Posted 10 November 2005 - 09:54 AM
Say its 320T so i just rate it to 125 for D with the 85 filter on...
or rate it the same : because -2/3 to -1 stop darker for the filter cutting out light and +1 stop brighter for the ASA decrease... evens out?
Am i far off here?
Posted 10 November 2005 - 10:54 AM
Posted 10 November 2005 - 10:56 AM
Posted 10 November 2005 - 12:01 PM
Edited by asparaco, 10 November 2005 - 12:02 PM.
Posted 10 November 2005 - 05:45 PM
ASA (that is actually ISO normed, by now) is only for still picture photography.
Motion picture film manufacturers only give an Exposure Index (EI)...
It's you, operator, to decide how you want to rate your film. One usually makes "key-light tests" as to determine this personal rating.
The EI is a good basis for you to compute in your light meter if you want.
If you have a 85 filter in front of your camera, loaded with tungsten stock, you can set it to the rating that will compensate for the filter. Usually, manufacturers consider that 85 filter cuts 2/3 of a stop, so that you can set your meter on this basis. It is this value that is written on the can.
How is this rating calculated ?
Since rating are in a geometrical progression, a 2/3 compensation corresponds to a factor of 2^2/3 that is approached by the value of 1.5625
So that the rating of 500 then becomes 500/1.5625 = 320
Posted 10 November 2005 - 06:01 PM
It's still 500 ASA whether it is in 3200K light or 5500K light.
Sorry, David, I don't really agree with this...
First, as I said, the rating is only the one that one determines, there is no rule for determine this.
If you take the ASA rating, for still photography, it's based on the HD
curve of the film. If it is a tungsten film, this curve will be done
with tungsten light.
Since the film has a spectral density (it's curve depends on the light
source), the curve one would obtain with a daylight source would be
diffrent, so that the calcution of the asa rating would give a
Posted 10 November 2005 - 07:41 PM
If you have a 500 EI filmstock and you put a filter that cuts 2/3's of a stop, then for convenience's sake you can set your meter to 320 EI to compensate. The daylight rating that Kodak puts on their film cans for tungsten stock has NOTHING WHATSOEVER to do with the film having a different exposure response to 5500K versus 3200K -- it's only the rating you use to compensate for the light loss from the 85B filter. Period. THE FILM DOES NOT HAVE A DIFFERENT EXPOSURE INDEX FOR DIFFERENT COLOR TEMPERATURES.
Obviously a film balanced for 3200K will not expose the three layers evenly if shot in 5500K light without the correction filter. But that doesn't mean it needs a different EI rating in 5500K light versus 3200K light.
Posted 10 November 2005 - 08:57 PM
to take a reading from a spot meter with the 85 on it and set the difference as an
EV compensation on a seperate setting on the meter. That way you expose for exactly
for what you want to see rather than counting on the calibrated EI that's on the can.