Now that I've gotten the basics of medium format out of the way, let's dig in a bit.
As far as I know, there aren't very many producers of medium format (MF) sized sensors out there. Not anymore, anyway. The big names in MF digital are Phase One and Leaf. Which, as I understand it, are now both owned by the same company. Hasselblad has their own system, which is exclusive to their current H system.
In the short amount of time that I've been into MF digital (less than two years), I can say that it seems that the most commonly used MF digital backs come from Phase One. I've spoken to several very knowledgeable people about this, asking them "why Phase One over Leaf?", and the answers I get are very similar to what you might hear with the "why Nikon over Canon?" question - it's a personal preference.
The pricing of all of these systems are relatively comparable with one another, generally starting at the $20,000 mark for the lower-end models. Phase One has made some interesting choices over the past few years, and seems to have the biggest company with the most products out there. Let's talk about some of them.
For starters, there are the H series digital backs. These have been around for over ten years. These backs were only usable in-studio, as they required tethering to a computer in order to work properly. The physical sizes of the sensors were actually not MF back then; they were 36mm x 24mm, which is actually what we know as 35mm "full frame" nowadays. But remember, ten years ago, that was pretty huge. These sensors started out at 6 megapixels, and eventually got up to about 25. As the megapixels increased, the sensor sizes did, too, and they got up to about 45mm x 35mm - a decent increase, but still not quite as large as a piece of 120 film.
These first generation backs were kind of finicky (I'm told), and marked the debut of Phase One's now famous digital capture software, Capture One. Capture One basically acted as the user interface to the digital back, providing all of the controls (setting ISO, white balance, etc), showing captured images on screen as they were being shot, and then processing the RAW files into TIFFs.
The next generation of Phase One backs were the P series. This is where things started to get interesting - the P series backs had integrated LCD screens, and CF card slots (in addition to their FireWire ports for tethering). This meant that you could actually take the camera out of the studio and even shoot hand held. Not very many people actually do that, but I think that this move really made a big impact.
As the P series evolved, additional features were implemented, such as faster capture times, better sensitivity, and the ability to do LiveView, as well. The most popular backs used today are the P40+ and the P65+, followed by their predecessors, the P30+ and the P45+. The P30+ and P40+ have nearly identical sensor sizes (about 44mm x 33mm, which would be a MF crop factor of about 1.25x (think APS in the 35mm terms)), and produce images that are about 30 megapixels (on the P30+) and 40 megapixels (P40+). Photographers choose these backs because they tend to be the most responsive, speed-wise. Because the sensors are a little smaller and the files aren't as big, everything moves a little faster. By "fast", I mean that you can capture an image almost once every second. I know, it sounds slow when you compare it to the insane 10 to 12 frames per second that the high-end 35mm size cameras can do, but remember - these files are close to four times the size of those. AND the mechanics inside these cameras just aren't built for that kind of performance. The mirror inside a 35mm camera is a lot smaller and more manageable than the giant pieces of glass inside MF cameras.
The Top end P+ backs are the P45+ and the P65+. The P45+ has a _near_ full-frame sensor (about 49mm x 36m), and the P65+ is the first back to get to (what Phase One calls) 100% full-frame - 54mm x 40mm. It's still a bit shy of the "645" MF film format (60mm x 45mm), and there still aren't any digital backs that can do the 60mm x 70mm or 60mm x 90mm formats that are sometimes used on MF film cameras. But it's progress, right?
The files that these backs produce are no joke. For one thing, the dynamic range is very impressive. These sensors are collecting so much light and so many pixels, that in the right hands, you can process terrific-looking images from what might otherwise be considered unusable exposures. I'm talking about 3 stops - and in some cases, even more. Additionally, these are 16-bit files, which means that in addition to the massive number of pixels, they are incredible deep, giving you even more flexibility when processing the files.
Earlier this year, Phase One announced a new line of digital backs - the IQ series. These come in two sensor sizes; the smaller one is similar to the P30/P40, and the larger ones (there are two) are the same size as the P65, "full frame". There are several significant innovations with these backs, including an iPhone 4-esque "retina" class LCD display, with an iPhone-esque multi-touch user interface. From what I'm told, some people feel that this improvement might actually be good enough to confidently shoot untethered, which is pretty cool. Additionally, they added a USB 3 port and a FireWire 800 port (both, but you can only use one at a time) for higher-speed I/O.
These new backs are just now hitting the market, and I haven't had a chance to play with one, but I'm sure they're pretty darned cool.
One of the neat things about the Phase One digital back system is that they are essentially camera platform independent. As I'd mentioned before, there are quite a number of different flavors of MF camera systems out there, many of which use interchangeable backs. Phase One has wisely made their backs available to work with just about all of the major camera systems.
Recently, Phase One decided to release their own camera system, the 645DF, based on the Mamiya 645 format (by which I mean you can use Mamiya lenses with it). What's interesting to me is that there are two other very popular MF digital cameras that look exactly the same as the Phase One 645 - the Mamiya DM and the Pentax 645. The difference is that these companies are making their own digital backs, which are considerably less expensive than Phase One's offerings. I can't really say what the quality is like, but I'm guessing it can't be that bad.
Capture One has seen a lot of development over the past couple of years, and I think it's safe to say that it's the most commonly used software for working with MF digital systems. They've worked with a few other camera manufacturers to produce software/plug-ins/drivers to allow cross platform compatibility, which is quite cool.
The latest version of Capture One is 64-bit compatible, and can process files using pretty much every ounce of CPU power than you can throw at it, which is pretty sweet. Watching it burn through a folder full of say, five hundred 65 megapixel images on a 12-core Mac Pro is something to behold. The other nifty feature that I think will catch on quickly is the ability to stream the contents of a specific folder (like the one you're capturing to, for example) to an iPad or iPhone, wirelessly. The iOS app is a free download, and once you get it set up, you can have as many clients connect to it as possible. The latest version even allows you to apply star ratings and color labels to the images instantaneously (it's kind of creepy, actually).
With the Phase One P series backs, the most commonly used camera systems are from Hasselblad and Mamiya. Hasselbad's 500 series film cameras use a what's called the "V mount". I think this is my personal favorite system right now. I really love the way the older manual Hasselblad lenses look and feel, the image quality is amazing, and the overall form factor and design is extremely tight, simple and efficient. The only thing to be aware of is that you need to use a sync cable to connect the lens (where the shutter lives, since these are leaf-shutter systems) to the digital back. A reasonable compromise, if you ask me.
Hasselblad also has a line of automatic cameras, commonly referred to as the H system. The first two iterations of this camera (the H1 and H2) are effectively identical; as far as I know the only differences between them are in software, and most people who have H1s send them into Hasselblad to have them upgraded to H2s. The successor to the H2 - the H3 - marked a decision by Hasselblad to enter the digital back game themselves, effectively ending compatibility with Phase One backs. The fact that photographers are still using the now nearly 10-year-old H1 and H2 systems (so they can use Phase One) says something to me about how well-accepted the Hasselblad digital backs are liked. I'm sure the H3 and H4 are fine cameras, but I've never gotten to play with one.
These cameras use autofocus lenses, which are also quite sharp, but they are made by Fuji in Japan. Because the camera is electronic, all of the manual controls are integrated into the body, and adjustments are made using buttons and dials, very much like modern 35mm DSLRs. These lenses only have one giant ring, for focusing. This is a good thing, because I have yet to see an MF camera system that can autofocus worth a damn. One of the nice features of the H system is that the backs do not require the sync cable; that connection is handled internally. This makes for a pretty nice, streamlined camera. The thing still weighs a ton, though, especially with a longer lens on it. Not really practical for hand-holding, if you ask me. But the fewer cables you have to deal with, the less likely something will fail, right?
In the fashion world, when you say "Mamiya", you're talking about the RZ67. The "67" in the name refers to the size of the image on a piece of film - 6cm x 7cm. Phase One makes a special adapter plate that fits onto the back of the RZ, allowing it to accept V mount digital backs. This is a good thing, as the V mounts are generally the most popular and easiest to find. The downside to this system is that there is an additional wired connection that has to go from the adapter plate to the camera body (since this system uses an electronic shutter), in addition to the sync cable to go from the digital back to the lens. And as if that weren't enough, these suckers are BIG. A fully automatic RZ rig - lens, motor winder, prism, body, and digital back with plate - weighs a good four or five pounds. And it's easily the size of small cat or a puppy. Not the kind of system you see people walking around with. But man - the images that this thing makes are pretty fantastic. Which is why there are still folks out there using it.
Mamiya has recently updated their camera systems, including a successor to the RZ - the RZ 33 - which uses a digital back from Leaf. I've played with one once, and it looked pretty sweet.
In addition to the obvious benefits of much larger images, higher dynamic range, and the ability to use all of that amazing glass, there is one other key advantage to using MF camera systems - sync speed. Since this post is already ridiculously long, I think I'll save that bit for next time.
Thanks for reading, and as always, any feedback is welcome.