Superwide, light falloff and exposure calculation

[Chris Partti]

I recently acquired a superwide lens that has the expected amount of light fallo ff--I'm guessing as much as two stops from the center to the corners. I'll be at tempting to compensate for this in printing when necessary. I'm wondering, howe ver, about the proper way to deal with falloff in calculating exposure and devel opment. As an example, today I was shooting a scene that had shadows around the edges of the frame, which I placed on Zone III, and highlights near the center, which fell on Zone VIII. Is my calculated exposure (based on the Zone III plac ement of the edge shadows) correct, or do I have to increase it two stops? If I need to increase exposure (or even if I don't, come to think of it), wouldn't I also have to do an N-2 developemnt (assuming the scene was N to begin with). T his suggests that a rule of thumb with superwides might be to increase exposure and decrease development time. Is this right?

Thanks for any advice.

[fw]

You'll only be able to adopt the kind of approach you describe if the pattern of lighter and darker areas in the image matches the light fall-off profile of the lense, and obviously it will only be by coincidence that this will occur. As the difference is a highly visible 2 f/stops, then you will be better off buying a centre filter (which are unfortunately quite expensive), in order the equalise the light intensity, and then be able to conduct normal zone measurement.

[Trevor Crone]

Chris it sounds if things are getting a bit complicated so I think the centre fi lter(although large and expensive) is the way to go. Although I do a lot of wide angle photography, using lenses from 35 to 75 in 6x9 and 4x5 formats, the only lens where I feel a centre filter would be useful is when I use the 35mm Apo-Grandagon. Also consider if you are using a filter on the lens you are not getting vignetti ng. This certainly happens with the 35 Apo. Good luck,

[Chris Partti]

I should say that, although I realize a center filter is the best solution, buying one would about triple the amount of money I have in the lens, so I'm trying to work without one. If I absolutely need a center filter, I'll probably just give up. Also, I doubt I could find a center filter near enough the right size to work. By the way, I assume that when one uses a center filter, the correct thing is to increase exposure the amount of the maximum density of the filter. Put another way, the only place where the exposure measured by a spot meter would be correct without the filter is in the middle of the frame.

[David A. Goldfarb]

I've been dealing with this issue with my 120mm/f:14 Berthiot Perigraphe, which has about 1-1/2 stops of falloff from center to corner with 8x10" by exposing normally for the center and dodging the corners when printing, usually shooting TMX rated at EI80 for normal processing in D-76 1:1. Thus far, I've found the film has enough latitude to get away with it, though I've ordered a center filter (which will cost more than the lens) for the sake of convenience, since I find that I use the lens quite a lot and a center filter should produce more uniform results.

[Doremus Scudder]

If you are shooting B&W negative film and can correct the light fall- off problem in printing, the only real problem you have is making sure that the film gets adequate exposure in the corners to render the shadow detail that you want there. Since you calculate your fall- off to be about 2 stops, placing corner shadows in Zone III would render them a detailless Zone I unless you intentionally overexpose or make some compensation to your exposure index for this lens (a "fudge factor" for this one lens so to speak). If you don't want to use a center filter, this will result in the center being that much more overexposed, but it's easier to get detail from overexposure than underexposure and the film latitude will probably hold detail well into Zone X or XI depending on the film you are using. I'd recommend calculating your exposure for the corners and letting the center get a bit overexposed for negative films. The edges can then be burned in to compensate during printing. For color chromes, break down and get the center filter. Hope this helps. ;^D)

[Ellis Vener]

I am curious about which lens you are using. I use the Heliopan centerweighted filters instead of the Rodenstock or Schneider CW filters and have been very happy with the results, and with the savings.

[Chris Partti]

Ellis, it's a Berthiot Perigraphe 120mm for an 8x10 camera like the one that David Goldfarb mentions. Even the Heliopan is a bit pricey given what I paid for this lens, but David tells me he's ordered one, and I'll be interested to find out how it works for him.

[David A. Goldfarb]

And as I told Chris in e-mail, the filter is backordered, so I won't be able to report back for a couple of months, most likely. When I had the lens shutter mounted by Steve Grimes, I asked him to space out the front filter thread (which extends beyond the front of the lens) with a center filter in mind, so in theory it should work.