Dilution Factor & Developing Time Relationship

[Vincent.G]

Hi guys

I am going to ask a silly question and hope you guys can help me with this. Does halving the concentration of the developer equates to doubling the developing time at the same temperature?

I am using TMAX developer. The bottle label recommends 1:4 dilution and 6 minutes at 20 C for TMAX400 at EI400. I tried to develop a roll of TMAX 400 shot at EI320 with 1:9 dilution and 12 minutes at 20 C, and it seems ok so that is why I am asking you guys whether I got the equation correct with respect to the dilution factor and development time.

Interestingly, I have also tried Chris' developing time for TMAX 400 at EI320 using 1:7 dilution for 9.5 min at 20 C. The results seem similar though I have never done a proper comparison of all 3 development times with the same scene at same exposure.

Am I missing something here?

 

[payasam]

A mathematically constant relationship is unlikely, Vincent. Do you need greater dilution to reduce contrast? If you don't, it's best to follow the label.

 

[Vincent.G]

That's partly the reason, payasam. The light down here at the equator is very contrasty so I am experimenting with ways to reduce contrast. Another reason is to see how much I can optimise the usage of my developer to stretch the number of rolls per bottle.

 

[payasam]

If you cut a roll into parts, you can develop each part in the same dilution but for different times. I'd suggest the same target, given the same exposure, on all the frames on the film. Once you find the right combination, stick to it.

 

[Chriscrawfordphoto]

Depends on the developer. With D-76 going from full strength to 1+1 requires only about a 1.5x factor, but Rodinal generally works out where the time and dilution are proportional to each other with most films.

 

[Freakscene]

Depends on the developer. With D-76 going from full strength to 1+1 requires only about a 1.5x factor, but Rodinal generally works out where the time and dilution are proportional to each other with most films.

Yes, some developers display interesting characteristics with dilution. Xtol uses a borate buffer solution as it's source of alkali. Mixed from the package the pH is 8.2; diluted to 1+3 the pH is nearer to 8.4-8.5. The increase in alkalinity emans that the developing agents will be more active even though they are more dilute. This explains some of the oddities about changing time for development with dilution of that developer.

Marty