Re: Eclair Cameras: Behind lens filter - Use - Question

[Wade Ramsey <wramsey@bju.edu>]

This is true, however, I've shot thousands of feet of film on our ACL
using a gel behind the lens for daylight, no filter for tungsten, and
have never had a focus problem due to lack of the gel filter in the
slot.  Reflex focusing automatically gives you the true focus, what you
see is what you get, since the focus screen is also behind the gel.  

If you scale focus, the gel is so thin--.0045" by my measure-- that 1/3
of its thickness still isn't a problem.  Short lenses would be the most
affected.  If my calculations are correct, the lack of a gel filter in
the slot when using an 8mm lens would bring the focus back from infinity
to 3591 feet, about 1095 meters.  And the closer you focus, the less the
change.

Its depth of field will cover that easily, even wide open.  (The
hyperfocal distance of an 8mm lens at f/2 is about 2 meters.)


Wade K. Ramsey, DP
Dept. of Cinema & Video Production
Bob Jones University
Greenville, SC 29614

> > > hansfilm@algonet.se 7/16/03 2:50:44 AM >>>
Filter behind the lens ??

If the camera is collimated for using filter behind the lens you shall
always use a filter. If you don t need a correction filter use a
clear or UV
filter. 

"The Filmmaker's Handbook," authors Ascher & Pincus note that "when a
filter
is used behind the lens, it refracts light and moves the focal plane
back
about 1/3 the thickness of the gel." They go on to say that if a
camera/lens is set up for a behind-the-lens-filter then one must
ALWAYS have a filter (not just a filter holder)in place, even if it's
just a clear 1A type.

Hans Hansson, FSF, Sweden

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