A minor question: are some (especially modern) lenses built with intentional mechanical vignetting, possibly to enforce reasonable coverage claims?
A minor question: are some (especially modern) lenses built with intentional mechanical vignetting, possibly to enforce reasonable coverage claims?
Dan,
Think of it this way; let's say you've shifted enough so that the center of the lens can't "see" a corner of the film. However, you've got the lens wide open, and the aperture is wide enough so that the outer parts of the iris do "see" the corner. Then you stop down... oops, suddenly the part of the aperture that was illuminating the corner is gone. Result, dark corner. I've observed this often in the field; (partially) illuminated corners when wide open, dark corners when stopped down.
Maybe it's lens-specific, but, think about angles for a minute. The angle from the far side of a wide-open aperture to a corner on the opposite side is surely more acute than one from the center of the lens. If there is anything in the way that is mechanically vignetting the center part of the aperture already, stopping down can only make things worse.
That said, I'm going to go double-check to make sure, since I respect your knowledge and expertise, especially when it comes to lenses. However, I've drawn the scenario out on paper and it seems to confirm my observations.
I'll report back.
Doremus
Update: Just checked with my 90mm SA f/8: At extreme rise, and when looking back through the lens aperture for the corners of the ground glass, I can only see the far corners through the edge of the iris wide open. When stopped down to f/32, the corners are not visible at all: vignetted. If I were to introduce a filter into the equation, the effect would be much more pronounced.
Doremus
Seems I have observed the opposite, Doremus. Looking thru the cut corners, with the lens wide open, one can see vignetting (the aperture is partly blocked), but as one reduces the aperture, one can see the entire aperture -- no vignetting.
The corner does not get darker as one closes down relative to the rest of the GG. In fact, the corner gets brighter relative to the rest of the GG since only the corners were getting vignetted.
"Landscapes exist in the material world yet soar in the realms of the spirit..." Tsung Ping, 5th Century China
Vaughn,
What you observe is true as long as light from the lens' center is still reaching the corner in question. In that case, the near edge (i.e., on the same side as the corner) is vignetted wide open, but not when the lens is stopped down. And, as you say, the corners in question end up being illuminated more evenly.
Carry the thought experiment a bit further though, to where light from the center of the lens aperture and then some is being blocked and not reaching a corner or corners (due to rise/shift or a combination), but a crescent of the wide-open aperture is still able to illuminate the corner(s) in question. So, when observing the ground glass, the corners are illuminated, albeit only partially. The darker corner, however, is not always so readily apparent, especially when dealing with a hot spot, wide angle lens, etc. Now, when you stop down, you'll effectively eliminate the illumination from this crescent, leaving only light coming the center of the lens, which never reached the corner in the first place, thereby darkening the corner entirely.
Plus, it's really common to be on the edge of coverage like this and then add a screw-on filter to the front of the lens without checking. The effect I'm speaking about is only exacerbated when a filter comes into the equation.
I hope I'm making sense here. And, I think this could be Tim's problem.
Best,
Doremus
What I want to know is what exactly are the lenses that are causing this? It doesn't sound like movements are extreme, so maybe you just need to buy lenses suited to the job. I know I have a couple of lenses where my bellows would tie in a knot before I'd run out of coverage.
Thanks, but I'd rather just watch:
Large format: http://flickr.com/michaeldarnton
Mostly 35mm: http://flickr.com/mdarnton
You want digital, color, etc?: http://www.flickr.com/photos/stradofear
No, if you can see the entire opening of the lens thru the cut corner, that corner is receiving all the light possible thru that aperture. If you see only the cresent, then you are not seeing all the light that is passing thru the lens (while all the light is still hitting the center of the GG). The light from the center of the lens does hits the corners.Now, when you stop down, you'll effectively eliminate the illumination from this crescent, leaving only light coming the center of the lens, which never reached the corner in the first place, thereby darkening the corner entirely.
If you add a filter that vignettes, you will see the filter thru the lens via the cut-corners.
"Hot spots" (brighter areas on the GG than the surrounding areas of the GG) are not due to the amount of light hitting the GG -- but instead it is the light hitting the GG and not all going towards our eyes. The film will not 'see' that hot spot.
"Landscapes exist in the material world yet soar in the realms of the spirit..." Tsung Ping, 5th Century China
Vaughn,Originally Posted by Vaughn
We're saying the same thing. I, myself, have made the mistake of thinking my lens was covering when it was not and the corners were only illuminated by a crescent of the aperture when wide open. This then disappeared entirely when I stopped down, resulting in real vignetting, not just light fall-off in the corner. I'm talking about a situation here where the photographer is not checking through clipped corners or by looking back through the taking aperture at the corners. Certainly, if the OP had been checking, he would have seen the vignetting. It's when you're not checking and not really aware that that detail in the corner is only coming from a sliver of the aperture that this mistake is easy to make.
Best,
Doremus
I think what Doremus says makes sense geometrically, as changing the location of the far edge of the aperture will change the angle that light passes through the barrel. Of course, not checking through the clipped corners is my error, but I can certainly imagine seeing part of the aperture when wide open and none of it when stopped down. This seems to me the most likely scenario, as I was applying extreme movements for the lens in question (yes, I need to buy lenses with more coverage), and evaluating the whole image on the ground glass wearing readers, then buttoning up and stopping down to 45 or 64, then adding a yellow filter.
This discussion has been very illuminating for me, and my takeaways are:
- when using movements, check the clipped corners with the lens stopped down to taking aperture and with any filters attached
- buy lenses with more coverage
Thank you gentlemen. Its 2:45 am in Bangkok (I'm still jet-lagged from my US trip), but at least I am making productive use of my time!
There is mechanical vignetting from lens edges, filter edges, and lens hoods edges - these can be checked for (as said previously) by checking through the cut corners of the ground glass - after you stop down - you should see a full circle of the aperture, and not one that is cut off. Also, there are lenses that get "soft" toward the edge - or said differently the focus goes down. My lenses are from the 1920's through the 1980's or so. Some of my older lenses do get soft toward the edge of coverage. I believe the later lens designers cut back the area of the "circle of confusion" from the edge of lens design through mechanical means. This is different than light fall-off toward the edge - that is compensated for in wide angle lenses by center filters. I have heard as one lens with focus fall off at the edge - that some photographers have mentioned angulon lenses having more focus fall off at the edge of coverage - otherwise these can be very sharp lenses - as documented by tests. It seems that out of focus edge areas in prints are dependent on how large a print is that you are making. 4x5 to 16x20 prints show more issues than a smaller print. A larger print will magnify visual perception of out of focus areas more.
Tim, I re-read your original question where you used shift to correct the composition. If your front standard and rear standard were parallel than only the lens edge focus quality could be involved. If you used swing (which I have done) and used too much swing, than it can really change the plane of focus - and throw another area out of focus area.
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