[LargeFormat] A Note on Rangefinder Cameras
Richard Knoppow
dickburk at ix.netcom.com
Thu Jul 7 18:57:38 EDT 2005
Sometimes rangefinder cameras seem to produce less than
sharp pictures even after careful set up of the rangefinder.
I've found one cause of this is focus shift of the lens.
Most lenses have a little focus shift but some have enough
to make the range finder inacurate under some circumstances.
Focus shift is the change in position for best focus as the
f/stop is changed. It is caused by zonal spherical
aberration. Spherical aberration causes light transmitted
through the edge of a lens to focus at a different distance
than light coming through near the center. Generally the
light from the edge focuses closer to the lens.
Usually rangefinders are set up with the lens wide open
since it has the least depth of field/focus then. However,
when the lens is set to moderate stops the position of best
focus may be enough away from this to cause some slight
blurring. Its easy to test. Focus the camera carefully with
the lens wide open. Then stop it down to some mid way stop.
For typical f/4.5 press camera lenses a stop of around f/8
to f/11 will get rid of most of the spherical but not make
the lens difficult to focus. If there is focus shift you
will find that the lens must be moved slightly away from the
ground glass to get the best sharpness. After optimising the
focus at the smaller stop open the lens up again. If there
is much focus shift the image will be noticably blurry. The
distance does not seem to matter.
There is no cure for this but it can be mitagated on
cameras like Speed/Crown Graphics, which have easily
accessible infinity adjustments, by adjusting the RF to read
correctly with the lens at the stop you are going to use. If
you know you are going to shoot wide open set the infinity
stop so that the ground glass and rangefinder agree at that
stop. If you know you are going to be shooting stopped down
adjust the infinity adjustment for around f/8. At that stop
it should be sharp from around f/6.3 to the smallest stops.
I find this works pretty well. I still set up the RF with
the lens wide open.
The infinity adjustment I am talking about is the
eccentric cam on the focus track that contacts the actuating
arm of the rangefinder. You can adjust it with a penny or
dime. The change is very slight.
Kodak Ektars have very little, but some, focus shift,
older Tessars have more.
Other cameras may not be so easy to adjust, particularly
those using older versions of the Kalart rangefinder. I
think also the top rangefinder is not so easy to adjust.
Note that this same effect takes place in SLR cameras
when they are focused with the lens wide open and then
stopped down. I've noticed the effect in my Super-D Graflex
altough its small. For sharpest focus the camera should be
focused with the lens stopped down to around f/8 to f/11. At
smaller stops its too hard to see optimum focus and there
will not be any additional focus shift. The same thing
certainly happens with 35mm and 120 SLR's and even with TLR
cameras but the shift for the shorter FL lenses is much
smaller plus the Planar type lenses usually found on 35mm
and some 120 SLR's have very little zonal spherical.
I rather think that this focus shift is the real reason
people have trouble with roll film adaptors on press cameas.
The small blur from the focus shift might not be noticable
for 4x5 but will be more evident if one is shooting 6x7 or
something similar on the same camera because the images will
be magnified more in printing.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com
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