Correcting Enlarger Light Fall With Homemade"Center Filter"

[Andre Noble]

I'm thinking of trying a method alluded to in an earlier post. I'm trying to c orrect for slight, but annoying light fall off in my B&W and Ciba prints from 4x 5 negs printed on my Omega 4x5 condenser enlarger. (I have previously converted it to a diffused source with opal glass). The B&W prints with light fall-off I can burn in, but try that with CIBA, and very hard to match exposure necessary t o avoid color shift change as you go center to corner of image...

Anyhow, the method involves exposing either photographic paper or perhaps even 8 x10 B&W film directly under the enlarger ever so briefly to the light of the cir cular fall off pattern. (If paper is exposed, then a second recording of this p attern onto sheet film would be needed - but I can't do it this way because I do n't have the necessary 8x10 camera.)

Anyway, then take the developed film, cut a 6.5" centered circle of it to match the condenser diameter, and place it on top side of the opal glass in the conden ser assembly. Will I never have to think of an expensive dichroic or cold light head again? Will it work?

If so, helpful details to consider in pulling it off? Thanks. Andre

[Pete Andrews]

I'm not sure why you'd consider using paper for this correction mask Andre. There are plenty of slow graphic-arts copying films to choose from, in sizes up to 20x16.You just have to experiment a bit to get a gamma of 1 in the development, then adjust the exposure for a nearly clear edge to the mask.

Incidentally, a cold cathode, or diffuse light source wouldn't cure fall-off. It's a natural optical affect caused by what's called the cos^4 law. When an image is projected, the light has a greater path length from the edge of the negative to the edge of the paper, compared to the shorter central path. This, together with the fact that light hits the paper at an angle at the edges, combine to progressively reduce the exposure away from the centre of the image.This fall-off goes as the fourth power of the cosine of the angle subtended at the lens. For example; if the corner of the paper is at an angle of 23 degrees from the lens, then the fall-off will theoretically be cos23 (=0.9205) to the fourth power (= 0.718). In other words, the light will be nearly half-a-stop weaker in the corners of the enlargement than in the centre, and this is assuming a perfectly even light source to begin with.I've seen cold-cathode enlargers that have had the reflector modified to compensate for this effect.So you see, the problem isn't limited to condenser enlargers, and your idea isn't original, I'm afraid.

[Pete Andrews]

Hi Andre.Lith film is just one type of graphic arts material, but I think what you really need is a slow, normal contrast copying film, such as this stuff from Kodak.I don't recommend a camera film like FP4+, because it's too fast to easily control your exposure in the darkroom. The Kodak duplicating film is about the same speed as printing paper.

[Andre Noble]

Thans for the tip and link. That's more what i need. Andre

[Steve Feldman]

OR

You could try this: (and it's almost free)

Cut a piece of frosted drafting tracing paper to the dia. of the diffusion opal glass. In the central area where you believe the hot spot resides, lightly draw a cross hatch pattern with a sharp pencil. A stipple (dotted) pattern may also work. Place this on top of the opal glass. The frosted drafting paper will reduce overal exposure about 1/4 of a stop and the pencil dots perhaps another 1/2 stop. Then, expose through a piece of clear developed film (for film base and fog) on to B & W paper, a light enough exposure to develope to a pale gray image. (Doing this beforehand shoud show you where your hot spot is). When the edge to edge gray print is even throughout, you've balance it. You may have to do this a few times to achieve the right dot or cross hatch density and location. But - what the heck - can't hurt.

When finished - drink beer.

[Pete Andrews]

Whoooops! Sorry Andre, but I double checked, and that Kodak duplicating film that I linked to is a direct positive film. It'll give you a mask that's the opposite of what you need.I can't believe it, but Kodak are discontinuing most of their slow copy film range. The only B&W duplicating film they're keeping is on a blue tinted base!Neither Agfa nor Ilford have anything listed either.You might be lucky and get some Kodak 4125 professional copy film before the last stocks are gone.

[Pete Andrews]

Hi Ernie. Yes, it's true that light fall-off occurs in the camera, but the effect is doubled in a projector or enlarger, since there's fall-off both from the corners of the negative to the lens, and from the lens to the corners of the projected image.The amount of fall-off is also dependent on the back focus of the lens used. So a long focus lens used on the camera gives very little fall-off, while a wide-angle falls off a lot.Any fall-off in the negative would only exactly compensate that in the enlarger under quite limited conditions, and the negative would have to be developed to an unusually high gamma as well.It really is better to assume that the negative is 'perfect', and to customise the enlarger to compensate for its particular lens and illumination system.The fall-off will also change slightly with the enlargement magnification, BTW.

[ernie gec]

There is no question but that light fall-off toward the edges is a fact of enlarging. But is it not equally true that the same phenomenon occurs in the camera? If so, the "thinner" densities of the emulsion edges will allow more light to pass upon enlarging, exactly where there is less light passing through the negative; in other words a self correcting effect.

[Andre Noble]

Come to think of it, you're right. Using paper in way described would create a mask compounding the problem.

What's Graphic arts film? I'm a young timer. (I came into photography as a kid at about the time Kodak's disk cameras were the rage).Is it like litho? Would it be better than using normal B&W film such as an 8x10 FP4+?

Besides light fall off, I think I'm also correcting for a slight central hot spot, ultimately due to internal reflections set up when the white opal glass bounces alot of light back through the condenser assembly into the bulb housing, where once reflected, act as a light source in addition to the bulb itself. (But since this additional source is located in a position that the condensers were not designed for, it is not evenly focused onto the negative stage, hence a need to correct this. Andre