Rolleiflex Automat: checking focus

[Joe Galloy]

I've read several threads on Photrio regarding focus calibration of Rolleiflexes, but I'm a bit perplexed and had a few questions for the knowledgeable folks here. I've got a klunker Automat K4/50 that takes decent photos stopped down, but I noticed some overall softness when focusing at distant objects. Test photos at minimum focusing distance, wide open, revealed a backfocus issue, maybe a couple of inches. The viewfinder and ground glass appear to be seated correctly.

1) When I align ground glass on the film rails (ground side to the rails), the focusing knob indicates 3' at a distance of exactly 36" from the target to the ground glass. Is this the correct way to measure the 3' distance?
2) Switching to the viewing lens, the target is out of focus. I need to turn the focusing knob to 2 2/3' for the target to appear sharp in the viewfinder. Does this make sense regarding the backfocusing in my photos?
3) When using another camera as a collimator to check infinity focus, the taking lens seems perfect. The viewing lens focuses to infinity somewhere before infinity (maybe 60' - can't recall exactly). I used this method: http://bit.ly/2Hiqoz4 Is this method reliable, and is it consistent with the discrepancy at short distances?

Thanks!

Joe

 

[shutterfinger]

1. yes
2. yes
3. never done it, not familiar with the procedure. May try it.
It sounds like your camera has the view screen infinity focus set to 100' or there about, a shortcut some use as they assume you will be using the camera stopped down and on subjects 15' or further away. Set infinity focus at the film plane then adjust the view screen to match.

 

[Joe Galloy]

1. yes
2. yes
3. never done it, not familiar with the procedure. May try it.
It sounds like your camera has the view screen infinity focus set to 100' or there about, a shortcut some use as they assume you will be using the camera stopped down and on subjects 15' or further away. Set infinity focus at the film plane then adjust the view screen to match.

OK, thanks for the info!

 

[Joe Galloy]

I decided to check the Rollei again, this time taping it down to a table and measuring everything precisely and with absolute attention to detail. I believe that I erred in my first time round when measuring and when checking taking vs. viewing lens.

1) I used distant jet contrails to focus on, and infinity focus appears to be identical on the ground glass at the film rails and in the viewfinder. It also appears to be the "optimal" focus compared with 'slightly less than infinity' but that can be hard to judge. Either way, it's very close.
2) At 3' on the focusing knob, the taking and viewing lens match.
3) The collimation method showed each lens to be in focus at infinity.

What I remain confused about is the distance markings on the focusing knob. If I measure 3' from the target to the film rails, the focusing knob is set at around 2-2/3'. However, if I move my target to precisely 3' from the lens board, both the viewfinder and ground glass on the film rails are in focus. This was also the same at other distances, such as 2-2/3' and 4'.

Anyone know what might be going on?

I'm going to shoot another test roll to check the focusing and see if the back focusing is still apparent.