Scott Ackerman on wed 16 jul 03
As a professional photographer I encounter the "depth of field" =
confusion
among other photographers all too frequently. Depth of field is strictly =
a
function of image magnification all other things being equal. To =
illustrate
if you were to use a 35mm lens (assuming a 35mm film format) and got =
close
enough to fill the frame with your subject and used a particular =
aperture to
adequately cover that depth of field say f16 and then you were to switch =
to
a 200mm and moved back far enough to fill the frame you would see that =
f16
would still be the appropriate aperture. Of course the 35mm lens would =
give
you a greater angle of coverage and include a lot more background than =
the
200mm lens would. This is because it is camera to subject distance that
determines perspective, not the focal length of the lens (this is =
another
widely misunderstood subject). So pick the camera to subject distance =
that
gives you the perspective you want use the appropriate lens to give you =
the
desired reproduction size on film and then use the appropriate aperture =
to
get your desired result. Of course if you are using a view camera, or
tilt/shift lens the rules change, but I assume that you are not using =
such
critters.
As to reciprocity failure in order to get any substantial shift on a =
modern
emulsion (assuming color reversal film - ie. Slide film) you would have =
to
shoot at a shutter speed shorter than about 1/10,000th of a second or a
longer one of about 4-5 seconds (note I said substantial). The only way =
to
get a shutter speed shorter that 1/250th of a second on any modern 35mm
camera is to use electronic flash which can be as short as 1/50,000th of =
a
second. I know some of your are going to say "my camera goes to =
1/2,000th of
a second", to which I will respond "go investigate the mechanics of a =
focal
plane shutter and your question will be answered".=20
The only reason I can see that you might be getting a blue shift is that =
you
are shooting outdoors in open shade. Your subject is now being =
illumninated
by open sky which is anywhere from 8,000 to 12,000 degrees kelvin (can =
you
say blue light?).
I guess in short the question is "Is there any way I can get predictable =
and
repeatable results without having to control lighting?" to which I would
answer "Is there any way I can get predictable colors in my glaze =
without
control over the ingredients?". Like all of us, you are going to have a =
make
a compromise.
"Every improvement in the standard of work men do is followed swiftly =
and
inevitably by an improvement in the men who do it" - William Morris
=20
Scott Ackerman
1133 Riverside
Suite B
Fort Collins, CO 80524
970-231-9035
<<<<light being used to illuminate the subject-- OR--B. Reciprocity
failure--which simply means the film does not respond to very long =
exposures
or very short exposures as predicted. The information sheet that comes =
with
the film will tell you to NOT expose the film longer than some specific
shutter speed or any shorter than some specific speed.
I get the Provia out of the "cooler" with nada - no box, no instructions =
- I
can see the need to get the specs on any film I use.
<<<<f/stop--the smaller the number, the larger the opening, and the smaller =
the
amount of depth of field. Camera to subject distance also controls depth =
of
field--as camera-to-subject distance decreases so does depth of field.
So, we can get better depth of field using a 105mm or a 135mm lens than =
the
"usual" 50mm lens?
Mel mentioned telephoto adapters that he puts on the front of his lens, =
I
assume to go from 55mm to 110mm. I have a 2X teleconverter that goes
between the lens and the camera body. A book I have mentions =
teleconverters
by saying: ".......the depth of field with the teleconverter attached,
though, will be only 40% of that effective f/stop."
Do you know if that means that there is no gain in depth field by using =
a
teleconverter lens?
Thanks,
Bob Bruch
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