f-Stop Printing Calculator – User Guide

How do I use the app?

Enter your base time in the input box at the top of the screen. Then click the GO! button. The new times and adjustments are now displayed for +/- 1/4, 1/2 and 3/4 stops.

How do I select the number of seconds?

Using your touchscreen, click the field at the top of the screen next to the text that says "Enter time, then press GO!". You will see a thumbwheel selector showing units in the format 000.0. These units represent seconds and tenths of a second, so for example if you want to enter 12.5 seconds you should select 012.5 using the thumbwheel. Then click "Done" to return to the screen and click "GO!" to generate the new times.

How do I enter times longer than one minute?

The thumbwheel selector allows you to enter times in the format 000.0, so for example an entry of 180.0 equals 180 seconds, or three minutes.

How do I switch on/off the red light mode?

Click the lightbulb icon in the bottom right corner to switch between red light mode and normal mode. Please be aware that many devices, including all iOS devices, use a white backlight that cannot be turned off. This means that it is impossible to use the red light mode as a genuine safelight. The red light mode is designed to reduce the white light as much as possible, but you should still turn the brightness down to its minimum level on your device and be sure to keep the device as far as possible (minimum 1 meter) from any light-sensitive material such as film and paper. We strongly recommend that you perform your own tests to determine light safety. The red light mode is provided for occasional convenience: the best practice is to determine your exposure in normal mode, then set your timer and load the paper with the screen off. You may want to employ Airplane mode when in the darkroom to avoid incoming text or calls from lighting up the screen.

NB: the number picker and other menu items on the Android version and pre-iOS7 versions of the app are not masked in red, so the device should never be in close proximity to any light-sensitive materials.

General Usage

The F-Stop Printing Calculator assumes you need to establish 1/4 stop increments (+ or – up to 3/4) of the base time you have entered and, either generate a new time, or an increment of an F-stop if you are burning in, or making a more precise test strip. The basic way to work would be to generate a test strip with intervals one F-stop apart (say 4, 8, 16, 32 seconds). If you think a correct exposure is half way between two that you have, enter the lower time in the F-Stop Printing Calculator, click GO! and the calculator will tell you the new time to use. So, if 8 seconds were too light, and 16 seconds too dark, the half way point is 11.3 seconds. You have the option of using 1/4 stops if you think it is not exactly halfway. If you are not sure why this works better than a straightforward linear time increase read on.

Background to F-Stop Printing

F-stop printing was originally made popular by the award winning printer Gene Nocon in 1987. If you want the full explanation, you can find it in his book Photographic Printing (now out of print). Put fundamentally, F-stop printing is the conclusion to all the other methods of exposure control you have made in getting towards your negative, because an F-stop is a unit of Exposure.

When you make an exposure in your camera, if you make a change from F8 to F5. 6, you are letting more light in by increasing the aperture by 1 F-stop. You are doubling the amount of light reaching the film. If you changed your shutter speed from 1/ 125 to 1/ 60, you are also doubling the amount of light reaching the film, by 1 F-stop, although you are using time to achieve this. If you load a 400 ISO film instead of a 200 ISO film, you are increasing the sensitivity of the film by doubling it, or 1 F-stop, but this time using the sensitivity of the emulsion to achieve this.

The idea of F-stop printing is that you continue this process through to the exposure on your photographic paper. An initial test strip could end up being 4, 8, 16 and 32 seconds, with each step exactly one F-stop apart from the next (doubling or halving the overall exposure). A linear strip might be 5, 10, 15, 20, 25 seconds giving a test strip of equal time intervals, but diminishing exposure intervals with 5 steps giving a shorter overall range than the 4 step strip. The linear approach, rather than the F-stop approach (sometimes called logarithmic or geometric), does not give even results and makes judging and burning harder.

If the shorter F-stop strip gives two steps that are too far apart (one too light and one too dark), you will need to find the smaller 1/4 or 1/2 stop times using the F-Stop Printing Calculator.


F-stop timers use a logarithmic rather than a linear interval and, therefore, produce an easier to read test strip as the exposures are exactly equal.

When you get used to F-stop printing, it will deliver bonuses for you. The relationship within a print from its base time to a burn time remains constant regardless of the size of the paper. So if a burn is +3/4 stop on a 10 x 8 sheet, it will always be +3/4 stop on any size paper. Once you know that relationship, then when you do the same print at a different size, you just work out the base time and use the F-Stop Printing Calculator to work out the burn time. If your print has several burn times that are different, no problem, the relationship applies, you save loads of time in the darkroom and get consistent prints. Magic!

If you are using variable contrast paper, it is possible to work out the difference that a modification in grade (on a multigrade or colour head) will make in terms of exposure time using F-stops. That relationship will apply to that enlarger always. So, if you discover that changing from grade 2 to grade 3 requires a 1/4 stop time increase, it always will regardless of paper size.

Copyright © Digitaltruth Photo Ltd & Bruce Tanner. All Rights Reserved.

Copyright © Digitaltruth Photo Ltd, 2024. All rights reserved.