Medium format cameras have long been considered the tool of choice for professional photography. Sure there are tons of professionals out there using 35mm-sized camera systems, but if you were tally up every owner of a 35mm system and compare that list to a tally of every owner of a medium format camera system, I'd bet that there are way more pros on the medium format (MF) list. A large part of this has to do with the cost associated with MF; with the bigger size film/sensors, physically larger lenses and build quality, things can get pretty pricey. This is generally considered "the cost of doing business" in the industry.
One of the coolest feature of MF cameras is that they have a tendency to be very modular. Sure, there are some simplified "closed" systems, but the most common pro cameras can be broken down to their basic elements - camera body, lens, finder, winder, and back. For those of you less familiar with MF, I'll explain a bit.
35mm-sized cameras are small and self-contained. In the old days, with film cameras, you'd pop open the back of the camera, load your film, unwind the film out of the canister frame after frame as you shoot. Once you hit the last frame of film, you'd rewind the film back into the canister (using a little crank with a flip-out lever; that was always one of my favorite things to do when I was a kid). Pretty straightforward stuff.
Now, let's say you've got a shot all set up, and you wanted to so some basic bracketing, just to make sure you got everything exposed right. You'll need maybe 8-12 shots to do this. Oh, and let's also say that you wanted to have that same image on both black & white and color film. WIth a 35mm system, where the rolls of film would come in lengths that would give you 24 or 36 exposures (frames), once you had the film in the camera, you'd have two basic options. You could either shoot all the way through the roll, or you could just end the roll early, wasting the remaining film.
With a MF camera, things are different. First of all, the film used is close to four times the size of the 35mm film. To keep costs reasonable, a typical roll of MF film will yield about 12 exposures. I say "about" because different MF cameras have different frame sizes and dimensions. For example, Hasselblad cameras make square images which are 5cm by 5cm, and Mamiya cameras make rectangular images that are 6cm x 4.5cm.
Rather than building the film transport mechanism into the camera body, MF cameras have a separate, removable mechanism. It's kind of like how a gun works, with interchangeable clips. You don't need to fire all of the bullets in a clip; you can swap out at any time without losing any shots. The mechanism that contains the film is referred to a "back", since it sits on the back of the camera. You load the back just like you would load a 35mm camera, but there's no camera on the front. Instead, there's a light-tight piece of metal called a "dark slide", which keeps the film from being exposed.
The interchangeable back system has some really great advantages. First, you can have several backs all loaded and ready to go, making continuous shooting a lot easier than having to pop the camera open and change film. With two film backs, you could have an assistant handle the loading and unloading of film, and as soon as you finished the roll in the back that you've got on your camera, you simply put the dark slide back in, pop the back of (just like changing a lens, only faster), hand off the back, grab the next back, attach it to the camera, pull the slide, and boom, you're back to shooting. Experienced pros can do this in like, five seconds.
The other obvious advantage of interchangeable backs is the ability to have different film types available on a per-shot basis. In high-end studio sessions, it is very common for prelighting and test shots to be made with polaroid film, so you can get a good idea of what you're going to get before you commit it to film. Polaroid backs are available for all of the major players' cameras. So you'd shoot your polaroids to check your exposure (not unlike how we "chimp" our LCDs on our cameras these days), then pop on your back loaded with black and white film, fire off a few frames, and then maybe swap that back with another one with some color film in it. This is just an example, obviously; I'm sure there are some pros who never went that far with it.
Here's where things get interesting. As the digital sensor (CCD or CMOS) evolved, and got more sensitive, bigger, and capable of producing higher and higher resolution images, camera designers began to develop digital backs for MF camera systems. Think about it - it makes a ton of sense - you've got thousands of dollars already invested in a great MF system of camera bodies, lenses and accessories, and now you can instantly turn your entire rig into a digital system, just by slapping a new back on your camera. Just like changing from black & white film to color film.
There are a few things to take into consideration, obviously. The most significant is that the sensor's size isn't as big as the area on a piece of film. This means that your lenses will behave/look a little differently, similar to how "cropped" sensors differ from "full frame" sensors in the DSLR world. What's more is that there are a couple of different size/resolution options out there from a couple of different manufacturers. While this does complicate things, I think it's an advantage to the consumer, in that you can get exactly what you need.
Another consideration - and this is a big one - is that these digital backs are really expensive. You know how a current top-end Macintosh has always been the same price, year after year? Year after year, they just make the machines faster and more capable, and the pricing pretty much stays the same. It's the same with digital backs. A current top-end back will run you upwards of $40,000. Yeah. Forty thousand. There are cheaper options out there, obviously, and there's a pretty active used/second-hand scene, but still - you can't really get into the MF digital scene for much less than $10,000.
Suffice to say, the majority of digital backs out there in use today aren't privately owned by individual photographers. Rental companies and studios tend to have full camera packages, which, all things considered, are pretty reasonably priced to rent.
The final thing to consider is how dramatically the digital back changes the workflow when taking pictures. In the early days of MF digital, the backs didn't have LCDs. If you wanted to see what your shots looked like, you'd have to connect the back to a computer via a firewire connection, and use some special software to render the RAW file on screen. Modern digital backs all come with LCDs now, but they are pretty small when you consider that you're going to be reviewing an image that is probably bigger than 40 megapixels in size. Because of this, the general practice when shooting MF digital is to always have a computer on hand for image review (talk about chimping!). In the studio, the camera stays tethered to the computer, and special software pops the image on screen a few seconds after it's shot, full-screen. This is imperative for checking focus. Out in the field, some people shoot to CF cards, but many shoot to laptops.
This has become such a standard that there is a whole niche industry surrounding it, called "digital capture". There are folks out there who specialize in putting together whole computer/camera rigs and workflows to facilitate this otherwise tricky and failure-prone way of doing things. I used to work at a company that specialized in this, and now I do that kind of work on my own, as a freelancer. Folks that do this kind of work are commonly referred to as "digital techs".
In part 2, I'll talk a little about some of the digital backs themselves, and what goes into using them.
Thanks for reading this, and let me know if you've got any thoughts or questions about this stuff.