OnePhase One is the largest brand and the easiest to find second hand backs for. Phase One P25 and P25+ are good entry level backs (22 megapixel), both based on the Kodak KAF-22000 sensor. Early P25 may be buggy, so get one with the latest firmware. The P25+ has the rare ability to support long exposures of many minutes (unlike the P25).
The P25 LCD has very low resolution and cannot really be used for sharpness/focus check, but the histogram works ok. Quite ok visibility in daylight. The P25+ has a higher resolution LCD which is supposed to be good enough for basic focus checking (although likely not for absolute sharpness check).
Still somewhat pricey are the KAF-39000 based P45 and P45+ (39 megapixel). P45 is hard to come by, and early versions had some issues, so it is probably wiser to go for a P45+. It is still today one of very few MFDBs that can do very long exposures (up to one hour in cool weather), and partly due to they it can be a bit expensive compared to other alternatives.
All these kodak-based backs need to be woke up before exposure (unlike Dalsa-based backs). The wakeup cables from Phase and the one-shot cable from Kapture Group are not cheap, so don't forget to calculate with that cost.
If you can live with the 44x33mm format, the P40+ is coming down in price, which is a crop of the larger full-frame 60 megapixel Dalsa sensor. Personally I think the P45+ is a better choice though for technical cameras, due to less crosstalk issues and a better balanced sensor size in relation to the lenses.
Concerning reliability in various weather conditions the passively cooled P+ series is probably the best medium format backs produced so far. The backs have very good reputation.
LeafCompared to Phase One Leaf backs are a bit less expensive, which also may be mirrored in the second hand market, but not always. The backs are based on Dalsa chips which means there is no need for a wakeup cable, and there is also a bit less color cast on them with wide angles (if we compare 22 and 33 megapixel sensors). The drawback is that there is no Dalsa-based back that can do long exposures, most are limited to 32 seconds.
The best price/performance models are Leaf Aptus 22 and Leaf Aptus 75. The newer models Aptus 54s and 75s are faster but actually has more noise issues (only at higher ISOs though), so for technical camera use the 22 and 75 can be better deals.
The Aptus 22 has only ISO25 as base ISO, and with the 32 second max shutter speed it can be tight at f/11 and center filter in low light. The 33 megapixel Aptus 75 is ISO50 like most backs. The LCD of these backs is large but only 320x240 and relatively poor visibility in daylight, some use a focusing cloth or similar to shade the display. If you use the ground glass you will not be disturbed by the screen visibility since ground glass is certainly dimmer. The screen is good enough for focus checking (better than P+ and older Hasselblad), which is rare for older backs. It's not fast though, zooming in to 100% takes about four seconds.
These backs have a cooling fan which is a bit noisy. The electronics are sealed off though, so it does work well for outdoor use with one exception: it's quite likely that they start to behave strangely in cold weather, especially when mounted to a technical camera as the fan is then always running at full speed and can cause the electronics to become too cold (may be due to poor quality components rather than the fan cooling too). There are copies that work well though, impossible for me to know how common.
The Aptus series even when working well is not 100% bug free (strange things may happen when running out of battery, the beep sound may stop being emitted and things like that), and are very sensitive on that you use a compact flash card from the supported list or else strange things can happen, write errors on the CF card etc.
The more recent Aptus-II series is coming down in price and can be a good choice. The models that share sensors with the older Aptus series will have almost exactly the same image quality, but a bit brighter screen and faster operation.
HasselbladHasselblad has during a period been less interested in integrating with technical cameras than other brands. Some H3D backs have problems with wide angles and sync problems at shorter shutter speeds. It can be made to work but may require being sent to the manufacturer, thus high risk to buy second hand. If you get a really good deal of a Hasselblad back (usually with camera body) it may be worthwhile to investigate further if it works with a technical camera, but otherwise I would not recommend it, there are simply too many question marks.
(Here in Sweden it is quite common with good second hand deals on Hasselblad SLRs with a back.)
The 39 megapixel CFV-39, same sensor as Phase One P45+ but no support for long exposures, can be used with techical cameras. It's generally sold at a lower price than the P45+, but is harder to come by. Sometimes CFV backs are sold at higher prices than one would expect because it matches the V system in look and therefore has some attraction by V system fans.
Today the 50 megapixel Hasselblads can be affordable, sometimes actually cheaper than 39 megapixel Phase One P45+. These use the Kodak KAF-51000 sensor and has the unique property to work well with symmetrical wides (Schneider Digitar range) while having this high resolution, which makes them highly interesting for technical camera users looking to use the more economical and lighter Digitar range. The CFV-50 is the only self-contained unit if we disregard from the current H5D-50 (which has a battery adapter accessory), but both the H3DII-50 and H4D-50 can still be used by using an external battery, as sold by Silvestri and others.
Despite being fairly recent the screens for the 50 megapixels backs are not great, the H4D-50 (if not an upgraded H3DII-50) has a bit better screen but I have not personally seen them so I cannot say anything for sure about how good they are for focus check.
SinarSinar digital backs are typically used in the studio and many of their models are tethered only. The 22 megapixel Sinar eMotion 54LV can be used untethered, but be prepared for that it has a really poor LCD.
MamiyaMamiya has had a few backs that were simply rebranded Leaf backs, like DM22 and DM33. These appear quite seldom on the second hand market so I have not investigated them further.
I would not recommend the "budget" Mamiya ZD back though which seems to have problems with multi-second exposures and some reliability issues.