How to compensate for shooting at half the ISO rating

dickbromberg · Feb 25, 2008

I am trying to emphasize motion and want to expose for longer periods of time.
I'm using Tmax 100 film and and shooting at about fstop 150.

I received a suggestion that I should compute the exposure time as if the film were rated at ISO 50 instead of ISO 100.

How would I compensate for the fact that I will be using the wrong ISO rating?

Thanks for any help.

Steve Smith · Feb 25, 2008

Shooting at half the ISO rating means giving twice as much exposure. The compensation for this is to reduce development by 20% to 25% of whatever is normal.

However, check the reciprocity chart for that film as if the exposure is long enough, extra exposure may be needed anyway

Steve.

Ian Grant · Feb 25, 2008

Most people shoot Tmax100 at 50 - 64 ISO anyway, so it's no problem, I wouldn't cut development by bery much.

Ian

Pinholemaster · Feb 25, 2008

There is no 'wrong' ISO, just different than the manufacturer's suggestion.

Going from 100 to 50 means you are doubling the amount of light that would strike the film. Overexposure usually results in a flatter contrast range because the highlights are all pushed to the top of the contrast curve. By under developing, you are bringing down your highlights towards a more acceptable area of the contrast curve, while at the same time not affecting the shadow detail.

Basically you are overcoming the reciprocity failure of Tmax 100 @ f/150.

Lee L · Feb 25, 2008

Pinholemaster said:
Overexposure usually results in a flatter contrast range because the highlights are all pushed to the top of the contrast curve. By under developing, you are bringing down your highlights towards a more acceptable area of the contrast curve, while at the same time not affecting the shadow detail.

TMX has a characteristic straight line response with little to no shoulder, so pushing the highlights up the curve in order to lower contrast in the highlights doesn't work with TMX as it does with traditional films. You can cut the contrast with reduced development, but you'll be lowering overall contrast, not just the highlights. Many people do that with TMX, rating it at EI 50-64 and reducing development. This lowers the slope of the contrast curve and makes it easier to print without highlight density getting as far out of hand compared to more traditional films.

Lee

rkmiec · Feb 25, 2008

neutral density filter,available in many levels and no worrying about developing times,just adds time to you exposure but reciprocity may still be involved if the exposure is long enough.they are really useful things to have.good luck.

CBG · Feb 26, 2008

If you can use ND filters as mentioned before, or cheat and use a polarizer as ND, then you can use your T-Max 100 at the speed you want, rather than overexposing. You will benefit by not needing to fiddle with developing times.

C

Christopher Walrath · Feb 27, 2008

Curious question, Dick. You pinholing the TMX or what? I make matchbox pinholes and use Efke 25 to give me longer exposure times. Slower films, not much adjustment needed other than accounting for reciprocity failure. Just an idea. Last year, the first one I made was with TMY 135 roll film. EV13 was 1/2 second at about f/100. Kinda hard to control. With the 25 I can put EV16 at one whole second. Maybe more depending on my mood.

Rich Ullsmith · Feb 28, 2008

Tmax 100 is great for this. No reciprocity correction up to about seven minutes.

Sino · Mar 7, 2008

What if I want to do the opposite; expose a roll of 120 film with box speed of 100 ISO at 160 ISO and develop longer? Is it 20-25% longer development time I should be going for? I'll probably be trying TMAX 100 too.

Excuse my ignorance, I'm asking because Ilford Sporti's recommended film speed for bright [summer] days is 160 ASA whilst I do not know of any film that comes in that speed today. I will of course try a roll this weekend and develop for recommended times to see what I'll come up with, just thought I should ask in order to learn what I should expect/do.

-Sino.

rwyoung · Mar 7, 2008

Kodak and Fuji sell color films rated at 160.

Otherwise that isn't much of an underexposure, only 2/3 stop assuming your Sporti is spot on. Good chance it is running a bit slow so Tmax100 would probably be just fine. One way to find out for sure, isn't there? Or just use a 200 speed film which would be 1/3 over exposed (again, assuming everything else is perfect).

Sino · Mar 7, 2008

I'm not sure if there only exists one way of finding out, but I will give it a try this weekend to see what I'll come up with. I really doubt that it is working correctly, I had to lube almost everything in it -- the shutter wasn't working and it was making some pretty scary noise. Everything seems to be working now, from what I see and hear...

I guess what I am asking here is, since it only has two shooting modes with no aperture or shutter speed mentioned anywhere [Ilford only mentions using a 160 ISO film at summer and a 400 ISO at winter, and marks 'sunny' and 'cloudy/flash' aperture positions on the camera], can I play with the film development times in order to compensate for that and if yes, is there a base to start upon? Or will I hit some sort of limit and lose in quality by doing so? I don't expect miracle images out of the Sporti, but I have no experience in pushing/pulling, I always used and developed at box speed until now. Newbie questions!

Cheers,
-Sino.

DannL · Mar 7, 2008

dickbromberg said:
I am trying to emphasize motion and want to expose for longer periods of time.
I'm using Tmax 100 film and and shooting at about fstop 150.

I received a suggestion that I should compute the exposure time as if the film were rated at ISO 50 instead of ISO 100.

How would I compensate for the fact that I will be using the wrong ISO rating?

Thanks for any help.

To wrap my brain around this question, I would like to know more specifics.

1. Why f/150? and why not f/22 or f/37?
2. How much emphasis of motion do you want?
a. Do you want slightly blurred moving objects?
b. Do you want moving objects to dissappear?
3. Can you shoot shutter priority with your camera?
4. Are you shooting at night or in bright light conditions?

Bobby Ironsights · Mar 15, 2008

Oh, well then, what you want is to PUSH! Shoot at ISO 160, That's easy, just increase development time by about 25% per stop of increase. So, for .6 of a stop increase 14% of development time....very little indeed.

How to compensate for shooting at half the ISO rating

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