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I was printing from one negative last night and had the 8x10 landscape print down pretty well using split filtration, then decided I wanted to crop in on it a bit more and orient it in portrait mode. I ended up raising the enlarger head probably from 10" to 15" or so. It was getting late and I didn't want to bother with another set of test strips, so I just ballparked it a bit and increased the time about 50%. The print came out OK, but definitely too light. I probably should have taken better notes before posting, but I do recall from a failed experiment in the past where I doubled the height of the head and I increased the time by 4x figuring the inverse-square law would work, but it just toasted the print. Has anyone done this experiment before and have a good factor for this sort of thing?

2. I use the area of the print: double the area, double the light (twice as long an exposure, or 1 f-stop); quadruple the area = 4 X the exposure or two f-stops.

3. You're dealing with the Inverse Square Law. Moving a light source farther from the subject it diminishes proportionately to the distance.

I've worked out a simple Excel program that I simply plug in three variables and come up with New Elevated Exposure

New elevated exposure equals:

((New elevation / Old Elevation) ^2) X Original Exposure

Supposing your 8x10 elevation from lens to paper was 24 inches @ 16 seconds and your New elevation is 30
inches, then: ((24/30)^2) = 2.44
2.44 X Original Exposure (16 seconds) = 39.4 New Elevated Exposure.

4. Why not just do a new test strip and see how much of a change it is? Better to do that and get the same visual results -- then do all the dodging and burning proportionally. One test strip should tell you everything you need to know.

5. I have always used an Ilford EM-10 for things like this, it's a nice handy baseboard light meter..... just remember to use it with your safe light off for greater accuracy.

6. Thank you all for your insight.

Bruce,
Does that formula work out pretty well for you? That is what I thought I did once and it just ended up being way overexposed, so I figured it might be a more linear function.

Michael,
The test strip is probably best, but I can punch in a formula a lot quicker than running more test strips every time I move the head a bit. That is if the formula can be relied upon to give equal resuts. I am certainly not in a hurry to make shoddy prints.

7. Good Afternoon, DE,

I use the formula Bruce cites. It works very reliably. JUst be sure to hit the right calculator buttons in the dark!

Konical

8. Originally Posted by declark
Bruce,
Does that formula work out pretty well for you? That is what I thought I did once and it just ended up being way overexposed, so I figured it might be a more linear function.
It works pretty well but I must admit I do some tweaking before I get what I really want. I think it may be due to the original exposure not being as true as we might think. Any error, even 1 second, when magnified will distort proportionately. In the example above, if the 16 second exposure could have been done in 15 seconds the elevated exposure would be almost two seconds different, thus some tweaking is required.

9. Off the top of my head, I think what may be missing here is a bellows correction factor. At least in my experience, there is a fairly significant change in bellows extension at typical enlarging set-ups. (I'm sure I could add that to a spread sheet, but for my limited printing activities, an EM-10 and/or test strips suffice.)

DaveT

10. I had a friend introduce me to the EM-10 who was using in a commercial application with sports photos. Having to make multiple prints from the same negative at different sizes.... the EM-10 saved him lots of time and paper.

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