pfogle
Well-known
there are three sources of backlash in the RF camera (for those unfamiliar with the term, it means that a lens focused so that the correct focus point is approached from the distance will end up focused in a different place than when the correct distance is approached from the near point of the scale).
One is machanical slack in the lens. Another is mechanical slack in the cams, levers etc that move the mirror in the rangefinder. The third is human, and variable - do you tend to slightly overshoot, or undershoot the focus point?
I suspected my 40mm of front focusing, so I tried a simple test.
Two lenses, focused on the same point (there's a picture of the ruler below):
focus distance on lens scale is 1.1m
each number is the average from 3 separate frames (negative numbers are front focus distance in inches) rounded to the nearest 0.5 inch.
Nokton 50mm @ f1.5 from dist: 0
Nokton 50mm @ f1.5 from near: -1
Rokkor 40mm @ f2 from dist: -4
Rokkor 40mm @ f2 from near: -3
Now, the wierd thing is that you'd expect the backlash from near to always leave you on the near side, and vice versa, as the slack in the system means the lens is always lagging behind the driver (you!); so why is it the other way round with the Rokkor??? Approaching from the front gives LESS front focus not more...
The only thing I can think of is that my personal habit is to overshoot the focus point slightly, and this has biased the results. Any help?
One is machanical slack in the lens. Another is mechanical slack in the cams, levers etc that move the mirror in the rangefinder. The third is human, and variable - do you tend to slightly overshoot, or undershoot the focus point?
I suspected my 40mm of front focusing, so I tried a simple test.
Two lenses, focused on the same point (there's a picture of the ruler below):
focus distance on lens scale is 1.1m
each number is the average from 3 separate frames (negative numbers are front focus distance in inches) rounded to the nearest 0.5 inch.
Nokton 50mm @ f1.5 from dist: 0
Nokton 50mm @ f1.5 from near: -1
Rokkor 40mm @ f2 from dist: -4
Rokkor 40mm @ f2 from near: -3
Now, the wierd thing is that you'd expect the backlash from near to always leave you on the near side, and vice versa, as the slack in the system means the lens is always lagging behind the driver (you!); so why is it the other way round with the Rokkor??? Approaching from the front gives LESS front focus not more...
The only thing I can think of is that my personal habit is to overshoot the focus point slightly, and this has biased the results. Any help?