47 SA XL vignetting on my Ebony SV45U

[Jan Becket]

I found I needed wider coverage than my 75 Grandagon offers and sprung for a 47 Super Angulon XL and the corresponding center filter. My Ebony is advertised as able to mount a 47 SA XL, but I have tried numerous configurations and still get vignetting in the corners, about 1/4 to 1/2 inch. I cannot seem to get a full 4X5 neg. This is strange because I came up with a method of focusing that involves a small LED flashlight placed 6 or 8 feet away. That makes it possible to focus accurately and then to check that the point of light appears in each of the 4 corners. However, the fact that the light appears does not seem to indicate that there will not be vignetting, as in a totally clear corner, no exposure. Weird.

I just concluded (another) test in which I
1) Set the asymetrical rear tilt to the max
2) Raised and tilted the lens on the front standard to make it parallel with the rear standard and centered
3) Used the LED flashlight to focus (8 feet, f/16 @ 1 sec)
4) Removed the rear ground glass to check that the bellows is not blocking the rear element of the lens (which it can do sometimes)

I see that I could somewhat balance the corner vignetting, since the lower corners on the image have more than the upper ones. This means lowering the front standard - I think. However, there will still be some vignetting in all corners. I have previously tried the standard rear tilt rather than the asymetrical rear tilt, and have tried a combination of the two. I called Bruce at Badger Graphic Sales, where I purchased the Ebony, but he could not offer specific advice, and commented that 47 SA XL lenses can have varying coverage. Anyone else run into that issue?

Iʻve used 4X5 for 30 years and have a book out - I have the feeling that I should be able figure this out. Or maybe not. This is beginning to remind me of when I first started and needed to figure out where in the film holder to place those notches ...

[Dan Fromm]

Interesting. If the lens axis is centered on the film then falloff should be the same in all four corners. So make sure the lens is centered. No tilt.

According to Schneider the lens reaches full coverage at f/22. This means that at larger apertures mechanical vignetting will be a problem. So f/22.

How are you metering, including adjusting for the CF? That the negs are completely clear in the corners indicates no exposure at all. By cos^4 the corners should be ~ 3.6 stops down from the center if a CF is not used. With negative film's latitude if you metered correctly and the subject isn't darker in the corners than in the center there should be some image in the corners.

Which CF are you using? Schneider recommends the IIa for the 47/5.6 SAXL. Two stops central density, 4x exposure time or set the lens to f/22 and tell the meter it is at f/45. With it the corners will still be ~ 1.6 stops down. You might want to use 8x exposure time, not to make exposure more even across the frame but to make best use of the film's latitude.

[Jan Becket]

Thanks for the excellent suggestions, Dan. I did an additional test, which included exposing at both f/16 and f/22. The problem turned out to be something I should have caught much earlier - the Lee filter adapter ring. Iʻm not using Lee filters on the 47 of course, but was using the white Lee lens cap, with an adapter ring that extends out less than a quarter inch - apparently enough to cause the slight vignetting in the corners I was experiencing.

The f/22 exposure does appear a better choice, at least when the lens is set at an 8 foot focus distance. It catches the close ground in front of the tripod, which resolves noticeably better at f/22. I might do some additional tests shifting between f/16 and f/22 while focusing at six feet, to check the resolution of very close and very far objects.

Iʻm using a IIIc center filter, which completely takes care of light falloff, and gives a nice even exposure corner to corner. I compensate for the filter by opening one stop, and have been getting negs with densities very similar to others made on my other lenses.

[Jac@stafford.net]

The outcome of this thread is a good reminder to consider the difference between mechanical and optical vignetting.

Thanks, Jan, for bringing it up.