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I'm using a Nikkor 18-105mm kit lens and it doesn't have a distance scale on its focus ring, is it possible to create your own distance scale by knowing the 0 and the infinity focusing points on the ring, measuring the space between them and putting the ticks from 0 to infinity? What function do those ticks follow (it's clearly non-linear)? Would the method be accurate or should you measure each tick by focusing an object at various known distances? Would the distance have to be measured from the body? Hope someone finds all these questions interesting! Thanks to all in advance

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Focus distance is measured to the focal plane mark, a circle with a line through it, on body, near rear edge of the top LCD. This marks the location of the digital sensor or film.

I have to say, "forget it". It could work, but only at one zoom value. Long story, but you will have extreme problems marking it, because zooming changes things considerably. On lenses even with a distance scale, the same focused distance (on very many lenses) will most likely be reported as a different lens rotation with rather different numbers at each zoom (different for the same focused distance). So you would need a new scale for every zoom value.

The reported focused distance is already in the Exif data (manufacturers section, but you may need a pretty good and recent Exif viewer to see it - ExifTool.exe is a good one). And that Exif focused distance is very often blatantly wrong, because many lenses usually report different distances with different zooms at the same distance.

My own complaint about it (a pet peeve) is how the Nikon D-lens distance is inexcusably wrong, sometimes terribly wrong, but is still allowed to seriously affect default flash mode exposure of TTL BL direct flash. We need a new menu option to refuse to allow that.

http://www.scantips.com/lights/ttlbl-d.html (half way down page)

Third party flashes seem able to sidestep the TTL BL error (no head tilt switch)

http://www.scantips.com/lights/ttlbl-cords.html

but most Nikon flashes suffer from it. It seems unimaginable. But only TTL BL direct flash is affected... which is default mode, but not bounce, not non-balanced TTL flash mode, etc.

  • you wrote It could work, but only at one zoom value. I though that some lenses were able to keep focus at the same distance (same object) while zooming. Are they doing it by rotating the focus ring ? – Olivier Sep 11 '15 at 6:13
  • Possibly some few can, but better check yours before putting much work into trying to calibrate one. My bet is no. That feature is more common in expensive movie lenses (called parfocal), but most photo lens are vari-focal instead of zoom. Vari-focal is not much problem, still camera AF focuses before each shot anyway. But the distance scale becomes a crapshoot. And worse, Nikon default TTL BL direct flash mode uses it anyway, which is worse than a crapshoot. – WayneF Sep 11 '15 at 13:44
  • My canon 17-55 2.8 seems to keep focus even while zooming. I don't have the tools to be sure of it, but to the naked eye it seems OK. – Olivier Sep 11 '15 at 16:42
  • Measure a few distances accurately, maybe 2, 4 and 6 meters, and take focused pictures at both 17 and at 55 mm, and maybe an intermediate too, and look at the Exif distance. I have $1800 Nikon lenses that do agree better at different zooms, but agree about zoom or not, few of the numbers will actually be accurate. For the inexpensive zoom lenses, totally forget it. :) It's no big deal when auto focus takes care of it. – WayneF Sep 11 '15 at 19:49
  • The real problem is the lens distance number can affect direct flash exposure. Direct flash is often overexposed because inverse square law provides dark backgrounds which influences the flash metering to overexpose to help it. So lens distance is watched as a safety check - if the guide number says metering is overexposed at the lens distance, the flash is backed off to prevent it. Sounds good, but but when the lens distance is so simply wrong, this is a big problem. – WayneF Sep 11 '15 at 19:49

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