Petri Racer / Petri 7 Focus/Rangefinder Calibration

[Teemō1]

I own a Petri Racer and it wasn't until I shot a roll through it that I realised the focus needed calibration. It uses the same rangefinder system as the Petri 7\s. Frustratingly, I've spent many many hours trying to calibrate the rangefinder via direct secondary ("front") mirror adjustment as well as the follow-focus arm adjustment to no success. While adjusting for infinity greater than 200m is very easy by bending the frame that the mirror sits on, adjusting it to any accuracy beyond 2m is impossible without throwing the accuracy out towards the infinity side. I am completely unable to get accuracy anywhere near the 0.8m minimum focus distance on the lens. The best I managed was down to 1.2m at the expense of 3m to infinity being misaligned.

Being my first rangefinder, it honestly has me wondering whether the system can ever be properly or closely calibrated across the entire focus range and from what I understand, even in a Leica, the answer is no?

Inside I thought it might be possible to calibrate it accurately if I could move the "Negative Lens Assembly" (Part 88 - spring-loaded, connected to the lens coupling arm) further towards the user which would correct the closer distances and then I could use a suitable linear adjustment on the front mirror to correct for infinity but because the spring is used to push the follow-arm against the lens barrel to follow it and to move it any further, I would need to adjust the helicoid of the lens which also isn't anymore possible because the focus-knob would then contact the barrel. Besides, as far as I could tell using scotch tape on the film plane, the lens focuses at the correct distances according to the scale that is printed on the lens.
Also, the repair manual for the 7s suggests that to correct rangefinder close focus, you only need to bend the mirror assembly for infinity. Then it later states that to adjust the near-distance focus, I should place a ground glass on the film plane and check it at infinity (200m) and then at the 3m mark... I don't have a ground glass that I could place on the film plane and 3m is hardly close distance anyway so it sounds like that wouldn't work.

Should I just give up and go back to my SLR?
:bang::bang::bang:

 

[johnnyrod]

Normally there is a screw to adjust the rangefinder. For example, on my Konica S, there is a small plug by the viewfinder you can remove to access it (unless you already have to top off the camera):
Viewfinder/rangefinder by John Rodriguez, on Flickr
As you say, the lens focus vs. scale is good so leave it be.

 

[johnf04]

The Petri 7/7s doesn't have a screw for adjustment.



For a ground glass, I use a piece cut from a CD case, roughened with abrasive cleaner:



I don't think these Petri cameras are designed for precise adjustment of the rangefinder.

 

[charjohncarter]

I'll look at my 7s tomorrow and see if I see anything.

 

[Teemō1]

The Petri 7/7s doesn't have a screw for adjustment.

Actually, under the black paper light baffle, you see the Negative Lens Assembly behind the thick negative glass. This is where the Petri 7 and Petri Racer start to differ, but the principle seems the same. The cam follower support attached to the same plate as the small negative lens assembly has an "adjustment screw" that while isn't mentioned in the manual, or appears to do anything significant to the cam follower support besides rotating it around the spring loaded end that meets the cam follower, it does actually adjust the rate of change at closer distances with a lesser effect on the farther distances.

I don't think these Petri cameras are designed for precise adjustment of the rangefinder.

I agree, because I believe what I discovered above is definitely not an intentional part of the calibration design. Having said that, even the actual cam follower (the arm being pushed against the lens by the cam follower support) could be considered to have an adjustment as there is leeway built in with the screw to the chassis.

I think the problem may actually be with the lens contrary to what I thought, since the focus scale notch always lined up on the edge of the infinity marker which doesn't seem like much but it doesn't take much to throw the rangefinder out of alignment. I don't have anymore time to work on it but I suppose the only answer could be the lens. The CD ground glass is a good idea but while reading about alternatives, some say that it needs to be extremely thin to be accurate! Also may need to acquire a loupe to check focus, or perhaps I can use a macro lens.