Crown Graphic Top RF adjust: master cam or not?

[nonuniform]

Okay okay last time. I know everyone swears by setting the infinity stops.

One of my Crown Graphics with Top RF has weird behavior - either I can have the rangefinder focus accurately from 6-15 feet, or I have it focus accurately from 15-infinity. Checked against the GG.

So, I've done the infinity stop adjustment, and that seems to get me the 15-infinity focus range.

I'm guessing this is a job for the master cam method of making a mess of my rangefinder?

ps - I have the camera set to focus from 6-15ft, since that's how I use it anyhow, but it's bugging me!

[Peter York]

As I understand it some of these rangefinders have inifinity set at "hard stops," where the focus rail is moved back as far as it can go. Others have infinity set when the focus rails are moved a bit foreward (per the manual). Have you checked to see if you have hard stops or not?

[Corran]

I made a thread about this a little while ago that had a ton of info. Basically it sounds like either you have the wrong cam or the one specific calibration you need a master cam for is out of adjustment (I hope not).

I'm sure much more knowledgeable people will respond than me

[rjbuzzclick]

I had the same issue with my top RF SG when I bought it. I couldn't get the RF and GG to agree. Eventually I pulled out the cam to check that it was the correct one for the lens, and it was. When I put the cam back in, it went in with a very solid SNAP sound, it seemed much more solid than when I took it out. After that, it was easy to align, and everything agrees-RF, GG, and focus scale. At least in mine, I think the cam wasn't seated properly. YMMV.

Also, for the record, when set to infinity, the track is not at the hard (all the way in) stop on my camera.

[Bill_1856]

What is a "Master" cam for the TRF Crown? The only ones I know about are the interchangable ones.

[Ivan J. Eberle]

Does it have the balls-in-a-tube actuator like the Super Graphic did? If so, perhaps some of the balls went missing?

[nonuniform]

Do you mean, check and see where infinity focus is set - with rails all the way back, or with the rails moved forward slightly? Hmm. Good point.

As I understand it some of these rangefinders have inifinity set at "hard stops," where the focus rail is moved back as far as it can go. Others have infinity set when the focus rails are moved a bit foreward (per the manual). Have you checked to see if you have hard stops or not?

[nonuniform]

I made a thread about this a little while ago that had a ton of info. Basically it sounds like either you have the wrong cam or the one specific calibration you need a master cam for is out of adjustment (I hope not).

I'm sure much more knowledgeable people will respond than me

I totally read your thread! I just wanted to try a logical approach to figuring this out, rather than randomly pulling things apart (tm - how I fixed my Jeep in the desert).

Anyhow, thanks for the ideas. I'm going to try the simpler ones first, and then check for the correct cam, etc.

[Neal Chaves]

I have cut many of my own cams for TRF Crowns, and Linhofs before that. I have never heard of a "master cam" for TRF Crown. I do know that most factory TRF cams are not as accurate as they could be. The cam shape for most lenses will be a curve, which indicates that focusing is not linear.

When hand-cutting a cam, a number of focus targets are set up at distances from the closest practical to "infinity", and the curve is developed and smoothed out. When doing a careful job, it may come to two or three final passes of a very fine file at the end.

In answer to your specific question about the cam not being accurate throughout its range, you can try the following. At the point where the split image does not indicate the accurate focus on the GG, it the image is short of coincidence, the cam curve is low. If the image goes past coincidence, the cam curve is high. Get in there with a Sharpie pen and mark the location of the follower on the cam.

If the cam is high at that point, you can simply make a few passes with a fine file in that area and then check focus again. If the cam is low at any point in the curve, you can "stretch" it a bit by tapping with a hammer and punch on an anvil in that area and then finish with a fine file. If you ever see any old Linhof cams, you may find some with these "stretch marks" from the factory.

Attached is a photo of some of my TRF cams, cut from hobby shop brass stock. 250, 150, 90 and 65mm.

[Robert Meier]

Neal -- you should make these for all of us CG users. Say, 150 and 210 cams. Maybe a 90.