Meeting 19

Last critique of prints

Reciprocity Failure
Photographing at night or in very dim light presents challenges exposure-wise. Your light meter - whether built-in or hand-held - will likely suggest exposures that are insufficient. Film sensitivity drops in dim light, and should be compensated for. Reciprocity failure often appears around 1 second and longer shutter speeds. Every film is somewhat different in this regard, and some normally slow films like Ilford Delta 100 are 2x more sensitive to light at very long exposures than Tri-X.

Large format film users are especially impacted by reciprocity failure of the film. They must use small apertures to get sufficient depth of field, and that could extend exposures into the reciprocity failure range even during a daytime exposure.

Manufacturers often provide correction charts for reciprocity failure for their films.

Here is a chart that shows reciprocity failure corrections for various films

Because reciprocity failure affects dim parts of a scene more than brightly lit areas, the resulting dim-light pictures are often higher in contrast than normal. During a low-light exposure, the brighter highlights may continue to record on film normally. However the dimmer light from shadow areas does not record proportionally. Shadows end up thinner. Highlights, being more like a normal daylight exposure, expose normally. This increased difference between dense highlights and thin shadows creates more contrast than normal in the print.

Night exposures are often contrasty to begin with, and adding the effects of reciprocity failure only compound the high-contrast problem faced by photographers.

Ideally, photographers should reduce development a bit to keep the highlights in the proper range if the shadows have been properly exposed after taking reciprocity failure corrections into account.

Potpurri - answer any remaining questions, wrap up of loose ends. Go over final exam topics.