A question on focal length and image circle

[roland]

I am not a large format photographer yet but is seriously looking into it. I have a debate with a friend and I hope someone here who has real life experince with large format can give me a definitve answer.

My friend is of the opinion that for a lens to throw an image circle large enough to be used on a large format camera, it needs to be farther way from the film plane than on a smaller format camera like a 35mm camera. For example, he claims that if you use a 100mm lens on a 35mm camera, the theoretical centre of the lens would be 100mm from the film plane. But he maintianed that if you use a 100mm lens on a large format camera, the lens needed to be further away from the film plane to throw a much bigger image circle than on the 35mm camera i.e. the lens needed to be significantly more than 100mm away from the film plane.

I told him that he was wrong and the focal length of a lens and hence the lens to film distance is constant and doesn't change simply because you have changed format. I maintained that lenses designed for large format cameras have inherently a much larger image circle compared to 35mm. This factor is independent of lens to film distance. So a 100mm lens on a large format camera focusing at an object at infintiy would be 100mm away from the film plane, just as it would be on a 35mm camera.

I would appreciate it very much if someone give me me a conclusive answer. Thanks!

[Steve Hamley]

roland,

You are correct, within manufacturing specs of course.

Steve

[roland]

roland,

You are correct, within manufacturing specs of course.

Steve

Thanks Steve! Much appreciated answer!

[Nick_3536]

OTOH with 35mm a 100mm might be a telephoto while it might be retrofocus with LF.

[Ole Tjugen]

OTOH with 35mm a 100mm might be a telephoto while it might be retrofocus with LF.

A LF lens is very unlikely to be retrofocus. That would lead to lots of unnecessary complications like barrel distortion or greatly increased weight and cost.

Nor is a 100mm lens for 35mm cameras necessarily telephoto; there are quite a few which are really a "normal" lens mounted in a long barrel.

But anyway, whet "100mm focal length" really means is that when focused at infinity, the optical center of the lens is 100mm from the film plane. The complications arise from the fact that this center does not have to be inside the lens at all!

[steve simmons]

Simly put the size of the image circle is decided by the design of the lens and not its focal length.

Two 210mm lenses for example can have different sized image circles depending on their design.

steve simmons
www.viewcamera.com

[Emmanuel BIGLER]

Roland, you are perfectly correct. You should direct your friend to this forum and to this discussion so that he can see what's going on here.

There is one thing that has to be explained, namely that the evolution of 35mm cameras and lenses has been driven since the fifties by very different forces that pushed large format optics to what we enjoy today here.
35 mm cameras systems were first pushed to faster lenses when the printing processes for newspapers eventually accepted the 35mm perforated film as a standard for photo-journalists. Then came fast lenses, then zoom lenses, capturing all the energy of photographic optical design offices in the 35mm photographic world, with one model in mind : the hand-held picture. We could even add, cheeke-in-tongue : paparrazzi-type of pictures, but this would definitely be unfair to all good images that have been taken with small format cameras ;-);-)

Large format optics on the other hand, since the sixties, had no longer any requirement to be fast optics to be used hand-held (you can see hand-held LF cameras in the last movie by Clint Eastwood "Flags of our Fathers" !) , the last kinds of fast LF lenses were f/3.5 and f/2.8 like the tessars, xenars, ektars, planars and xenotars in the focal range of 80 to 150 mm that are no longer in use by photojournalists !

The real requirement for LF after the sixties was large image circles and ultimate image quality for camera used on a solid tripod. Short bellows draw was not a real constraint but LF telephoto lenses do exist.

All this long preamble to explain that 35mm lenses differ somewhat in their design to 35mm lenses because they serve different purposes and different professional needs. The majority of LF lenses are of a quasi-symmetric design, a kind of lens that exists in 35mm only for the standard 40 to 60 mm focal lengths. Their maximum aperture is hardly faster than 5.6; 6.8 and 9 are common. Tessar designs, a slight asymmetric design capable of apertures of 3.5 and 2.8, were common to all kinds of cameras, all formats, from ths miniature 18x24 mm to the 11"x14" camera, but are now being gently pushed to the collector's shelf : their angle of coverage cannot exceed 60°, this is considered too limited for LF jobs by nowadays standard.
No LF zoom and no fast f/2 LF lens ! a strange world indeed !!

The "distance" between the lens and the film is in fact defined by the laws of Gaussian optics, in a quasi-symmetric lens the exit principal plane from which the focal length is measured to the focal plane is usually inside the glass, but not too deep inside, so quasi symmetric lenses are not so far from a single lens element in terms of bellows draw . Image performace cannot be compared of course.

LF telephotos do exist in order to keep bellows draw short, exactly like 35mm telephotos, but many LF users prefer long focal lengths like symmetric apo-repro lenses which are much smaller and lighter for a similar or better image quality. Long focal apo-repro lenses are usually unknown to 35mm photographers ! only 4 lens element ! in a world governed by 12-element zooms, how can such a simple lens deliver good results ;-)

To give an idea, a 360mm apo-ronar, like a single lens element, has to be located approx 360mm from film in order to focus at infinity.
A 360 Tele-Arton will require only 210 mm of bellows draw from the lens board but will deliver the same image magnification.
This makes a significant difference in size and surface to the camera in windy conditions ;-)

I think you should see what kind of images can be obtained with a LF camera and whow your friend !

[Nick_3536]

A LF lens is very unlikely to be retrofocus. That would lead to lots of unnecessary complications like barrel distortion or greatly increased weight and cost.

I thought most/many? modern LF wide angle lenses are retrofocus? The SUPER-ANGULON XL 47 seems to have a flange focal distance of 59mm.

[Helen Bach]

I thought most/many? modern LF wide angle lenses are retrofocus? The SUPER-ANGULON XL 47 seems to have a flange focal distance of 59mm.

Nick,

That lens extends 30 mm behind the flange - ie within 29 mm of the film plane.

Best,
Helen

[steve simmons]

Originally Posted by Ole Tjugen
A LF lens is very unlikely to be retrofocus. That would lead to lots of unnecessary complications like barrel distortion or greatly increased weight and cost.


You wll always find exceptions to this statement. Doing so does not make this statement invalid.

If you throw in flange focal distance to this discussion you are only going to confuse matters. It is not relevant.

steve simmons