Lens image circles

[lacavol]

I am confused over image circles, F stops and light inside the camera. If one were to have a lens with a hypothetical image circle of 160mm and one with an image circle of 260mm would an F8 admit the same overall amount of light but just spread it out over different size areas? Or does F8 mean that the same amount of light is projected on each square millimeter? I’ve never seen the mathematical formulas and the books I have read so far are really vague. This would seem to have exposure implications. Also, does the light that doesn’t strike the film tend to reduce the contrast as the absorption can’t be 100% efficient?

I am thinking about this as I may buy a lens that has a larger image circle than needed now but it may be useful in the future if I go to a larger format.

[Ian Grant]

The image circle is usually focussed at Infinity and at f22. The Apertured is the amount of light reaching the film and an f8 lens with an image circle of 165mm is just a bright as an f8 lens of the same focal lenght with an image circle of 260mm.

With LF work the image circle is important when using movements.

Ian

[ic-racer]

I am confused over image circles, F stops and light inside the camera. If one were to have a lens with a hypothetical image circle of 160mm and one with an image circle of 260mm would an F8 admit the same overall amount of light but just spread it out over different size areas? Or does F8 mean that the same amount of light is projected on each square millimeter?

yes

Realize that the 'radiant energy' (jouls) and 'radiant flux' (watts) passed by these two lenses IS different, but we think of film exposure in terms of irradience which is watts per square meter.

I’ve never seen the mathematical formulas and the books I have read so far are really vague.

aperture number = focal length/entrance pupil

This would seem to have exposure implications. Also, does the light that doesn’t strike the film tend to reduce the contrast as the absorption can’t be 100% efficient?

It reduces contrast because of internal reflections. So you need a good big bellows, no shiny thing in the camera and a compendium shade to minimize this. Multicoating helps also. Adsorption comes into play only in emulsion making

I am thinking about this as I may buy a lens that has a larger image circle than needed now but it may be useful in the future if I go to a larger format.

Well, it (lens with large coverage) will be useful for any shift or tilt movements you may need now.

[pgomena]

You will never regret having a larger image circle. I can use three lenses with large image circles on my 4x5" camera and on my 6x8" whole-plate camera. It's the best of both worlds!

Peter Gomena

[Hikari]

There is no correlation between image circle and exposure. Image circle is only relevant in the format you can use or the degree of movement that is possible.

[mdarnton]

It's a smart question. Think of it backwards: you are a tiny spot on the film looking towards the lens for your light. No matter how many other spots like you there are on the film, no matter however large the film is (the image circle), no matter how many other film bits like you there are in the room, the lens looks the same to you from where you are. That's all that counts: what you, yourself, get from the lens.

[johnielvis]

the larger coverage lens lets in more light---but at the same INTENSITY---in other words--it's just as bright no matter where you are as long as you're in the image circle---so larger coverage does equal more total light, but it is the same brightness.

[lacavol]

Thanks, everyone for all your help. I have an old Kodak lens book but they were rather vague, I can relate to it now. It's sort of like voltage vs. power at least to my mind. I do think a compendium shade is a good idea as it could cut down on extra light outside the film area. I have a 6X9 now that I use in a daylight tank but I am thinking of going larger when I get the processing capability. This has given me the confidence to buy a lens with a larger image circle with the future in mind.

[k_jupiter]

It is a very interesting question and I don't believe you have gotten a correct or complete answer. Mr elvis is not correct in that the brightness is NOT the same as long as you are within the image circle. An image circle is a circle with an acceptable amount of light at the edges. Most times it does not matter, the difference between center and edge is pretty small. But some lens need a filter in the middle to decrease the amount of light at the shortest distance between the lens and film. A wide coverage lens does not have a greater field of view than a narrow one, it just disperses light in a wider angle inside the camera. The amount of light coming into the lens does not change per lens, just the dispersion pattern. My guess is that it is not terribly significant but it is different.

Don't accept answers that do not explain fully what they are trying to prove.

On the other hand... don't lose any sleep over it. You should be calibrating your process with every lens you own and take damned good notes.

tim in san jose

[Dan Fromm]

It is a very interesting question and I don't believe you have gotten a correct or complete answer. Mr elvis is not correct in that the brightness is NOT the same as long as you are within the image circle. An image circle is a circle with an acceptable amount of light at the edges.

Tim, there are two coverage concepts floating around. One defines the circle covered as the circle within which image quality is satisfactory. The other defines it as the circle satisfactorily illuminated. One would think that the second definition is laxer than the first, i.e., defines a larger circle, but this isn't always true.

The first concept is used by lens manufacturers and exacting photographers. It is conditioned on aperture; the image circle typically gets larger as the lens is stopped down. This because some off-axis aberrations get smaller, i.e., image quality off-axis improves. as the lens is stopped down.

The second one makes little sense. Two problems with it.

First, it ignores image quality in (typically) the corners. This matters for some photographers, not for others.

Secondly, there's a rule of thumb about decline of illumination at the film plane as the distance off-axis increases. The rule, which many, not all, lenses follow, is that illumination declines with cos^4(the angle off-axis). If you do the arithmetic, you'll find that old-school extreme wide angle lenses have large falloffs. At 45 degrees off-axis, this means that illumination is two stops down from the center. The Goerz Hypergon (with star) has claimed coverage (image quality concept) of at least 120 degrees. The sharp image it puts on film is 4 stops down at the edge. This is why we use center filters on extreme wide angle lenses; they let us get better exposure at the edges, the better to use detail that's there even though it is hard to see on the ground glass.

Some modern w/a lenses use an optical trick to improve illumination's decline to only cos^3(the angle off-axis). A modern 120 degree lens is down only 3 stops at the edge. Every little bit helps.