Digital technique to correct Light fall-off : LUSENET : Large format photography : One Thread

With a wide angle lens on large format, the corners are dark due to cos**4 light fall off. The center filter fix is expensive. I will be scanning the negatives, so a digital fix should be easy. I need a Photohop filter or other software to lighten the corners, something easier than manual dodging. Any suggestions?

-- James E Galvin (, October 24, 2000


Response to Light fall-off

Use the radial gradient tool with the "lighten" option. Start the gradient from the optical axis (may not be the center of the image if you used movements) and take it out to the corner. Tweak the opacity and/or the endpoints of the gradient range until you achieve the effect you want.

-- David Goldfarb (, October 24, 2000.

Response to Light fall-off

Sorry, I just looked at Photoshop, and that's called the "color dodge" option, and set the color range to "black,white" all in the options box you get by double clicking the radial gradient tool. The lens I usually use this with has about 1-1/2 stops of falloff, so I set the opacity for 75% and get good results typically.

-- David Goldfarb (, October 24, 2000.


-- Andre (, October 24, 2000.

James: Note that, because you are not "getting it in the negative," the results, post exposure (this has nothing to do with digital vs. analogue) manipulation may not yield the highest quality. Some areas of the neg will be underexposed relative to others, which will result in a grain/sharpness difference. You can't scan back what's not there in the first place. Since you are a large format photographer, I'm curious why you wouldn't spend the extra money eventually to make the most out of the format? Surely post-exposure dodging is a quality compromising and less than optimal solution-seems almost like a negation of the benefits of large format quality. I hope your solution works out for you in the short term. . let me know.

-- Josh Slocum (, October 24, 2000.

Josh: It depends on your film/paper combination, development techniques, amount of falloff with the particular lens, etc. Shooting chromes, yes, a center filter is really needed to get the most out of the narrow tonal range of slide film, but with B&W films, if the exposure is right the film has a wider tonal range than standard papers, so you can have two-stops of falloff, and still, depending on the scene, have more detail than the paper can record even with post-exposure dodging in the darkroom. Move to a printing technique like platinum with longer tonal scale, though, and you might want a center filter even with B&W.

-- David Goldfarb (, October 25, 2000.

David: Yes, you are largely correct. We are not talking, however, about having more detail than the paper can record, but about the quality of the detail present in the film. A negative with 1-2 stops of light fall-off at the edges is not "correctly exposed." This is not a silly semantic difference, it's a a truthful observation with practical consequences. Even with the forgiving exposure latitude of negative films, a neg with a one or two stop variation across the frame WILL show a difference in the amount of grain and shadow detail between the differently exposed areas. Shadows that fall in the corners where not enough light reached, for instance, will have markedly more grain, and the image will not be as sharp. Increased development will not help; it will merely increase overall density in proportion, leading to better shadow detail at the cost of blown out highlights. Dodging by hand, or electronically, will also not restore this detail, it will merely even out the tonal range in the final print. The difference in image quality may indeed be noticeable in the final print. Personally, I have never found a negative that's underexposed by two stops acceptable. Even one stop is pushing it for critical work. In either case, what is acceptable across the entire frame quickly becomes unacceptable when two different densities in the same frame are visible next to each other. No matter what James does in the darkroom or on the computer to even out the print's density, the edges with fall off will exhibit a different, and probably degraded, quality of grain and sharpness- REGARDLESS OF WHETHER THEY'VE BEEN DODGED TO MATCH DENSITY (Sorry for the caps, I just can't make italics show up). The edges will just look "washed out," and increasing contrast will only add to the problem, as it will emphasize the different grain structures. As I said before, this has nothing to do with analogue vs. digital- it's about understanding how the film medium works, and remembering the old adage "garbage in, garbage out."

-- Josh Slocum (, October 25, 2000.

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