Light Falloff

[tomczak]

Photographing a white wall and then using Compute in Light Falloff often gets me a 'Fail to fit a good fit' message. That is the case even if I blur texture. Would it be possible to add manual slider(s) (akin PWP3.5, if I recall correctly) so that the falloff could be corrected manually still?

There is a way to do that with Gradient (Oval, Curve, Filter), but it's quite cumbersome.

[JvdW]

I suppose the 'light falloff' function is looking for a circular brightness pattern in the image. When photographing a wall you are likely to have a lightsource above or sideways. Applying a gradient will adjust for this assymetry, but I think that the resulting falloff correction is probably not a good match to the actual falloff of your lens.
I checked my lenses by photographing a sheet of paper oriented to the sun (paying attention not to photograph my own shadow).

I also found that some lenses have no detectable falloff, resulting in the same 'no fit' message. It is a bit confusing, but it makes sense if you think about it.

[jsachs]

It is critical to photograph an evenly lit field if you want light falloff to work. A uniform flat wall evenly lit by sunlight can work or you can use a strong diffuser such a opal glass or an Expo Disc in front of the lens. Wide angle lenses as especially tricky since you need a larger uniform area. Sliders would be very hard to use manually since there is no intuitive meaning to the coefficients. Some point and shoot cameras have asymmetrical falloff that cannot be easily corrected.

[tomczak]

This may be much more primitive than the calibrated method used by PWP now, but I recall PWP 3.5, while not having sliders, had 'Focal Length' and 'Film Size' parameters. Between the two, it was fairly easy to adjust for fall off in the corners. Could some similar manual method with one/two parameters be resurrected somehow? Regardless of my difficulty with the calibration image that I mentioned, there are images that one can't use that method at all - such as the images taken with a camera that I don't have anymore.