wiltw
Subscriber
Dylan, the problem today is that the internet with its many advices and even rages easily can puzzle a beginner.
Looking back to the old days, I would say there are these reasons for setting at a camera a different film speed:
-) At cameras that do not yield a true exposure correction switch the film speed dial is the means to do so instead.
One might do so
-) when the scene luminosity is different from the "average grey" (for example when white skin being dominant in the metering field, then a about +1 stop correction is needed, the film speed setting thus reduced by 1 stop, or with people in snow, then even more)
-) when using filters (to keep a grey object still rendered at same density)
-) when homeprocessing b&w films, as often developers other than the standard D-76 in effect will change film speed
-) when using long stored films
Other reasons to rate color film differently from its official speed rating (ISO)...
- Using Velvia (ISO 50) as a specific example that was rated differently by most photographers...ISO 50 rating was generally felt to yield a bit too dense (dark) a photo. So most photographers came to rate Velvia with EI 40 rather than its official rating, so more exposure (due to the lower rated speed) yields a brighter (less dense) photo. While most photographers also know that more exposure = less saturated colors in transparencies, Velvia already had very highly saturated colors, so giving more exposure simply made the colors slightly less saturated and a bit more 'real'.
- Using most color negative films (like Fuji NPS, ISO 160) it is known that underexposing it will cause the shadow areas to become 'muddy'. So many pros might rate NPS at EI 100 rather than ISO 160, so there is less tendency to underexpose in the shadow areas, and become muddy. Since most color neg film is very tolerant of overexposure, rating it lower does little harm.
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