June 2001
President's
Message
Portrait of Will Klein
Photo FAQs
Photo FAQs
Eric Kissa
Below are some of the questions
asked at the "Experts" session, answered now in more detail.
Q.:My
Canon 300mm lens does not focus to infinity in the autofocus mode, although
I can turn the focusing ring manually to the infinity mark. What is wrong?
A.:Perhaps
nothing, if the image of a distant object is sharp in the autofocus mode.
Some lenses focus to infinity before the distance scale is at the infinity
mark. This is not a fault of the lens but a deliberate design to allow
for thermal expansion and the tolerances of the mount. The actual infinity
position of the focusing ring is the same for autofocus and for manual
focus.
However, if autofocusing does not make a sharp image, then the most likely
causes are: (a) an excessive curvature of the lens which may mislead the
autofocusing system and necessitate manual focusing. This condition is
rare for telelenses, however. (b) The autofocus system may be the culprit.
If on the same camera body other lenses behave normally or if the suspect
lens has the same problem on another AF body, the defect is likely within
the lens. Electronic problems, such as malfunctioning of the CPU, result
usually in a gross failure. Small errors are more likely caused by a mechanical
misalignment. The accuracy of autofocus can be tested by photographing
a line on a ruler at a 45 degree angle. The autofocus test results can
save time and money if the lens has to go to the camera hospital.
Q.:At the last slide night a flower slide had been modified by the Kodalith process which made parts of the flower black. What is the Kodalith process and how is it done?
A.:Kodalith
is an old process which uses a high-contrast orthochromatic film (Kodalith
Ortho Film 6556 or Ortho 2556). The film is not sensitive to red and can
be handled in a dark room illuminated with a red safelight. A slide is
removed from its mount and placed on the Kodalith film, emulsion to emulsion.
If a printing frame is not available, a glass plate can be used to hold
the film flat. The assembly is exposed to light and the film is developed
to obtain a Kodalith negative. Because the film is not sensitive to red,
a red flower will become black. The Kodalith negative can be used to make
a positive (by contact printing from Kodalith to Kodalith) or mounted
onto the original slide in a registered position to create a "derivative".
In our Club Val Luedeke is a derivation specialist and he can give you
the details of this process.
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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.