Hi Everyone -
This is rather long, and somewhat of a cross-post (I put it in rec.photo.darkroo m, too). On the other hand, it's of general interest, and the test speaks speci fically to 4x5 film.
A while back, I posted a question (in rec.photo.darkroom) about the capacity of Ilfosol. I wanted to know how much film can you develop in x amount of Ilfosol? The data is not on their website or on the bottles. I wrote to Ilford, and th ey never answered, so I decided to do an experiment.
I mixed up 1 liter of developer at 1:9 (100 ml of concentrate out of the bottle) . I then exposed twelve pieces of 4x5 (HP5) under the enlarger through a step t ablet. I ran the sheets through the developer (in 5x7 trays) three at a time, k eeping the temperature constant (20C) with a water bath. I punched holes in the negatives to keep the batches straight. I gave all the films the same agitation .
After the films were dry, I measured the densities of the first twelve steps wit h a densitometer (my new eBay toy?..whoo hoo!). The first two batches were iden tical. The third batch showed a consistent drop in density in all measured step s, and the fourth batch even more so. For instance, a step that measured 0.75 i n the first batch was down to 0.64 in the fourth. This 15% drop in density from batch 1 to batch 4 was about the same in all measured steps. This corresponds to a drop in contrast and slightly lower film speed.
So now I know that 100ml of Ilfosol concentrate will develop between six and nin e sheets of 4x5 (120 - 180 square inches of film), or about 1.2 to 1.8 square in ches of film per ml of concentrate (love those English/Metric ratios). One roll of 120 has about 68 square inches of surface, so two rolls is just over the 120 square inch boundary, probably in the safety zone. It's interesting to note th at Ilford lists times for a 1:14 dilution, which would get you 67 ml of develope r for two rolls of 120, a ratio that was PAST the point of exhaustion in my expe riment. Maybe the increased development time makes up for it. Maybe someone el se wants to do that experiment!
So why is this important? Surely no one develops that much film in that little developer. But then I thought about all those JOBO processors out there merrily spinning away. It doesn't take much liquid to cover the film (about 600ml as I recall), and those two reels will hold (you guessed it), twelve sheets. If you 've never done the math to decide if you were over-extending your developer, it might be worth a few minutes of your time. I think Kodak, unlike Ilford, publis hes capacity data, so you might not have to actually do this experiment.
Please, this isn't a knock on JOBO...they're great gadgets. But it is possible to use them improperly, and you can certainly make the same error with trays or tanks.. And obviously, not all developers have the same capacity as Ilfosol. Y our mileage may vary, as they say.
Maybe you're thinking, "I've always developed x sheets of film in y ml of develo per, and I've never had a problem". Well, unless you've done an experiment like this one, you wouldn't know you had a problem. You might not know your real fi lm speed and contrast, because you have nothing to compare it to. And, getting back to JOBO, I understand that exhausted developer can be a source of streaking .
If you're curious about your own situation, you don't need a densitometer or a s tep tablet to repeat this experiment. Find a negative with a good range of dens ities. Put it on top of a piece of film, and place both films on a piece of bla ck paper (to cut down on reflection) and expose it in your enlarger. I was usin g an exposure of 2 sec at f32 with the head 32 inches over the baseboard. Devel op normally, in batches, and see if the negatives vary from first to last. The d ifference I quoted earlier was obvious when the negatives were held side by side . A difference big enough to see makes a big difference in your print. No densi tometer needed!
Thanks for reading a long post. I'd love to hear from someone else who has done similar tests. But now, I'm going to go shoot some pictures, and completely f orget about densitometers and data points.
Peace and good light.