App to calculate time for a different head height

Hello everybody!

Few months ago I found a tool (app) that calculate the time increase it's need it to make a bigger print. You give the initial head height and the time need it for ex: 8x10 print, then the head height for the bigger (or smaller print) and it gives you the time.

I didn't write it down and I can't find it anymore... Any help?

Thanks!

Here's one way of calculating this below.

But If I'm going from 8x10 to 11x14 (or the other way around), I just use the same exposures. The prints look the same to me.

You don't really need an app for this, just a calculator.

New height divided by original height, squared, multiplied by the printing time. I just have that taped to the wall.

Measure from the easel to the lens board. If it's 16" high and your new print size is 20" high and your exposure is 10 seconds, you get 1.25. Square that (1.25x1.25) for 1.5 (essentially) - think of that as the "factor" to multiply exposure times. 10 x 1.5 = 15, that will get you very close. It works in reverse as well, for going smaller. Same formula, new size divided by old, but your factor will be less than 1.

If you have a complex print with dodging and burning, just refer to your print map and multiply each exposure time by the factor, do a test print and you'll likely find a couple small tweaks.

Are you saying:

20/16=1.25
original time was 10 seconds

1.25x1.25x10 = 15

Usually when I go from 8x10 to 11x14, I’ll open the lens one stop. That way the time remains the same for all exposures, dodging, burning and even split printing. Sometime I may have to make a quarter-stop or so adjustment as papers may vary from batch to batch and size to size.

Pieter12 said:

Usually when I go from 8x10 to 11x14, I’ll open the lens one stop.

Click to expand...

Which makes sense. After all, 8x10 = 80, 11x14 = 154 which close enough to twice the area. With the same light intensity this would require twice the exposure to reach the same density, ignoring reciprocity failure.

So an easy approximation (IMO) is to calculate the change in the surface area you're illuminating and adjust exposure accordingly. If someone wants an app for that: a calculator is a default app on every phone!

There is an app called “enLarge” by Andrew Wittner.