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Topic: photos (kind of long, sorry) (3 msgs / 118 lines)
1) From: Gary Zimmerman
Lubos wrote:
<Snip>
I'm going to jump in here, even though this might be considered slightly 
off-topic (though we're discussing how to compare roasting notes, so I 
guess it's fair game).
I'm not a professional photographer but I play one on... no wait a minute, 
I meant to say I've taken a few photo courses.  In one of them, the teacher 
gave us an excellent assignment to demonstrate exposure and how film 
camera's and exposure meters "think".
When an exposure meter reads "okay to shoot", it's really saying that it 
will expose whatever it's metering as 18% gray (assuming black and white 
film).  (At that time, it was still all based on 18% gray so that's what 
I'm using.)
The exercise was a simple one:  Go out on a bright, sunny day, and 
photograph a white cloth and a black cloth (or towel or shirt).  Rumple the 
fabric a little bit, make sure it's in the sun, and have it fill most of 
the photo viewfinder.  Adjust the camera settings until the meter reads 
"okay" and shoot.  Use black and white film.
After we developed and printed the pictures, it was difficult to tell the 
pics of the white fabric from those of the black fabric!  They all looked 
gray... 18% gray, to be specific, because that's what standard exposure 
meters are set to think is "okay".
We bracketed the exposures as part of the assignment, so we had also taken 
"overexposed" and "underexposed" versions of each shot.
This taught us that, to get a white thing to "look white" in a photo, we 
needed to slightly "overexpose" beyond what the meter said for that white 
area.  Maybe one stop.  Just the opposite for black or dark things like 
dark complexions or dark coffee beans.
If you meter just the beans, and shoot, you'll get 18% gray beans, no 
matter how dark they really are.  If you stick 18% gray card on the beans 
and meter for THAT, then remove the card, the meter will tell you you are 
underexposing the beans.  Ignore it and shoot!  You want the beans 
"underexposed" to get them to look darker than 18% gray, just as they do in 
real life.
I believe the "white balance" in video cameras and digital cameras isn't 
for exposure so much as it's for color balance.  In that case, you take a 
pure white card and hold it in front of the camera under the same light you 
intend to use when you shoot. Then you hit the white balance button.  If 
you don't do this, whites (and other colors) will drift, and you'll get 
unwanted tints that are not true to life, even if the exposure is 
correct.  It's like shooting with normal film under fluorescent lights - 
things turn greenish, even if they are properly exposed.
As for Cathy getting pics that show her roasts as more uneven than she 
really sees them, it might have something to do with the "latitude" of the 
film (or CCD?).  I know some types of film are more sensitive to 
differences in light and dark in the same picture.  I don't know if the 
same is true of digital cameras.
Some films (those with "wide latitude") will give you good pictures even if 
there are very light and very dark areas in the same picture, while others 
will not be able to capture both.  Light areas will be overexposed or dark 
areas will be underexposed.  (For film you generally want to overexpose, 
because "if the information (light) isn't in the picture in the first 
place, there's no way to make it appear later."  Darkroom manipulations can 
mitigate the overexposure, while bringing out the details in the darker areas.)
Hope this helps in some small way, and that I haven't offended any "real" 
photographers out there or given any inaccurate information.
-- garyZ
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2) From: Photogal1966
In a message dated 8/19/2002 9:41:37 PM Eastern Daylight Time, 
garyzim writes:
<Snip>
Gary, you pretty much nailed it, IMO. This is not a photography forum, so I 
will not elaborate further.
Excellent explanation.
Andrea

3) From: Ryuji Suzuki -- JF7WEX
From: Gary Zimmerman 
Subject: Re: +photos (kind of long, sorry)
Date: Mon, 19 Aug 2002 18:38:56 -0700
<Snip>
If the fabric is white and dark, you would probably need more like
2-1/2 stops or more to compensate for them. (Of course, depending on
how bright/dark they are, but about 4 stops below gray card reading
would be black black, almost nothing registered on film. Of course,
this depends a bit on the choice of film and developer, and how you
rate your film speed.)
This effect is also actively used to create high key or low key effects.
<Snip>
It's just a matter of metering light falling on the subject or light
reflected by the subject. If you measure incident light you need not
worry about the reflectance of the subject in determining the baseline
exposure, while if you are using reflected meter you must consider
that factor in order to get the correct exposure on the film. Gray
card is just an inexpensive way to turn a reflected meter into an
incident meter.
<Snip>
You must be doing more than taking a couple of classes... :-)
--
Ryuji Suzuki
"I can't believe I'm here.
People always say that I'm a long way from normal."
(Bob Dylan, Normal, Illinois, 13 February 1999)
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