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# Thread: Time calculations when switching print sizes

1. Originally Posted by Steve Smith
Dan is quite correct.

I was not suggesting that doing a square law conversion would give a highly accurate new exposure time for each size of paper but it will give a very good indication of a starting point without having to do a full range test strip each time.
Indeed. The OP was asking if there's a simple way, and the inverse square law is a simple way if the size change is not too large. By way of a check, the difference between what the formula predicted and what I actually measured with an Analyser when changing from a 10" width to a 16" width was less than a tenth of a stop - barely noticeable except for the finest quality work, and if you're working to that quality level you'll want to do another fine test strip anyway

Regards
Richard

2. See if you can get a copy of the Kodak Darkroom Handbook. There is a calculator for this purpose in it, sort of a circular slide rule, which is very handy. Lots of other stuff too.

3. Richard

Just use the inverse square law with the lens to paper distance, and there will be no difference to your analyzer measurement. You can check in 'Way Beyond Monochrome' on page 291 (same graph as I posted here before). There are no short cuts required for this. It's pretty simple as it is.

4. Originally Posted by Troy Hamon
If you are working from 35 mm negatives and using
your calculation for the change from 5x7 to 8x10, it
will be closer in accuracy than printing from a 4x5
inch negative and going from 5x7 to 8x10, as
pointed out by Michael Briggs and others.
I'll point that out also. The reason the calculation for
35 mm is more accurate is due to less of a change in the
speed of the lens when going to 8x10. To 5x7 a 35mm is
5 times enlarged while the 4x5 is only 1.4 times enlarged.
For the 4x5 little more than a same size print. Think of
the bellows extension. At a 1:1 ratio a lens, camera
or enlarger, is down 2 stops. Dan

5. Originally Posted by RalphLambrecht
Just use the inverse square law with the lens to paper distance, and there will be no difference to your analyzer measurement. You can check in 'Way Beyond Monochrome' on page 291 (same graph as I posted here before). There are no short cuts required for this. It's pretty simple as it is.
Just done a rather more scientific check and you're absolutely right - if you substitute lens-paper distance for print length in my previous post then it will work accurately.

6. Originally Posted by RH Designs
Just done a rather more scientific check and you're absolutely right - if you substitute lens-paper distance for print length in my previous post then it will work accurately.

7. Could I expand on the above a little by suggesting that it is the image size, and the lens node to paper surface that are the important dimensions. Paper size is not important.

8. One can mark the negative to paper distance on the enlarger column but can not do so for the lens to paper distance, but lens to paper distance is what we really need. Now can we calculate the lens to paper distance if we know negative to paper distance as well as the lens focal length?

9. Originally Posted by Chan Tran
One can mark the negative to paper distance on the enlarger column but can not do so for the lens to paper distance, but lens to paper distance is what we really need. Now can we calculate the lens to paper distance if we know negative to paper distance as well as the lens focal length?
Chan, why do you want to calculate the lens/paper distance when it is quicker to measure it?

10. Because I want to install an encoder in the enlarger head lift and display the height directly. If I can calculate the lens distance then I can make the gizzo display the exposure time automatically. Just for fun really AS we all know it's much easier to measure the light intensity.

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