Lens performance & "tricks" in the field

[Maris Rusis]

Maybe things aren't so difficult!

Using your example, if maximum rise @f22 for infinity subjects is 9.35mm then at 1:1 (double the bellows extension) the maximum rise is double too; namely 18.70mm. In-between values operate proportionally.

Mechanical vignetting happens when the exit pupil of the lens is cut by the format mask or lens barrel. The actual size of the exit pupil, on axis and off axis, depends on lens design but for a simple 47mm lens @f22 it is about 2.14mm diameter. For the same lens @f32 the exit pupil is about 1.47mm diameter. This means you can place the exit pupil closer to the edge of the format by 2.14mm minus 1.47mm equals 0.97mm. Not much of a gain in lens rise movement!

Optical vignetting is a non-linear artifact of lens design and it cannot, in practice, be calculated by regular trigonometry.

I have never seen any photographer use this sort of arithmetic in any picture taking strategy with a view camera except in a very approximate way. The ground glass (and film subsequently) is the absolute arbiter of image integrity.

If the arithmetic says the image is clear and the ground glass says it is vignetted then it is vignetted!

[aduncanson]

Using your example, if maximum rise @f22 for infinity subjects is 9.35mm then at 1:1 (double the bellows extension) the maximum rise is double too; namely 18.70mm. In-between values operate proportionally.

Not so fast. Going from infinity to 1:1 doubles the diameter of the lens' image circle and lets you increase your rise by a factor of two, PLUS an additional one half of the of the negative height.

You can use
image distance = 1 / ( 1/focal length – 1/object distance)

and
image circle (@close up) = image circle (@infinity) * image distance (@closeup) / focal length

to estimate the increase in image circle when close up.

To find the allowable shift or rise use
Rise = square root (image circle diameter ^2 – horizontal dimension of negative ^2)/2 – vertical dimension of negative/2

Mechanical vignetting happens when the exit pupil of the lens is cut by the format mask or lens barrel. The actual size of the exit pupil, on axis and off axis, depends on lens design but for a simple 47mm lens @f22 it is about 2.14mm diameter. For the same lens @f32 the exit pupil is about 1.47mm diameter. This means you can place the exit pupil closer to the edge of the format by 2.14mm minus 1.47mm equals 0.97mm. Not much of a gain in lens rise movement!

Not quire right. The effect that stopping down has on image circle varies quite a lot with the lens design but does not principally depend on reducing the exit pupil diameter. Stopping down reduces aberrations and can significantly increase the usable image circle. The angle of coverage of a Dagor goes from about 60 degrees to about 80 degrees as you stop down from F/6.8 to F/22. That is a huge increase in image circle.

Image circle = 2* focal length * tangent (angle of coverage /2)

I have never seen any photographer use this sort of arithmetic in any picture taking strategy with a view camera except in a very approximate way. The ground glass (and film subsequently) is the absolute arbiter of image integrity.

If the arithmetic says the image is clear and the ground glass says it is vignetted then it is vignetted!

I agree. It is not hard to model the math above in a spreadsheet on a PDA or small computer, but I would not want to use it in the field. You may want to consider improving your viewing system with a better dark cloth and loupe. Silvestri make a tilting loupe that is supposed to help you see into the corners. I have recently gone to a 4x clip-on magnifier (bought at Cabellas on line) over drugstore reading glasses that I chose to allow me to carefully view the whole focusing screen. This works very well for me.

Good Luck.

[Gene McCluney]

One of the simple "tricks" to do, and one that Pros do all the time is to shoot an instant film test with Fuji FP100C45 (color ISO 100) or Fuji FP100B45 (b/w ISO 100) pack film, the image area is very very close to a 4x5 negative in coverage. This can provide all the info you may need. Exposure, coverage, sharpness.

[Doremus Scudder]

Heroique,

I'm not sure exactly what your problem is, so pardon any of the following brainstorming that does not apply.

I gather from your remarks above that you have a hard time seeing the vignetting when you exceed the amount of rise (or other movements) that your image circle will allow. If you cannot see the vignetting on the ground glass, but can see well enough to look back through the lens at the corners or through the clipped corners of your ground glass back at the lens aperture, then that will be the quickest. If you use these last methods, remember to be sure to check at taking aperture, not wide open, since stopping down can actually reduce the image circle in some cases and cause more vignetting.

However, I get the impression that you need a way to know how far you can shift/raise/etc. without having to check visually in the field. If this is the case, then you can simply make some tests in controlled conditions and put together a table that you can carry with you in the field. If you can see your ground glass well enough in bright light, then you can do your tests under those conditions and simply make notes for each lens you have, e.g., 135mm max rise = xx.x mm in landscape orientation; yy.y mm in vertical... etc.

If you cannot see the vignetting on the ground glass even in the best of conditions, then you will have to shoot film to tell when your lens is vignetting. Include a note with the amount of rise, etc. you are applying in the exposure. Develop the films and collate the results.

I also take it that you often need this information for working distances closer than infinity. Again, you could simply follow the above procedure. By testing at a couple of bellows extensions per lens, you can then make a rough table (or graph) that lets you extrapolate in-between values. If you work mostly in a certain magnification range, then be sure to test for those values.

Unfortunately, as mentioned above, the amount of acceptably-sharp image circle changes for a number of different reasons as the lens is stopped down. Most plasmat design modern lenses change very little when stopped down, but Dagors and G-Clarons as well as others gain a lot in usable image circle when stopped down. Again, testing and making tables for the lenses you have and use will be your best bet.

Once you have tables made, then you can simply refer to them when needed in the field. I, for one, hate doing calculations when I should be concentrating on the artistic aspects of my photography, and calculating exposure is about as much as I like to do. I carry tables for bellows extension, E.I. changes with different developments, filter factors, and so forth and find that consulting them is quick and effective.

If you are unsure about the characteristics of any of your lenses as for as image-circle improvements when stopped down, etc., you can get information here.

Hope this helps a little.

Doremus Scudder

[Heroique]

Maybe things aren't so difficult!
Using your example, if maximum rise @f22 for infinity subjects is 9.35mm then at 1:1 (double the bellows extension) the maximum rise is double too; namely 18.70mm. In-between values operate proportionally.

Maris, I wish I had phrased my question as quickly and elegantly as you have addressed it. Even the geometric relationship you describe is as elegant as your answer. (I also appreciate the modifications in aduncanson’s later post.)

Indeed, I appreciate all the posts here sharing creative “tricks” for the field. Many had never before occurred to me. Paul’s idea about flashlights, Gene’s about instant film, and Doremus’ about tables are just three inventive examples – but really all the suggestions are helpful.

Once you have tables made, then you can simply refer to them when needed in the field. I, for one, hate doing calculations when I should be concentrating on the artistic aspects of my photography, and calculating exposure is about as much as I like to do.

You share all of my sympathy here. I enjoy doing the conceptual math in front of my computer – then, if necessary, applying the math with grace & ease under the shade of the trees. Zen and the art of 4x5 lens maintenance.