Quote:
Originally Posted by dancqu  Apart from all that I'm now struggling to understand
why precisely filter factors are necessary. Any clues?
I think it may have to do with an averaging of negative
densities across the spectrum. Filtering out some portion
of the spectrum does not affect film sensitivity yet most
filters require an increase in exposure. Dan |
Dancqu - It's not a loss of sensitivity on the part of the film. It's that less total light gets to the film. It's probably not very helpful to think of it in terms of some averaging. It's just a loss, and unless you want to reduce total exposure you must compensate for the loss.
A filter factor is designed to get you back in the ballpark when using an external - handheld - meter or the like, in fairly normal conditions. It tells you roughly what fraction of total light outside the camera is getting to the film. A 2X factor says half the light is absorbed by the filter.
Filter factors can be a conundrum. One you start thinking about them a lot, life can get more complex than you probably want.
Why?
1. Because many of us have in camera meters - life gets confusing. Were an in-camera meter truly matched in response to the film, a factor would be un-needed for most general uses. Best to simple meter externally when using strong filters.
2. Because not all subjects are "normal". If you photograph a bright red object in a bright red room - with only one blue detail you just want to turn black by using a #29, your filter factor may screw you up. The dominant red image will pass largely unimpeded, much as if no filter were present. The blue item will go black. If you bump up the exposure several stops, you will probably overexpose.
Best,
C