I had initially planned to write a huge blog post on how I develop film at home. But after jotting down all the points I wanted to cover I realized that it would take me a long long time to write it. And therefore I would never actually do it. So I decided to split it up into many small pieces.
This time I'm covering two formulas that I use all the time when I develop film. I'm honestly not sure where on the interwebs I've found them so please excuse the lack of peferences.
Formula 1: How to adjust the development time according to temperature
As I only use stand-development (at topic for another installment of this series) and therefore have development times of at least 60 minutes I can't really control the temperature of my developer. It will always change unless it's exactly room temperature. So instead of going to great lengths to keep my developer at 20°C I will just use it at room temperature and adjust the development time accordingly. For this I use the following formula. It is probably an approximation and should be adjusted for each film stock, but it has worked out fine for me:
new_time = old_time * Math.exp(-0.081*(new_temp - old_temp))
Tip: If you use the Massive Dev Chart Timer to time your development you can use this formula to your advantage as this is more or less the formula used by this app. The problem is that you can only enter development times of up to 59 minutes which is not long enough if you use stand development and want to push. But, the app does support longer development times. So you can just calculate a development time shorter than 60 minutes using the formula from above and enter it together with the correct temperature. After that you can use the temperature compensation feature of the app and voila you have now entered a development time that's longer than 59 minutes!
Formula 2: How to adjust the development time for push and pull processing
If you want to push or pull a film there is also a rule of thumb that you can use to adjust the development time:
new_time = old_time * (1.5^N)
Where N is the number of stops you want to push (use negative values for pull). Apparently Kodak Tmax films need less adjustment, but as I only scan my film I've used this formula with Tmax 400 with success, too.
So much for installment one of this series. I'd love to hear from you how you adjust the development times. I'm really not an expert in this and I probably could me more careful with my development times. But as I don't print in the darkroom using just these formulas has worked out for me just t