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Re: [Rollei] Incident v. reflective
- Subject: Re: [Rollei] Incident v. reflective
- From: Richard Knoppow <dickburk >
- Date: Sat, 13 Apr 2002 13:15:47 -0700
- References: <B8DDC83E.8287%bob
At 09:12 PM 04/13/2002 +0200, you wrote:
>>and that it is not
>> > for the Gossen?
>>I was hired by Kodak. I wrote the instructions for the Kodak Gray Card.
>>The standard of 12.5% for meters has been in place since the 1950s.
>Bob, as my both Gossen measure the same value for reflecting light on my
>"Non Kodak grey card" and for incident light, does this mean that the grey
>card has 12.5 % or does it mean that the Gossen are calibrated to 18 %?
>By the way, for incident metering, I thought, too, that the domes of the
>meters are transparent to 18 %. Or should they also on 12.5 %
>That makes my crazy. I gues I go to the next shop and by the Kodak Card
>with your instructions.
18% gray cards are 18% but meters may be calibrated to produce an average
gray of 12.5%
A white card can also be used. For copy work a white card is often used
but the indicated exposure multiplied by five times. That's approximately
the same as 18%, given that the white card is not 100% reflective.
A big problem with gray cards is specular reflection. They are assumed to
be lambertian surfaces but are far from it. So the angle of incidence of
the light makes a substantial difference. There is an ISO standard for
meters but I don't have a copy.
The ISO has several standards for film speed. There are different ones
for black and white and color, for negative and reversal, for still and
motion picture film. The one for black and white negative still film has no
safety factor in it. That can result in underexposure under some
conditions. Also, the ISO speed is valid only for the sensitometic
conditions specified in the standard. If you develop to some other contrast
than that in the standard, or use a different developer, the speed rating
is no longer valid. The current ISO standard does not specify a developer,
any can be used, but it must be specified along with the film speed.
Kodak somewhere has a bulletin describing the difference between ISO
speed and Exposure Index.
Los Angeles, CA, USA