Lens RF coupling

[uhoh7]

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Edward, do you try to compensate when shooting? I'm surprised how well the brain can learn and apply the correct "english" for a calibration that is consistently off on one lens. It's not as good as perfect, but it does seem to work. My worst of the good glass is the 135/3.4 which backfocuses very slightly. But if I do some practice i get pretty good at compensating.

 

I'm very lucky, as you are with the 85/2, in that most of my crucial high speed glass is spot on: CV 35 and 50 superspeeds are perfect and so is my 75 Lux, a lens notorious for cam issues.

 

Anyway my point: it used to bug me if a lens was off, but now I realize it's often easy to deal with.

[edwardkaraa]

Hi Charlie. Yes indeed, I do compensate and I'm actually very good at it I would prefer if the lens was perfectly calibrated but it's just slightly off. No big deal.

[Lenshacker]

I don't understand why adding thickness to the focusing cam will fix front-focus, but there are many thing I do not understand.

 

 

The OP had a problem with Back-Focus, increasing the thickness of the RF cam corrects back-focus, decreasing it will correct front-focus.

 

The preferred way of doing this is to change the thickness of the shim for lenses where the optical module sits on a shim that acts as a stand-off. On lenses with an indexed-cam, rotating the cam is preferred. The quick-and-dirty DIY fix that can be done without disassembling the lens- adding the copper tape.

 

 

Thank you Brian for the reply, do you recall what kind of 3M copper tape you use? I will try to get 1181. 

 

 

 

Question did you ever tackle front focusing issues? Are you using a diamond-file or take lenses apart? 

 

Stefan 

 

 

 

I use 1181 copper tape, just checked. 1 roll bought ~20 years ago or so. I have worked on some Zeiss and Jupiter lenses that were cemented and the optical module could not be removed from the focus mount, and the lens was front-focusing. This means the optics have to be moved closer to the image plane. One of them: carefully filed the RF cam down a little at a time.

 

On the Summarit 5cm F1.5: moved the rear module back 0.2mm. Again- trial and error, the Summarit is a "special case". The focal length is actually 51.1mm but it does not use an indexed cam, and does not use a shim. It's set that infinity focus is good at F1.5 and close-focus is good at F2.8. It seems to be a method to compensate for focus-shift, I've taken 5 apart. The "0.2mm" extra spacing between front/rear section, also slightly increases the focal length. It's a Hack, it worked! Turns out a Shim from a Jupiter-8 was just the right diameter. The Xenon: seems to be optimized for F1.5 close-up, at least mine is. The Hexagon shaped aperture helps minimize effects of focus shift.

 

- Brian

[Erik Gunst Lund]

On new lenses with floating elements it's not optimal IMHO to fiddle with tape on the cam.

The reason for the lens being off could be the position of the floating element...

 

A new lens like this one should not back or front focus unless it's calibrated as a compromise to 'adjust' for focus shift.

 

A new lens not focusing correct - send it back for calibration!