PDA

View Full Version : Reciprocity?


Mike Meal
8th January 2009, 12:16 PM
I've been using Foma 100 sheet film while I've been learning LF and I've noticed how alot of people have mentioned how terrible it is for reciprocity. But if I WANT a long exposure is it really a bad thing?
If I was using the same speed Ilford film to get the same actual shutter speed then my options would be either stopping down or using ND filters both of which would give you a slight loss of picture quality.
If your metered=actual shutter speeds are correct and you shorten development times accordingly then why is reciprocity such a bad thing?

B&W Neil
8th January 2009, 12:35 PM
I always understood reciprocity falier to be only a concern with long exposurs on colour films (changes the colours) and that with mono films it could be ignored.

Neil.

Barry
8th January 2009, 01:13 PM
Reciprocity failure occurs with all films. FP4+ metered for 10 secs needs around 30 secs for correct exposure. Fuji Acros has the best reciprocity characteristics of any mono film.

I think the usual problem is forgetting to take this into account and ending up with a very underexposed neg. :(

Niall Bell
8th January 2009, 01:13 PM
B+W film suffers indeed from reciprocity failure. For example, each of the technical support sheets published by Ilford for their film says that no adjustments to exposure are necessary where metered values range from 1/2 to 1/1000th second. At exposures slower or faster than these, additional exposure is required. The additional time can be extensive, for example, a metered exposure of 30 seconds requires to be adjusted to over 150 seconds, and it's an exponential curve (greater exposure needed as metered value increases).

When shooting in low light (not infrequent in daytime in NE Scotland) I frequently have to increase the exposure. Strangely, I've never had to shoot at faster than 1/1000 second.

Niall

Dave miller
8th January 2009, 01:27 PM
And a 30 second exposure can easily become 5 minutes, as I found out last weekend when making some test exposures in cloud inhibited sunlight; and the negs were still a stop under exposed.

lee l
8th January 2009, 02:50 PM
Howard Bond did extensive tests on five films in 2003, published in the US magazine Photo Techniques. He found that his tests didn't agree with manufacturers' published reciprocity correction data (especially the "generic" curves that Kodak and Ilford provide to cover all B&W films), and that with many of the modern films there was no necessity for adjustments to development to compensate for increased contrast. I don't have permission to post the article here, but I made up a table of his data and will try to attach it in .pdf format. At minimum the Tmax 400 film has been changed since these tests were run.

There is a line for indicated exposures out to 240 seconds with 1/3 stop increments, and then each film has an adjusted exposure time. Note that the adjusted times become "minutes:seconds" format with longer exposures.

Lee

Mike Meal
8th January 2009, 03:30 PM
Thanks lee, interesting stuff. I was suprised when I first saw that Ilfords reciprocity correction charts applied to all their film!
I have found that Foma films dont need any development correction funnily enough.

Argentum
8th January 2009, 03:42 PM
I've been using Foma 100 sheet film while I've been learning LF and I've noticed how alot of people have mentioned how terrible it is for reciprocity. But if I WANT a long exposure is it really a bad thing?
If I was using the same speed Ilford film to get the same actual shutter speed then my options would be either stopping down or using ND filters both of which would give you a slight loss of picture quality.
If your metered=actual shutter speeds are correct and you shorten development times accordingly then why is reciprocity such a bad thing?

It has been frequently stated by some people that reciprocity increases contrast. However, understanding this requires careful evaluation of the subject which in some cases may be true and in most is not.
Take the following image for example:
106
I exposed this image for the highlights which were the church spires metered with a spot meter and corrected accordingly. If I had metered the shadows, I would have required a much longer exposure after reciprocity correction. Different parts of the subject are into different degrees of reciprocity and this is especially true where the subject is high contrast to start with. i.e. some parts of the image require reciprocity correction and others don't. Depends on the subject and picking the correct part of the subject to meter from is critical. If I had metered shadows and corrected for that, then the highlights would have been blown out and you could be forgiven for thinking that reciprocity had increased contrast. But the fact is that for nightime shots at least, it is my view that metering highlights and correcting for reciprocity of the highlights is a far better way to go about it. Strangely the shadows still have plenty detail in them. The neg shows branches of trees clearly but that is not what the main subject is and the main subject is what I wanted exposed correctly.
Next there are some films which have short straightline line curves before they shoulder off quickly. Delta 100 is like that. Foma may be for all I know. If you are metering for shadows and foma has a short curve, then it may well be considered as not good for subjects falling into reciprocity. But if you metered for a zone 6, 7, 8 or 9 my bet is that it would be fine. Metering deep deep shadows fools you because you don't really know how black they really are and trying to put them on zone 1 2 or 3 is often to high. The result is that you apply too much reciprocity correction and get blown highlights.
Far better to meter highlights and let the shadows go where they go. That way you will end up with great mid to high tones set against really dark shadow tones. I have found Ilford provided reciprocity correction figures work very well when metering this way.

the image here was given my normal development.

Richard Gould
8th January 2009, 05:30 PM
I've used foma 100 and 400, the 400 for still life indoors lit by a small window with time down to 4 to 5 seconds and I haqve never had problems or extented dev times,Richard