What defines camera format?

What defines the “format” of a camera system? Film/sensor size? Size of the image circle created by the system on the image plane? What?

I realize, historically, the film size was matched to the image circle. But what if the image circle doesn't match the film/sensor size?

Like the 10D. Is it a 35mm DSLR or an APS-C DSLR? And do EF lenses on a 10D have the properties of 35mm-format lenses or APS-C system lenses?

as a newcommer to this discussion clearly the area of exposed film defines the format 10d etc are always described with a conversion factor (back to 35mm) to allow the less experienced to relate to something they currently understand . this cropping factor is probaly the most confusing (irrelavant) statistic applied to dslr after all the image in the view finder is by and large the image on the film or sensor

The size of the recording medium determines the format... Canon 20D, 10D, 300D, 350D and 1D are not 35mm DSLR's where as the 1Ds is...

That was a good thing concerning lenses for view cameras. The bigger the circle, the more swings & tilts you could use. (You REALLY paid for that feature)

So if I put 35mm film in an 8X10 camera, it becomes a 35mm camera? And when I do this, the lens attached to the camera take on the properties of lenses designed for 35mm film?

Help me with this...

It seems to me that the format of the camera/lens system is independent of the size of the recording medium. If I alter the back of a 35mm film camera so a piece of 8X10 film sits on the original film plane, does that make it an 8X10 camera? If so, it would be an 8X10 camera that wasted much of the surface area of the film on the film plane.
So, it seems the camera format is determined by the size of the image circle the camera/lens system was designed to fill. If I take an AE-1 and put APS film in it, the camera and lenses I attach to the camera remain a 35mm system--and, importantly, behave like a 35mm system. In this case, I'm not wasting film--I'm wasting image circle (which is not nearly as costly!).

Where does this position fall apart?

Frame size. That's all.

You mentioned elsewhere that putting an APS-sized sensor in an 8x10 camera would not make it an APS camera. But, in fact, it would. The lenses would be really long, of course.

I do it all the time, actually. I have a 6x9 rollfilm back for my 4x5 view camera. When I put that rollfilm back on my view camera, I've changed it to a 6x9 view camera. I still use the same lenses, but they mean different things. The 90mm/5.6 Super Angulon that is such a stunning wide-angle lens in 4x5 is merely a normal lens when used with the 6x9 back. Sure, it lights up a lot of the inside of the camera that I'm not using, but that's true for all view-camera lenses in order to allow movements.

People used to put 35mm adaptors in their 6x6 Rolleiflexes all the time (i.e. the Rolleikin). Is it then a 35mm camera? Yes. The 80mm lens on the Rolleiflex is now a short tele rather than a normal lens, because we've changed the format.

We will also enlarge the images more. I rarely made prints from 4x5 more than a 4x enlargement, and the usual degree of enlargement was more like 3x. But the 11x14 print that requires a 3x enlargement from 4x5 requires a 5x enlargement from 6x9. Thus, any artifact in the image gets enlarged nearly twice as much. That 11x14 print requires a 19x enlargement from an APS-sized sensor. That's why format is king.

People use lenses intended for 8x10 cameras on 4x5 cameras all the time. A 165mm Super Angulon will cover 8x10 easily, and it is merely a normal lens on 4x5. But that extra large illuminated circle means you can tilt that sucker just about on edge and still light the 4x5 frame. But it's still a 4x5 camera it would be mounted on (even if the bellows, lens board, film back, ground glass, and everything else except the film holder also worked for 8x10). In fact, many view camera systems can exchange many items between the various formats, just to facilitate the use of lenses and other parts across different formats. A "4x5 reducing back" is a standard item for 8x10 view cameras.

Rick "thinking a little view camera experience goes a long way to sorting out format confusion" Denney

So what does it mean that a camera is a "35mm camera" if it's format designation is dependent on the size of the recording medium?

I get that you can rig/adapt a body to accept different film sizes, but as you point out, when you do this, the lenses "mean different things." But while they MEAN different things, they remain the lenses they were before you adapted the size of the recording medium. And they behave the way they did before you adapted the size of the recording medium. So what has changed is your interpretation of how the lenses work--not how the lenses actually work. Right?

I'm curious--does the 4X5 reducing back of an 8X10 camera utilize the same film plane as the 8X10 does? Or does it move the film plane forward in the camera body?

Yes, the 4x5 reducing back uses the same film plane as the 8x10 version. Of course, view camera lenses are symmetrical and mount at the nodal points (with the exception of a few telephoto designs), so the notion of "back focus distance" doesn't mean much--every lens has its own distance from the lens board to the film plane. That's why the cameras use bellows instead of a hard box. But if I focus a 165mm lens on 8x10 ground glass, and then put a reducing back on it with 4x5 film, it will still be focused. In fact, when I use my 6x9 rollfilm back on my 4x5 camera, I focus on the 4x5 ground glass first.

The only difference is that the smaller film frame doesn't include as much of the scene. And that's the only reason the lenses mean different things. If I focused that 165mm lens on 8x10, and made an exposure on 8x10 film and then on 4x5 film, I could take that 8x10 negative and trim off all but 4x5, and have an identical negative to the one shot on 4x5 film. When I did so, that wide-angle lens (in 8x10) would now have the effect of a norma lens (in 4x5).

The lenses therefore produce the same image, but you are using different portions of the image they produce. So, it's not my interpretation that changes, but something more important at play. And that is:

We define lenses as short of long based on their relationship to "normal". A normal lens is defined as the diagonal of the frame. Thus, whether a lens is short or long depends utterly on the format in which they are used. They focus the same, project the same, and illuminate the same. But with a smaller format we take a little out of the middle and enlarge it more to make a given print.

Back in the old days, camera makers used to take their widest rectilinear lens and make a picture of a scene. Then, they would draw rectangles in the scene to show the field of view of longer lenses. I used to think that was wrong, but then I learned that camera position alone determines perspective, and that focal length and format determine how much of the scene we'd actually see in the image. So, if we start with that wide-angle image of the New England town, and then see that the 600mm rectangle only includes the distant church steeple, we can see that we could arrive at that image using one of two methods: We could use a 600mm lens, or we could take that tiny rectangle out of the original image and blow it up. That's why changing format has the same effect as changing focal length, and that's the reason for the 1.6 equivalency factor between a 10D and a 35mm camera.

A 10D is not a 35mm camera. It is an APS camera to which you can attach lenses intended for 35mm camera. Let's apply a little reductio ad aburdum in the other direction. Let's say we had a digital point-n-shoot with a sensor 2mm by 3mm (I'm not sure such a thing exists, but let's imagine it does). We could mount that sensor in lieu of the APS sensor on a 10D and take pictures all day long. The lenses would focus the same way, and they would project the same image. But they would only provide you with a 2x3mm chunk of that project image. Since a "normal" lens for a 2x3mm sensor would be 3.6 mm, the 50mm lens you might put on that camera would be a 14-times-normal telephoto. That would like putting a 600mm lens on a 35mm camera. Thus, the 50mm lens on that 2x3 sensor would produce an identical image to a 600mm lens on a 35mm camera. You'd get the steeple both ways.

But the reason we use the long lens on a 35mm camera to get that distant church steeple is so that we don't have to enlarge the image more than necessary. I could get that steeple, and it would look just the same, if I took a 2x3 chunk of the 35mm frame and enlarged it. The difference is that I would also be enlarging all my lens faults and also my film grain (or sensor resolution). That's the only reason people bought long lenses--there was a limit to how much they could enlarge the negative before the junk that they are also enlarging causes problems.

So, the lens isn't any different. It focuses the same image on the same film plane. But we use a different portion of it, and the meaning of the lens, vis a vis wide-angle versus telephoto, has only do with the portion of the scene we use.

You asked why we couldn't put a 4x5 film behind a lens intended for 35mm film and get an image earlier. The reason is simple: The lens barrel is too narrow and it will cut off the corners. But if I took a Canon 50mm lens and put it on a MF camera, I'd get a circle in the middle of the film. Within that circle, the image would be absolutely identical to what my 50mm Zeiss medium-format lens would produce. But the glass and the lens barrel isn't designed to light up that much real estate, and it would cut off the corners. That' s why medium-format lenses are so much bigger and more expensive for what you get.

There was always the question of whether it's better to use a teleconverter or enlarge more. When a lens is effectively longer (with respect to normal) because we are using it on a 10D, we are adopting the "enlarge more" strategy. Using a 1Ds with a 1.6 teleconverter (assuming such a thing existed) would be the teleconverter strategy.

Hint: The teleconverter has to be pretty crappy not to be better than enlarging more, heh, heh.

Rick "format is king" Denney

The lens will still focus on the same plane as before

Yes. If you put a 35mm lens on a view camera
1. You won't be able to focus unless you mount it on a recessed lens board &
2. you'll get a stinkin' little 1-1/2" diameter pic on the 4X5" film. The angle of coverage is smaller 'cause 35mm cams don't need a bigger pic, except on PC lenses of course.

Steven & rdenney: Thank's a lot for saving me a lot of 2-fingered typing!
Edit: we cross-posted. Thank's again!

Yes--that makes sense. But also brings up an interesting point. 35mm cameras--and most other modern cameras--do not have bellows. They have a hard box. And in most cases, that box is designed to work with a particular size recording medium (acknowledging one can switch backs on some...). The format for these cameras are generally referred to by the size of the recording medium (35mm, 4X5, etc.). And the image circle created on the film plane is optimized for that recording medium. So, a 35mm camera--and lenses, are designed to project an optimized image circle to fill a 35mm film frame.

In my mind, that's the essense of a 35mm camera.

Now, changing the size of the recording medium doesn't change that essence. Nor does using a football to shoot hoops with change the essence of the football--it doesn't BECOME a basketball even though you are shooting baskets with it; you are just shooting hoops with a football.

You really should think of those boxes as a continuum. For example, the distance from the film plane to the lens mount is not at all constant across 35mm cameras. It ranges from as little as about 28mm for some rangefinders to as much as around 50mm for some SLR's. That's quite a range! The effect it has on the lens is how much barrel is included, and that's about all. One of the real advantages of the EF mount is that is has a fairly short back-focus distance (though, sadly, not as short as the old FD mount). Thus, lenses from cameras with longer backfocus distances are easy to adapt.

Medium-format cameras with interchangeable lenses have backfocus distances ranging from nearly as short as the bigger 35mm SLR's to nearly 70mm.

There have been (and still are) many cameras on the market with bellows focusing, so that the lens mount didn't have to have a focusing mechanism built into it. The Mamiya C-series TLR's and the RB and RZ 67's use bellows focusing, as do several Fuji models, nearly every large-format camera ever made, and even a few 35mm cameras with folding lenses.

A camera body with a sensor of a particular size or a film frame of a particular size has a particular format, no matter what else the sheet metal surrounding it might have been used for. It's quite reasonable for an APS camera to have the same distance from film plane to lens mount as a 35mm camera--this allows lenses to be used across formats, as we have with the 10D and 35mm Canons. APS and 35mm are closer to each other than are, say, 35mm and medium format, so it shouldn't be a surprise that the same backfocus distance would be usable by both. But it does put a strain on the wide-angle end, where the lenses require a more pronounced retrofocus design to get the glass out in front of the mount as needed.

I know several people who have taken old telephoto lenses intended for 35mm, cut off part of the barrel, and mounted them on medium-format cameras. One such that is commercially available is the Hartblei 500mm/5.6 mirror lens made in the Ukraine, and available in various medium-format mounts. It was originally a 35mm lens with a longer barrel.

There are two reasons why small cameras don't have bellows focusing, especially these days: 1.) Bellows are tender and these cameras are subjected to abuse, and 2.) modern lenses have internal focusing and moveable elements, not to mention auto focusing, that require the focusing to take place in a complex lens barrel rather than simply at the lens mount.

Rick "seeing a continuum with considerable overlap" Denney

All 35mm cameras, whether they are range finders, P&S, SLR, cinematography camera, or what not, have one thing in common and that's the film(sensor) size they are using. You'll notice that the lenses are all different, depending on the camera and the application but the size of the film/sensor are all the same. All formats are specified by film sizes, not by the type of body nor the lenses they use. Period.

This is always what struck me as odd about 35mm. Neither dimension is 35mm. The frame dimensions are 24mm by 36mm. Only the overall width of the film (with sprocket holes) is 35mm.

---Bob Gross---

This stems from its origins at movie film. The point of the original Leica was to make use of cine film that was measured across the gross width of the film.

Of course, rollfilm film designations have always been wacky. APS, 35mm, 120, 220, 620, etc.? Wacky stuff.

Rick "thinking it must have seemed like a good idea at the time" Denney

I know all too much about gross widths.

---Bob Gross---