• Originally Posted by Michael R 1974
I'm not following. Did you mean to say a one stop flare factor? Let's assume the two exposures I listed are both 4 1/3 stops below metered and that the film density under zero flare conditions would arbitrarily be 0.20 above FB+f in either case. In the first exposure, the intensity of light entering the camera is high, and so some of this light (from the portion of the image circle beyond the borders of the emulsion) might bounce around, eventually bouncing onto the film and adding some additional exposure. At least I think it might. Maybe the actual film density ends up being 0.23 above FB+f. In the second exposure, a much lower intensity of light enters the camera, but the shutter is open for longer. Would we still expect a density of 0.23? Or would we actually get the zero flare density of 0.20 because the light intensity is not high enough to cause significant reflections in the camera - even though the film is exposed for longer.
Same scene, same luminance range, and say shot at two different f/stops should have the same flare. And sorry about the "filter factor" typo. It was supposed to be flare factor.

Perhaps there is no definite answer. Here's another way to think about the same example. You take a picture of a bright, uniform, featureless mid day sky and expose it to produce an expected net film D of 0.20. In the evening you take a picture of a middle grey, uniform, featureless evening sky and expose it to produce a net film D of 0.20 (so obviously this would be a significantly longer exposure than the picture of the mid day sky). Assume some camera flare in the first exposure leads to an actual net film D of 0.23. Would that occur in the second example too, even though the intensity of light entering the camera is lower? I'm thinking there would be less, or zero flare in the second exposure. Not sure.
If the featureless sky takes up the entire frame, it's the same as shooting a target. It's virtually a flare free situation, or to put it more accurately, free of the effects of flare.

Keep in mind this question concerns an in-camera film test with as little flare as possible (aside from some degree of lens flare which cannot be totally eliminated). The aim is to use a brightly lit white card for the entire series of exposures. In actuality, the lens is masked to restrict the projected image to within the borders of the emulsion. In other words, there should be no camera flare. But even though I would be using this mask anyway as an extra precaution, I'm wondering if it is necessary. So my example above would be the equivalent of not masking the lens.
You're keeping stray light from entering the camera. The exposure of the white card shouldn't be affected either way. Even with an average luminance range the effects of flare at the point of the white card is inconsequential.

Camera Flare.jpg

Regarding the K-Factor discussion in Henry's book, as it happens I re-read that very section today, and I came away just as little understanding as when I first read the book. I wish he had started by explaining what the K-Factor actually is before getting into the ranges, calculations, standards etc. I still don't get what the K-Factor is. The only thing I think I understand is that it is not what Adams said it was. When you read Adams you come away thinking it is just a safety factor built into the meter.
With Adams you come away thinking it's a conspiracy. You should read the stuff I wrote n the K-Factor thread. Here's a page.

Defining K, part 2.jpg

Henry talks about the various values of K, but he doesn't explain what it is or how it relates to exposure. In Henry's book you come away thinking it's an arbitrary decision by the manufacturer. BTW, he never really defines exposure (or from what I've read so far). I like that he has a bibliography, but he could take the level down a notch and include the titles of the papers. The book isn't for scientific publication.