Yes thanks for that chuckroast but what counts in craigclu's link is what does he mean? I am sure he will clarify matters
pentaxuser
I don't think
@craigclu is the originator of this spreadsheet. It suggests that Francis Miniter was the author.
The math is clear enough: Take the original time and multiply it by the square root of the dilution component ratio.
It would appear that the author is taking the geometric mean of the two dilutions to find the new time coefficient.
This does not match the Ilford published times (
https://i.pinimg.com/originals/36/a4/31/36a431c3aed059f09542f6c4f96a68a6.jpg) for ID-11, as just one example.
So, I don't think it's a simple as this formula suggests. This has been discussed before here on Photrio to the conclusion that a general formula actually isn't that simple. See:
https://www.photrio.com/forum/threads/developer-dilution-vs-dev-time.75149/post-1041020
Now let me further muddy the waters, if I may. Shadows develop much more slowly than highlights. The more you dilute the developer, the more development time you'll need to achieve expected speed OR the more you have to expose the negative (lower EI) to maintain shadow detail. But either way, you are also going to be pushing the highlights up the H/D curve and run the risk of blowing them out and getting featureless white.
In my own practice, when I want the sharpness benefits of increased dilution, I pretty much always resort to semistand or Extreme Minimal Agitation for this exact reason. These extended development schemes let the film sit in the developer for a long time. This gets you to an EI that is full box ASA (or nearly so) thereby preserving shadow detail. But the limited amount of agitation combined with the highly dilute developer, ensure that the developer exhausts quickly on the highlights and keeps them from blocking up.
The more interesting thing is that - with EMA or semistand -
the actual time in developer is far, far less critical. You just need two things: Enough time in developer to get full shadow speed, and sufficiently dilute developer so that the highlights don't get over done. I've done semistand anywhere from 45 min to 24 hours without a great deal of difference in the final negatives (though the 24 hour development had a kind of funny finish to the negative).
Take a look at the picture of the ticket stand I uploaded in post #37. The light was just blazing on those white foreground surfaces, but the distant trees were in fairly deep shadow. A long, highly dilute development protected both nicely. It also created a lovely mid-tone local contrast.
With conventional dilutions and normal development, I would have had to increase exposure a bunch (reduce EI) and then underdevelop (N-) to hold the shadows and highlights respectively. But that would have collapsed the mid tones into a boring gray. I spent years chasing better negatives using Zone system exposure and development controls and never consistently got what I wanted. It wasn't until I understood better how film works, and how you have to control for shadows, midtones, and highlights properly when developing that I started getting better results. Long, dilute, low agitation schemes were a path to this.
I hasten to point out that this is not something that is appropriate for all subjects. For example, I would not do portraits this way.