Introduction: Film Developing Temperature Compensating Slide Rule

This is a developing time compensating slide rule.

I saw a picture of the original online somewhere. This is a copy I have made from memory. I have checked the times based on a calculator on the massive development chart and it matches up.

There is a contrast calculator on the bottom, use the modified times to increase or decrease the contrast. I was going to make a push/pull calculator on the bottom, but it varies from film to film.

So how does it work?
If you develop black and white films you normally do it at 20C or 68F. However it's easy to do at room temperature by adjusting the time. Saving you heating up or cooling the solutions.

Find the development time for 20C, slide that time to the 20C index.

For example 8 minutes:

Slide 8 minutes to the 20C index,

If the room temperature is 17 degrees, find it on the top row and read off the development time next to it. In this instance 10 minutes 45seconds.


Sheet of A4 paper


Laminator and one A4 laminator sheet (optional) or use clear sticky tape.

glue stick

Scissors or guillotine

Step 1: Download and Print Out This Sheet

There is a jpg and a pdf attached

Just print the whole sheet, everything you need is printed on it including layering instructions that you'll see on the next part.

Cut out all the numbered parts. There are 9 parts

Step 2: Layering the Slide Ruler

Starting with part 5 (its marked 6 on the image above - sorry). This is the slider. If you are laminating it, do it now and cut it out as close to the lines as you can. Or cover it on both sides with adhesive tape.

Cut out part 1. This is the base

Cut out parts 2 and 3 carefully.

Glue part 2 over the top of part 1, keeping within the lines. Do this with both part 2 layers

Place the Slider in the middle then glue parts 3 on the bottom as close as you can get them, allowing the slider to move with no wobble.

(If you live in a part of the world that prefers to use fahrenheit, you'll have to write the equivalent on the scale.

above the Celsius.)

Glue parts 4 and 6 (the real 6) to the top of the stack, making sure that the 20C indexes are aligned.

The slide rule will work at this point. But its a good idea to waterproof it somehow. I used a laminator

Cut out part 7, the window template, and mark it out on one side of a sheet of laminate. Use unheated laminate and cut the shape from that. Place the laminate (dull side up) in the middle between the raised sides.

Something I did at this point was cut a longer piece of stiff paper from the template and put it underneath the laminate just to lift it a bit and to leave a gap when it was pulled out. It should be longer than part 1.

When everything is in position, put the whole sandwich into a laminating pouch and laminate it.

The edges wont be completely done but I sealed mine with an iron later.

Cut the whole sandwich at both ends and slide the long template out. You may need a little help with a toothpick. Now, the slider should go smoothly through the channel in the middle.

Et Voila its finished.

If you don't have a laminator, you could use clear adhesive tape over the slider and two pieces stuck together to make the window over the slider. Then cover the whole thing with adhesive tape.