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Changing densities with development time

luxikon

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Germany
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Multi Format
I exposed two APX 100 (135) at 80 ASA. On each film I exposed test frames at Zone I and Zone VIII. The first film I developed at 17°C in 1+50 Rodinal for 21 minutes (rotary development) and achieved d 0.03 at Zone I and d 1.42 at Zone VIII. To get better results I developed the second film as the first but increased development time to 23 minutes.This time I achieved d 0.2 at Zone I but d 1.39 at Zone VIII where I expected a higher density compared to the first film too.
Can anybody explain this to me? Did I miss the basics of densitometry?
 
It's hard to comment on your densitometry from afar , but it would be unlikely for that small a change in development to affect the very low values, going in either direction, in my experience. I suspect exposure is different between the two rolls. Maybe a slight, and difficult to notice, change in lighting. You don't mention the shooting conditions (inside, outside, etc), but this is a change in light small enough that it might not be noticed when shooting. You also don't mention how many exposures on each roll of each value, or how much time lapsed between the rolls.
You might try repeating test, and on both rolls expose all frames, alternating throughout the roll between ZI and ZVIII (to get a good average of your shutter accuracy), then process both rolls together, placing one of them into the tank 2 minutes after the other one, especially since you now have a close sense of the right amount of time.
It's been recommended, when doing a test like this, to choose a sunny cloudless day, but in open shade, a blank wall, way out of focus (keeps the neg uniform, for reliable readings with the densitometer). This has worked for me in the past.
 
On your first attempt it looks to me like your Zone I is right at the Base+Fog level. Your second attempt has a significantly higher Zone I level, so I would start by trying to explain that. I think before anyone could help, you will need to tell us more about your exposure, development and densitometry techniques. Unless you have each of these under reasonable control your final readings will be all over the board. Your 17C temperature is lower than I am used to seeing. Is there a particular reason for that?

Denis K
 
Yes, there was a reasonable change in lighting. The first test exposures were made from a white wall in plain sunshine, the second from a white wall on a ship in shade because there was none in bright light. I thought the meter would read them both to 18% gray.
I think I've to repeat the test the way George proposes.

@ George
Yes, as I wrote I meant .2 at ZI.

@ Denis
I'm developing APX 100 at 17°C because that produces finer grain as at 20°C and I'm gaining a little more speed. Normally I even go down to 16°C but in summer my JOBO CPP2 doesn't cool down so deep.
 
Your threshold density is almost completely the result of exposure, not development. If it is too low, you need to give the film more exposure, other things being equal.
 

**************8
You should be photographing an 18% grey card; not a white wall.
 

I think you have an exposure problem. Zone I is usually seen as being at 0.1 over filmbase + base fog. It should *not* be the same as your film base -- it should definitely show some development -- that is, it should show some density above the density of clear filmbase. If your exposure won't give you a good Zone I density then your Zone VIII results from that same exposure are meaningless. That's why the books say you have to find your EI before you find your "N" development time.
 
Densitometry can be tricky. Comparisons need identically (or nearly so) exposed spots, and even then your results can vary from one part of the spot to another. I'm not sure what happened, but I would guess exposure changes accounted for the differences. The change in developing time is less than 10 percent, and it would be less than a +1 change with just about anything. In general, increasing development time will increase contrast, not Zone I density. It will usually also increase fog. But it takes 15 to 20 percent added development to shift the contrast by roughly one paper grade.
 
luxikon, Is this legacy APX-100 or is it Rollei Retro 100 film you are using. I noticed that you measured a density of 0.03 on your first roll. That's real low; hell that's crystal clear. I was just wondering in case I ever needed a 35mm film stock with that clear of a base. I'll bet it would do well as a reversal film; for example as a film for for the dr5 process.

Denis K
 
@ Denis

It's legacy APX 100. I zeroed the densitometer after measuring base+fog. Then I measured 0.03 for ZI.
 
luxikon said:
@ Denis

It's legacy APX 100. I zeroed the densitometer after measuring base+fog. Then I measured 0.03 for ZI.
Click to expand...
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You can reasonably expect that your exposure meter is calibrated to measure an approximate 18% reflectance. Are you taking your exposure reading off such a surface?
 
No, until now I didn't. I only used any uniform (near) white background because I thought the meter would give me a standardized reading of 18% gray. But I'll take a gray card for my next tests if that helps.
 
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