Film speed and development tests

[Stephen Samuels]

I may have got the wrong end of a stick here but i'm under the impression that received wisdom says you cannot sensibly conduct film speed and development tests across formats - that is, testing say HP5 on 35mm would not yield the same results as HP5 on 10x8. This seems to be borne out by the different developing times listed in things like the digitaltruth massive development chart.

The only reason i can think of for this would be the difference in agitation used in developing different formats because, surely, the emulsion is the same? But does it really make that much difference?

I'm proceeding slowly into 10x8 and am concerned about doing tests on film that is over £4 per sheet. I know everyone will say that using film for tests will save me money in the long run through reduced wastage but I'd still rather test a reel of 35mm/120 and use the results for the big stuff.

Is there a right end to this stick?

Thanks.

 

[degruyl]

I routinely do sheet film tests in 4x5 and use those numbers for larger sizes. It is all cut from the same sheet... But I do use the same developing methods with approximately the same ratio of film / developer. (1 gallon tanks with six full hangers, ideally).

On the other hand, 35mm, 120 and 220 are on a different base than sheet film, which might affect developing time.

 

[JRieke]

From recent experience yes it does make a huge difference. I couldn't find development times for the 4x5 sheet film I was using in R09 one shot but there was the equivalent in 35mm. My negatives came out with only a slight hint of an image. I added 10% but then it was over developed. I'm going to try 5% more time on the next negative. Luckily I anticipated this problem and set out with several film holders and took multiple shots with the same exposure of the same scene to test development times and dilutions.

 

[degruyl]

Particularly with Rodinal (R-09 One shot, etc) I have found the published times to be complete fiction. Did those times work well for you for 35mm?

 

[Stephen Samuels]

Thanks for the replies, guys, looks like i need to do it format by format.

I'd still like to understand the science of this phenomenon though, if anyone out there knows it?

 

[ic-racer]

Development time tests may be wildly off depending on how you process the film.
In terms of speed, many times film is cut from the same stuff. Even different batches are 'trimmed' to a specific ISO ( what I understand from posts per PE).

 

[ic-racer]

Thanks for the replies, guys, looks like i need to do it format by format.

I'd still like to understand the science of this phenomenon though, if anyone out there knows it?

I can say that with T-max developer, depending on how much you put in the tank or drum, the developer activity changes as it gets exhausted. So, processing a step wedge exposure on a single strip of 8 inches in 140cc winds up with much greater activity than processing a whole 36 exposure roll. The difference is pretty big in my darkroom. Like the high 0.8s for the single strip and around .65 for a step wedge exposed on the leader of a 36 exposure roll filled with pictures.


In terms of 'sensible testing' I'm pretty comfortable with my results that I'll just cut film to 6" strips and process it in 140cc, knowing the contrast index is going to be higher.
In real life terms this means I am not putting a step wedge on a single 8x10 T-max 400 sheet and processing it together with 4 other sheets in an expert drum each time I want to run a test, that is too expensive (I only do that once). If I want to re-test what I do is cut a 35mm strip of film from an 8x10 sheet and process it like a little strip of 35mm. I look for a contrast index around 0.8 to 0.9, knowing the actual CI is going to be less in the Expert drum.