About backlash...

[pfogle]

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?

 

[ZorkiKat]

At least three of my J-9s would focus accurately if their helicals move from minimum focus towards the correct distance setting, and would consistently focus erratically if the focusing movement starts from the infinity mark. In one lens this is very consistent.

Jay

 

[Sparrow]

If I focus my m3 on a ground glass the rf will look to be spot on but I can then turn the focus a little the opposite way without the rf moving, the chap who services it says that's backlash and its caused by an accumulation of small amount of wear on the various parts of the rf mechanism, and would need completely replacing to correct it.

 

[zuikologist]

Sparow - that's an interesting point. It is not just the lens that needs to be taken into account, the whole body/lens system and the interaction of different components needs to be assessed. The Nokton and Rokkor could be interacting with the camera body differently.

 

[Sparrow]

The technician was able to correct the “slack” in the contemporary leica lenses, I haven’t noticed it in my modern lenses and R2 and 3 cameras

 

[jlw]

It's not just the lens. Any clearances in a system of mechanical linkages (such as a rangefinder coupling mechanism -- all the arms, levers, etc.) will cause backlash. When everything is pushing in one direction, the slack is taken up, but when the system reverses direction, the clearance allows each piece to move freely for a short distance without moving the pieces linked to it.

Generally a rangefinder coupling mechanism is spring-loaded to keep all the pieces pressed together in one direction regardless of which way they're moving, eliminating (or at least reducing) this backlash. But since the spring can't act with equal force on all parts of the mechanism, there are bound to be a few spots that have a certain amount of free movement when the direction is reversed.

The designer's job is to keep this within small enough limits that it doesn't impair focus accuracy. But as Sparrow says, long-term wear can open up the clearances enough that you can notice the effects.

 

[LCT]

Got focussing issues with the Rokkor 40/2 (CLE version) that i reported here if i remember well.
No problem with the Leica 40/2 though so i don't use the Rokkor any more i must say.

 

[pfogle]

Got focussing issues with the Rokkor 40/2 (CLE version) that i reported here if i remember well.
No problem with the Leica 40/2 though so i don't use the Rokkor any more i must say.

'though I do like the Rokkor (CLE version) it has the worst front focusing problem of my lenses.

Anyone know where to get it adjusted in UK? I don't want to mess with the camera, as the focus is spot on for the Nokton 50/1.5, and that's more critical than the 40mm.