Lens focal length, its coverage, and its field of view are not the same things. The focal length affects the
magnification and the magnification only. And in that way, any lens with an 80mm focal length will magnify the scene at focus to the same degree as any other 80mm lens, no matter what camera it was intended for.
The coverage is an outcome of the design. Some lenses have wide coverage, and others have narrow coverage, and both might have the same focal length. For example, the 50mm f/1.4 lens for a Canon might only cover a 50mm image circle--wide enough to cover the 24x36 frame. A 47mm Schneider Super Angulon XL has an image circle of 170mm--enough to cover 4x5 film. If you could adapt that Super Angulon to your Canon (which would require camera sugery because the lens won't fit in front of the mirror box when focused to infinity), it would make the same image as the Canon lens. Okay, let's find some middle ground--let's say I adapt one of my Zeiss Jena 50mm f/4 Flektogons onto my Canon. That lens is designed for medium format and has an image circle of 90mm or so. It's easy to adapt to my Canon because it was designed for medium-format SLR's with big mirror boxes. It would make exactly the same image as the 50mm f/1.4 Canon lens, when both are used at the same aperture setting.
So, there are three lenses with vastly different image circles, but all with the same focal length. All produce the same magnification of the scene onto the sensor/film. The lenses with the wider coverage will just see more of the scene.
Let me draw a different example. Let's say I want to make a portrait on an 8x10 camera using a 300mm lens. That lens provides the "normal" view on that format, given that its focal length is about the same as the film's diameter. With that lens, I might make a full-face portrait, where the face is one-half life size (1:2) on the film. I could remove that lens, and replace it with a Canon 300mm lens, if I could make the camera machine hold it in the right place. It would magnify the image exactly the same. The only problem is this: That Canon lens will only have a couple of inches of coverage, and my portrait will only include the sitter's nose. But that nose will be the same size on the film as with the 300mm large-format lens.
So, do not try to apply correction factors--that way lies madness. Think of lenses in terms of their relationship to the image diameter (or diagonal--same thing), their focal length, and their coverage. That 47mm Super Angulon XL is an extremely wide lens on 4x5, but it would be a "normal" lens on 24x36. On a 15x23 APS-C digital camera, it would be longer than normal.
(Now, if you want to make the same
picture using different formats, then you have to change the
magnification. To make the same
picture on 4x5 as on a 24x36 camera from the same camera position, I'd need a lens with something like three times the focal length--but the equivalency is approximate because the two formats aren't the same shape. I'd need more magnification to make the image bigger to fill that bigger frame with the same picture.)
Most medium-format lenses have a mounting distance from the sensor sufficient to accommodate a medium-format camera's mirror box. That might be in the range of 60mm, or perhaps a bit less for some 645 cameras. If the 24x36 DSLR has a lens mounting distance of 45mm (or thereabouts), you might have 15mm to work with or a bit more. There is no view camera I know of that can get the lens to within 15mm of the mounting surface on a DSLR.
If you will use longer lenses, this is not so much of a problem. Unlike small-camera lenses, large-format lenses typically mount close to their optical centers (unless they are a telephoto design, which is a small and narrow category of lenses that mount closer than their focal length, not relevant here). So, when you use a longer lens, you'll separate the film and lens standards and stretch those bellows to get it into focus. That will give you more room. Lenses in the 100mm range and up might work okay, if the camera supports close spacing with a bag bellows. For example, my Sinar F2 will focus a 47mm lens on a flat lens board, if I use the Wide Angle Bellows 2. It will provide perhaps 6 degrees of tilt or swing. But if I suck up another 45mm of depth with a DSLR mirror box, that lens is now impossible (not to mention that the lens itself will physically run into the DSLR camera body long before it reaches infinity focus). So, if I need 48 or 50mm of separation between the 4x5 film plane and the lens flange to allow a bit of camera movement, plus 45mm of depth in the DSLR mirror box, then the shortest lens I'll be able to accommodate is 90-100mm. A 100mm lens in front of the DSLR will be just like the 100mm Canon lens. I could achieve the same thing using a 100mm medium-format lens with a tilting adapter (as was the case with the fence picture linked above, and described in the article I link further down), or using Canon's 90mm TSE lens.
The cameras that Bob Salomon is suggesting are really medium-format view cameras that have been specially designed to support medium-format digital backs. These backs place the sensor on the film plane of these cameras, not at the back of a camera body mirror box. And the lenses made for this application are reasonably short, and are designed with a bit of reverse telephoto (aka retrofocus) to provide a little more separation from the sensor surface. These cameras and lenses are top-end stuff--the best stuff made for production-level commercial work. And they are expensive.
If you want to use lenses that are shorter than over twice the diameter of the 24x36 frame, the conventional view camera just won't work with a mounted DSLR body. With longer lenses, you can have some fun, perhaps. Remember that the mirror box will cast a shadow on the film, so shift is limited to 12mm vertically and 18mm horizontally.
There are alternatives. The best one is to just buy the Canon TSE lenses, which come in 17, 24, 45, and 90mm focal lengths. These will also work (to a point) with a 1.4x teleconverter. The 17 and 24mm TSE lenses are exceptional. They are expensive, but not even on the same planet of expense as a medium-format digital back solution.
A second alternative is this: Buy some Pentacon Six-mount medium-format lenses. If you can find them, look for the Hartblei 45mm PCS (shift only is fine, but they also had a tilt-shift version), the 55mm Arsat PCS (an excellent lens), and any of the 80mm normal lenses. Then put them on a tilting adapter (available on ebay). I have tilt-shift capabilities at 45 and 55mm, plus shift capabilities at 75 and tilt capabilities at everything from 45 to 500mm. I also own a 24mm TSE lens (the first version, which is cheaper than the current Mk. II). All of that can be done for the price of either one of the 17 or 24mm Mk. II TSE lenses, but the quality won't be the same. I have written an article on this topic,
here.
A third alternative is: Buy the 4x5 camera, and put film in it. Then, scan the film in an Epson V750. Compared to 24x36 digital, the quality will hold up nicely even when scanned in that consumer flatbed.
The fourth alternative is to send Bob a LOT of money to sell you a Linhof camera and digital back combination, plus a bevy of Rodenstock Digarons. Bring your checkbook.
Rick "hopefully saving you some time" Denney
Originally Posted by GG12
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