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

[walter23]

Just to clarify things a bit, retrofocus and telephoto really refer to particular types of lens designs.

Retrofocus lens design allows the lens to sit much further from the film or sensor than a normal design. This is the design commonly used in wide angle lenses on 35mm SLRs; I have a 12-24mm zoom for my digital SLR, and if you look inside the mirror box you can see there isn't room for the lens to sit 12mm from the sensor! So the lens is designed to be retrofocus; it can sit much further out than would be requred with a normal lens design.

Telephoto lenses are designed using the opposite principle. In 35mm world, telephoto is used as a generic term to mean anything longer than about 60mm, but this probably also reflects the design. In large format (and the history of lens design), telephoto lenses are a specific design where the lens can sit closer than it's focal length; the important point from which focal length is measured is floating out in front of the lens. This lets the physical lens sit closer than would be required for a normal lens, so that a 300mm telephoto lens might only need to be 230mm from the film to focus at infinity. These lenses are preferred by people who have field cameras that can't extend long enough to take long lenses with normal design. Of course, in 35mm, telephoto lenses are likely to be a true telephoto design; is your 300mm lens really that long, taking into account the important node isn't sitting right at the end of the lens in a normal design?

So the short answer is that most large format lenses are of some kind of normal design that does not push the important node out in front or behind the lens elements, whereas the constraints of 35mm (compact telephoto lenses & wide angle lenses that can clear the mirror assembly) mean that retrofocus & telephoto designs are used... so in practice, there will be a difference. It has nothing to do with the large image circle needed for large format though.

[VerticalMan]

I understand that this thread is dated. However, this is exactly what I'm searching for an answer. I'm trying to get into large format photography and been looking into different knowledge bases.

The comment below raises another question for me.


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

So, if two 210 mm lenses have different sized image circles, do they need to have different angle of view as well?

[Bob Salomon]

I understand that this thread is dated. However, this is exactly what I'm searching for an answer. I'm trying to get into large format photography and been looking into different knowledge bases.

The comment below raises another question for me.



So, if two 210 mm lenses have different sized image circles, do they need to have different angle of view as well?

No, all 210mm lenses have the same angle of view. That is dtermined by the format size. Image circle is how large a circle is created by the lens. The larger the image circle the more movements the camera can do.

[VerticalMan]

Thank you very much Bob.

I may have used the wrong phrase in "angle of view". What I meant to refer to was the angle of coverage the lens itself can produce, regardless of the format.

So just to confirm my understanding...

For a lens to produce larger image circle than another of the same focal length, it must "see" wider than another. But the image captured will be of the same angle of view.

Is this correct?

[ic-racer]

Some lens manufacturers use "Angle of view" rather than image circle to classify their lenses. In this case the term is used differently than Bob was pointing out above.

For example Fuji old-style "W" lenses had an 80 degree angle of view for the whole series (http://www.thalmann.com/largeformat/page1.htm). Each lens had its own image circle (which can easily be discovered with trigonometry, or by reading the table in the lens brochure), but for convenience you can list "80 degrees" and apply it to the whole series.

[Ken Lee]

Lets say we use a 120mm Super Angulon, or other similar lens that covers 8x10.

If we mount it on a 35mm camera (or "full frame" digital camera), it becomes a moderately long lens: it's "angle of view" is rather narrow, just like any other 120mm lens on that format. We can use it for portraits or other moderately long shots.

If we mount the same lens on an 8x10 camera, it "becomes" a rather wide-angle lens, equivalent to 20mm on a 35mm camera.

The lens hasn't changed, and its image circle hasn't changed: by our choice of format, we have simply cropped the image, and determined the angle of view accordingly.

So there's the maximum usable angle of view that a lens can give (as ic-racer has described below), and then there's the actual angle of view that we use, by our choice of format.

Many large format lenses that allow a wide maximum useable angle of coverage, have the letter W in their names (or something equivalent line "angulon"). We may or may not want to use all that potential coverage.

[Bob Salomon]

Thank you very much Bob.

I may have used the wrong phrase in "angle of view". What I meant to refer to was the angle of coverage the lens itself can produce, regardless of the format.

So just to confirm my understanding...

For a lens to produce larger image circle than another of the same focal length, it must "see" wider than another. But the image captured will be of the same angle of view.

Is this correct?

yes