while printing some full-frame images with black borders, i just realized that my beseler 23CII-dichro isn't providing even corner to corner illumination. that is, the corners of my borders aren't as dark as the sides. i've always ended up burning in the corners of my prints, but i always thought it was due to aesthetic concerns--to direct the eye away from the corners--but apparently my corners have been exposing lighter.

is there a way that i can alter the enlarger to get it to expose evenly, or am i going to have to continue and refine my burning techniques.

thanks.

-- brad daly (bwdaly@hiwaay.net), June 20, 2000

Answers

How uneven is uneven? There will always be a little falloff at the corners, even if the enlarger light source is perfectly even. The light has to obey what is called the cos^4 law, which means that it cannot be as bright in the corners of the image as it is at the centre, for the simple reason that it has further to travel from the lens. The amount of fall off increases with the angle of the lens, and is usually "acceptable", but if you're using a wide-angle enlarging lens (say 40mm on 35mm, or 60mm on 6x6) it becomes quite noticeable.

With a standard lens, the light at the corners of the frame will be 3/4 of the brightness at the centre due to the cos^4 law. i.e about 1/2 a stop down. With a 40mm lens on 35mm the corners drop to 0.6 of the centre brightness, nearly a whole stop under. Maybe someone should invent the centre filter for enlarger light sources.

-- Pete Andrews (p.l.andrews@bham.ac.uk), June 20, 2000.

Brad: I can't offer you a solution. I can only say that your 23C suffers a defect apparently common to these enlargers. My old 23C, with its conventional lightbulb and condenser head, produces a pattern about a stop brighter in the center than in the corners, with the farthest edge brighter than the nearest. I have to dodge and burn many prints to compensate. The head is simply constructed and offers no adjustments. What you report has been reported by others on other message boards. I've seen no cure reported, though. I tried putting a circular piece of neutral-density material in the center of the filter compartment, but didn't see any improvement. I've just learned to work with the thing the way it is. If you can devise a fix, we'll all light an enlarger bulb in your name.

-- Keith Nichols (knichols@iopener.net), June 20, 2000.