Sept. 2001
President's
Message
Portrait of Bob Coffey
Photo FAQs
Photo FAQs
Eric Kissa
Q.:I have
bought a new camera (Nikon F100) and two new lenses. One third of the
slides, exposed under identical conditions, appear to be darker. How can
I find the cause of the problem, is it the camera or the lens?
A.:First
of all, you have to determine that the exposure variance is real. Shoot
a slide film with a camera in the 1/3 stop auto bracketing mode. For the
Nikon F100 the sequence is normal - minus 1/3 - plus 1/3. When everything
goes well, each slide will differ from its neighbors by a uniform 1/3
stop difference. (For the N90S with MF-26 the sequence is minus 1/3 -
normal -plus 1/3. For most cameras the smallest auto bracketing interval
is half a stop, which makes the test less sensitive.)
The reproducibility of exposure
depends on the (a) exposure meter, (b) shutter, or (c) the lens, mainly
by the stopping down action of its diaphragm. If the slides are not properly
exposed, consider the following tests. Mount the camera onto a tripod
and aim it to a clear sky or to a gray card on a concrete surface (sidewalk,
driveway). Use an effective lens hood to prevent flare. Take several exposure
readings. If the readings are reproducible, shoot three frames in the
program mode. To test the shutter, place the camera in the shutter preferred
(S) mode and shoot several frames while varying the shutter speed. To
test the diaphragm of the lens, have the camera in the aperture preferred
(A) mode. Shoot some frames with the lens at its largest aperture (open
diaphragm) and some frames at a small aperture (diaphragm stopping down).
To complete the test, shoot some frames with the camera in the program
mode, aperture preferred, shutter preferred and manual mode. Theoretically,
all the frames should be equally exposed. If you are still worried, repeat
the test with the gray card in a shade. When examining slides on a light
box, keep in mind that the illumination of the light box is not perfectly
uniform. Compare slides adjacent to each other and reverse their order
to verify the observed exposure differences, if any.
Modern shutters are highly accurate. Modern photocells do not have a significant
memory effect, like the old selenium cells had. The weakest link is usually
the lens - the exposure reading is made with the lens fully open and the
exposure is made after the diaphragm has stopped down, presumably to the
correct aperture.
Q.:Last week I went to a zoo and most of my shots have bars. How do I make the bars disappear when photographing monkeys, lions etc. ?
A.:Position
the camera as close to the barrier as safely possible. Open the lens to
its widest aperture. If the animal is sufficiently far from the barrier,
the depth of field will make the bars disappear. Obviously, a wide maximum
aperture of the lens (smaller f/number, longer focal length) is helpful.
Q.:How good is the new Fuji Provia 400F film?
A.:Very
good indeed, much better than the old Provia or Sensia 400. Undoubtedly
the best ISO400 transparency film. I compared the Provia 400F and the
PRO Ektachrome E200. The test was made using two Nikon F100 cameras equipped
with Sigma APO 70-300mm f/4.0-5.6 zoom lenses. No filters. One camera
was loaded with Provia 400F autobracketed at EI 400, 500, and 640. The
other camera was loaded with Pro Ektachrome E200, autobracketed at EI
200, 250 and 320. The subject matter was identical for both cameras and
photographed almost at the same time. The transparencies were examined
on a light table with a Kenko achromatic 9x loupe and a 14x Hastings triplet.
Although 400F is grainier than E200, the difference is less noticeable
than the RMS values (13 versus 12) indicate. The resolution of 400F is
high (specs. are 135 lines/mm with a high contrast target). Color rendition
is similar to Provia 100F. The real noticeable differences are the higher
contrast and lower color saturation of 400F. Even when 400F is underexposed
at EI 640, where it is getting too contrasty, the color saturation does
not match that of E200 at EI 250. This was evident also when the slides
were projected.
The question remains, can 400F replace E200? If a high image quality is
the main consideration, E200 or a ISO 100 film is preferable. However,
when a higher speed is needed, 400F is an outstanding choice. The lower
cost consumer version, the new Sensia 400, is said to be similar.
--------------------------------------
Send questions concerning photographic
equipment (cameras, lenses, accessories, filters), photographic techniques
(other than digital), and film, as well as information on international
photographic exhibitions, to: ekissa @aol.com.