[LargeFormat] A Note on Rangefinder Cameras

[Richard Knoppow]

    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