Developer capacity - an experiment

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.

Kevin

Kevin, congratulations on getting your new densitometer! Now, let me muck up your conclusions about how much film you can develop with a given developer batch. You might be surprised at how much effect the film exposure can have on this. In reality, the issue is not so much how many squared units of film were processed, but rather how much total silver was developed. The developing agent depletion and (generally) by-products released will be in proportion to silver developed. If the overall film density increases by 0.30, this is roughly double the silver being developed.

My point is that one can not legitimately specify developing capacity by units of film, alone. The best the manufacturer can do (short of recommending process control strips to monitor the processing) is to consider typical shooting conditions and then make very conservative recommendations based on that.

By the way, when you exposed your sensi wedges (under the enlarger), did you mask off the surrounding film (so that it was clear) or fully expose it (so that it was "black")? This should have a large effect on the your conclusions. The real world analogues to this might be night sky photos vs snow scenes. Just a little food for thought.

Hi Bill -

Thanks for the post. Yeah, I thought about that....clearly, a negative that's been lying around in the sun chews through more chemistry than one that's barely exposed. I tried to pick an exposure that gave me some blank steps as well as some very dark ones. But, I have no idea if my test negs correspond to a typical exposure.

Hmmm....maybe I need to get a grant from the NSF so I can study this for the next few years....

Tell them you have a densitomer and they will be very impressed and give you a blank check.

Kevin, your reply puzzles me.

If it was meant to be humorous, let me know and I'll be happy to laugh along with you (really!).

On the other hand, if you thought the experiment was flawed in some way, or didn't prove anything useful, educate me and I'll do a better job next time.

I'm hoping to uncover your intent while avoiding a flame war.

- Kevin (the other one).

Aaahhhhh................Pyro!

Kevin, I believe your experiment is slightly flawed. A 15% drop in overall density isn't the same as testing the developer to exhaustion. All developers exhibit a drop in activity with use, it's the nature of the beast, and with re-useable developers like D76 it's recommended to extend the development time by a given percentage per area of film processed.If you'd developed all 12 sheets together in a freshly made litre of Ilfosol, there'd have been far less than a 15% drop in density, because initially all the film would have been immersed in fresh developer. As a rough estimate, I reckon they'd only suffer a 15/12, or 1.25% drop in density - negligible.

The real way to estimate capacity is to find the point where a normal contrast can't be reached, no matter how much the development is extended. In other words, where Dmax starts to be curtailed for a given mid-tone density.The data you've got so far is useful, in that it tells us that the development time should be extended slightly when developing more than 6 sheets of film in one litre of developer, but it doesn't actually give us the limiting capacity.Kodak's stated capacity of a litre of D76, for instance, is 320 sq ins of film, but this can be increased to 640 sq ins if the development time is extended by 10%, while Ilford claim that 800 sq ins of film can be processed in a litre of ID-11.

Kevin, I'm not sure if you're interested in this or not, but kodak now has alot of their porcess manuals (Z-manuals) available online as pdf files. In the past, you had to purchase these and they weren't exactly cheap. The B&W Process manual, Z-133E, is one of them. here's a link:

http://www.kodak.com/global/en/service/library/chemicals/process.shtml

You might want to get some control strips, if you're into this type of process control. If you get into running plots on your process, you should be able to gauge the activity of the developer pretty easily.