The distance from the light source to your print is important when determining exposure time. I mounted my 20W UVLED exposure unit under the lens holder on my Minolta enlarger so I can easily adjust the height. I can move it up to cover a large print and down for a small one. The advantage of moving the light closer to the print is reduced exposure time. But just how much should I reduce the exposure time, you might ask. Well, here’s how to figure it out. It’s based on the principle that the intensity of light is proportional to the square of its distance from the subject. The inverse square law, IL=1/D2 applies here but keep in mind it’s relative.
For example, if your light is 10 inches from your print it would have 1/100th the intensity as if it were 1 inch away. That’s pretty simple but what happens when we have to deal with real world measurements. Let’s say you were exposing a print with the light source 18.5 inches above the print and you measured the standard exposure at that distance to be 68 minutes. Then you realize that you don’t need to cover such a wide area and you can move the light closer to the print. It looks like you’re getting pretty good illumination when the light is only 11 inches above the print, so how much is the new exposure time? We’ll calculate the exposure factor, a value we’ll call EF, by which you can divide the original exposure time to obtain the new exposure time.
Formula: EF = E12 / E22
Where: EF is the exposure factor, E1 is your original distance and E2 is the new distance
E1: 18.52 = 342.25
E2: 112 = 121
EF = 342.25 / 121 = 2.83
Divide the original exposure time (68 min) by 2.83. That’s about 24 min. See you just saved yourself a bunch of time. If you’re moving the light source away from the print, simply multiply the original exposure time by the exposure factor instead of dividing.