F/Stop Timer

DIY F/Stop Timer

This webpage shows how you can build an f/stop timer for B&W analogue printing. If you just want to buy one, both RHD and DA will sell you a very nice working and well-supported unit. I will not sell you a timer.

If you build this, please contact me (email address is in the headers of all the source files) as I'd like to have some idea of how many of these are out there. Of course if you have suggestions or bug reports, please send them in too.

WTF is an F/Stop Timer?

The term "f/stop timer" was coined by Gene Nocon, who is widely credited with inventing this approach. Nocon sold an f/stop timer and more importantly, called it an f/stop timer. Here's a video where Gene discusses f/stop printing and a bit of printing history.

Here's a brief article explaining what f/stop timing is all about and here's a primer from RHD (the manufacturer of another of these timers).

To take a step back, this is an enlarger timer. It controls how much light hits a piece of black and white photographic paper, thereby controlling how dark it gets. By expressing exposures in a log scale (stops, being log base 2), we can perform a lot of print manipulations such as dodging and burning, changing size, shifting the image by a fixed overall tone change, etc, just by performing simple addition/subtraction arithmetic.

While you can achieve the same ends with a linear timer, it's a lot more taxing, both in the number crunching you have to do in your head as well as needing to adjust the times for all exposures in a print when making a global adjustment, e.g. for print magnification.

Regardless of the f/stop thing, it's also handy to be able to program in a whole exposure sequence for a print instead of needing to remember the sequence and set the times one-by-one.

Features

What it does:

0.01 stop resolution, exposures of 1ms to 1000s
Drydown correction with 0.01 stop resolution
Direct numeric entry of decimal EV
Textual prompts entered by user to describe each exposure
Up to 8 exposures per program, save 7 programs in internal EEPROM
Pause, resume and skip exposures
16x2 red LCD and 4x4 keypad for user interaction
SMS-style multi-keypress method of text entry
Adjustable LCD backlight brightness
Generate test-strips and test-patches in equi-EV increments (new!)
Transfer stored programs to/from a PC (not yet implemented)

The "not implemented" features have placeholders in the code and they should appear in the next few months. Missing features that are likely to stay missing for the foreseeable future are:

Multiple output channels for additive colour enlargers and split-grade automation, e.g. tone-preserving grade-changes
Closed-loop mode (use of a photosensor)
Support for more dodging than the base exposure, i.e. concurrent dodges / operator-is-an-octopus mode
Lamp warmup/switchon correction

Construction and Use