 Corinne Walker |
Pushing and Pulling - The Yin and Yang of Film Hi - can you please shed some light on "Pushing and Pulling" film. I shot a roll of Ilford PanF Plus 50 at ISO 100 which to my understanding means the film is over exposed. How do I determine the number of f-stops the film has been over exposed to? And... What's the general rule of thumb on compensating developing times for over (or under) exposed film? I checked out Ilford's site for compensating over exposed film and it recommends double the amount of time (using Ilford's chemicals). I'm confused - I thought developing time would decrease. How do I calculate developing time using a non Ilford developer such as Agfa Rodinal?
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doug Nelson |
If you shot a 50-speed film at 100, you underexposed it one stop. Had you shot it at 200, you'd have underexposed it two stops. Had you shot it at 25, you'd have overexposed it one stop. See the relationship? BTW, "f-stop" refers to using the aperture adjustment. Plain "stops" means a standard measure of light adjustment that doubles or halves the light coming in, whether you use the aperture, shutter, exposure compensation dial, or ISO setting to make the adjustment. I'd take Ilford's recommendation to the bank here. When you underexpose, as you did, you compensate by over-developing. When you over-expose, you compensate by under-developing. Refer to Ilford's recommendation. There are pros here who can advise you on the Rodinal, as that's outta my league. Keep doing what you're doing, ride the learning curve, and take satisfaction that you're doing real photography.
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Jeff S. Kennedy |
Doug's absolutely right. Think of it this way. You were shooting PanF (good film btw) which is an ISO 50 film but you told your camera you were shooting an ISO 100 film. ISO 100 is twice as sensitive (fast) as ISO 50 so your meter figured it needed half the light since it was twice as sensitive. Therefore you underexposed the film by one stop. A stop is just a unit of light. When you add a stop you double the light. When you subtract a stop you cut the light in half.
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Corinne Walker |
Thanks Doug, thanks Jeff. I understand now - thanks for the clarification and the speedy response!
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Jeffrey Berkes |
Hey guys IF had 100 speed film in my camera and the iso is set to 400. what does that do to the film whilw its being shot. Doesnt it need more time to get an image or what?
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Andrew Laverghetta |
Kinda late but... When you shoot it at a ISO you're telling the camera that the film is more sensitive than it really is. That doesn't do anything to the film but underexpose it. Yes, since it's only 100 it should get the amount of exposure that 100 film should but you intentionally are telling the camera to forget about the 100 and you are underexposing it 2 stops. This is usually done either for effect, or when you just don't have the right speed of film and need to get a faster shutter speed for indoor photography or such.
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