Puzzling light blocking. Vignetting, lens hood? Please help to identify the source.

[Willie]

Might check for bellows sag or folds getting in the way.

[Vic Montaigne]

Hi Willie,

You are of course right. I had thought of this possibility. However, when putting together the same setup of camera, lens, movement, bellows and lens hood for the sake of finding the cause, I could not make out any sagging of the bellows.

How/where would it show? On the GG, when checking for vignetting through the GG corners, or both? I might well have overlooked it in the situation, but should sagging not show again with the same setup in place?

Does sagging show in any way different when a lot of rise is applied? (Sorry, I am better at visualizing images than the physics of light rays at work...)

[Maris Rusis]

...Does sagging show in any way different when a lot of rise is applied? (Sorry, I am better at visualizing images than the physics of light rays at work...)

Ordinary bellows vignetting due to sag cuts off the bottom of the picture = top of the ground glass image. With extreme rise the bellows can cut off the top of the picture = bottom of the ground glass. Bag bellows when available is a cure. Pulling several of the top front pleats of an "accordion" bellows toward the front standard can help prevent cut off with extreme rise. I always look through the lens (at working aperture) towards the ground glass to check for vignetting. If I can see all of the ground glass it can see me and all of the field of view.

[Vic Montaigne]

Hello Maris,

thank you for your very helpful answer.

Ordinary bellows vignetting due to sag cuts off the bottom of the picture = top of the ground glass image. With extreme rise the bellows can cut off the top of the picture = bottom of the ground glass.

This is exactly the kind of knowledge that would be very difficult to find in books. Thanks for the information, much appreciated!

Bag bellows when available is a cure. Pulling several of the top front pleats of an "accordion" bellows toward the front standard can help prevent cut off with extreme rise..

I will keep in mind your advice concerning the front pleats.

Talking about the ruined shots, I am quite sure that bag bellows were mounted, simply because with this camera/lens combo and standard bellows it would have been impossible to raise the front standard that high. Usually I lightly pull at the four corners of the bag bellows to make sure nothing will get into the way. But inmidst of the sophisticated LF-routine, which is not yet a real routine to me, it is still a possibility. I might have forgotten.

I always look through the lens (at working aperture) towards the ground glass to check for vignetting. If I can see all of the ground glass it can see me and all of the field of view.

Me too, I prefer this direction for checking, at least with wide angle lenses. But - have you ever tried to stick your head into a compendium-like lens hood and twist your neck to try and make out the corners?

[Maris Rusis]

.... But - have you ever tried to stick your head into a compendium-like lens hood and twist your neck to try and make out the corners?

Ah, compendium lens hoods. I love 'em and I hate 'em. And I think I've vignetted more negatives with bad hood adjustment than with saggy bellows. These days I don't put my eye in the compendium hood when looking for the ground-glass corners. I move further back from the front of the camera and look past the edges of the hood to make sure those edges aren't obstructing some of the picture. To make things easier to see I'll use the focussing cloth over the front of the camera. Admittedly this looks rather odd but works well.

[Vic Montaigne]

Maris, I am not sure I fully understood your technique. But you are right, when stepping back a little I actually manage to see the cut out corners even with the lens hood attached, although this gets more difficult with a stopped down lens and lots of rise.

There is however something that I do not yet understand concerning the two methods of checking - back to front vs. front to back. In forums I read more than once that this would always be vice versa, mechanical vignetting would always show in both directions. Yet I get different results:

I repeated the following many times because I just could not believe what I saw. When I check for vignetting with the SA 5.6 on the TK 45s, with bag bellows, with or without rise, at full aperture, looking from the back, I can most definitely make out that football or lemon shape of the lens opening, transforming to a round shape when stopping down. However, at the same time, when looking from the front through the lens, the cut out GG corners are freely visible at all apertures. Looking straight along the optical axis all one can see is of course a patch of the GG. Looking from different angles though, one can see each of the corners at some point when moving around. According to the understanding I had to this day this should not be the case when at the same time I see vignetting from the back. Have I got something wrong here?

On the other hand, if I stand in front of the camera, this time not looking for the corners but simply onto the lens opening, I can again see the football shape as soon as I am looking from any other angle than along the optical axis. By stopping down I get to see the actual blades and the round (or really polygonal) opening they form. Only problem, how would I know to what degree of wide angle my perspective needs to shift to be sure there will be no obstruction? I hope you know what I mean.

Could someone please explain to me which method of checking for mechanical and or optical vignetting is reliable under all circumstances! I know the one from back to front is, but, with very wide lenses and very high rise at small apertures this becomes really really difficult. An alternative would be very welcome. (By the way, photographer bent down low, twisted awkwardly, looking up to the back of the camera is a sight no less peculiar than the dark cloth over the front ...).

[Maris Rusis]

I'm afraid there's no easy way. A closely adjusted compendium lens hood that doesn't vignette at full lens aperture will almost certainly vignette strongly when the lens is stopped down. For example I can line up by eye (from in front of the lens) the right hand edge of the compendium with the left hand edge of the oval shaped (fully open) lens entrance pupil and the left hand edge of the ground glass. When the lens is stopped down the edges of the oval entrance pupil move inward and form a small round hole. The original line-up is lost and the result is vignetting.

The compendium hood must be optimised with the lens at working aperture. This is difficult because the aperture may be tiny, the ground glass might be dark, and the "looking" angle might be very oblique. In the studio I could have an assistant hold a Light Panel (Cabin CL-5000P for me) against the camera ground glass to make the corners bright enough to see through the stopped-down aperture. In field work I've abandoned the compendium hood and just shade the lens with my hand, hat, or dark-slide. This works ok but I have several negatives with hands, hats, or dark-slides just showing on the edge.

[Vic Montaigne]

I do not want to overstress your readiness to help, so I will try to sum up:

Checking for vignetting:

Like described above in # 16, looking front to back - spotting the cutout corners of the groundglass through the lens (no lens shade involved), produced a different result than looking from back to front the conventional way, that is through the cutout corners to the lens opening: Checking from back to front showed vignetting at full aperture, the lens had to be stopped down for a round opening. However, viewing front to back allowed for free vision of all four corners of the ground glass already at full aperture, without having to stop down the lens. This is an obvious contradiction. Maybe it occurs only under certain conditions, I have no idea. Should someone have an explanation for this I would be glad to hear it.

Otherwise I will just stick to checking through the open corners of the ground glass as described by Leslie Stroebel (View Camera Technique, 7th ed, p. 108). Even if this tends to become very difficult with wide lenses and extreme rise.

By the way, according to Stroebel stopping down the lens reduces both, vignetting by lens barrel and by lens shade. Usually never miss them, yet I wish I could verify any of this immediately using digital or polaroid...

What I have learnt regarding my four ruined negatives:

The first one (obstruction displaying some kind of pleated pattern at bottom of image) might have been caused by the compendium style lens shade which I had forgotten to adjust properly but have corrected following this exposure.

Light blocking in the three following images (pitch black broad stripe at top of image) might have been caused either by the holder issue described by Bob, or by interference of the bag bellows, gone unnoticed. I will certainly put increased attention to avoid those potential mistakes.

Many thanks to everyone who took the time to contribute!