 Timothy A. Chorlton |
Long Exposure Compensation with Ilford B&W Hi everyone I'm about to go on a photographic holiday to Orkney (nice) and I have planned to take some very long exposure shots (100 ASA film at f22 with 2x 2 stop ND filters, that kind of thing) with an old Rolliecord. However, whilst looking at Ilford's website, I have spotted that with B&W film (Ilford Delta 100 is my personal choice) long exposures need more time than my meter would suggest. Has anyone any experience of this or can I just ignore it and carry on as normal? Any tips or hints please!
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John A. Lind |
Yes, I have had experience with this (but with other films, not with Ilford Delta). The effect is called "reciprocity failure." All films exhibit this with long exposure times. The shutter speed at which it is encountered depends on the film. For Ilford Delta it affects exposures longer than 1/2 second. This can be as short as 1/10th second with Kodachrome 64 and as long as 128 seconds with Provia 100F (RDP III). [Kodachrome 64 is not the only film requiring compensation beyond 1/10th second.] The reciprocity failure characteristics and how to compensate for it is something to be aware of when making long timed exposures. With color films there can be a color shift as well, because the individual color layers will behave differently in reciprocity failure. I recommend copying Ilford's chart and taking it with you. The term comes from film generally having a linear relationship between subject brightness and exposure required. In other words, one EV of less subject brightness requires one EV more exposure. When reciprocity failure kicks in with long exposure times, this relationship changes and one EV change in brightness is more than one EV change in shutter speed. How much change is tested by the film maker and results in the table or chart you see in the data sheet for long exposure times. -- John
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