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Focus scale and minimum focus distance


MrBan

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I tried three lenses set to minimum focus distance:

1) 28mm Elmarit latest version.

2) 35mm Summicron V4.

3) 50mm Summilux (11891, so fairly recent).

The focus distance scale readout on the 35mm Summicron and 50mm Summilux clearly show a bit less than 0.7 meters with the lens set to minimum focus distance. The 28mm Elmarit shows exactly 0.7 meters. 

I tested the 35mm Summicron at minimum focus distance, showing < 0.7m on the scale (maybe around 0.65m roughly, as an estimate) and the image is bang-on tack sharp. I measured the distance from the subject to the flange/mount on my M10 and it was 0.63m. So considering the sensor is a bit further back inside the body, the subject to sensor distance must be roughly 0.65m. Certainly less than 0.7 meters. So the measured focus distance correlates with what is shown on the lens as less than 0.7 meter and the image is perfectly in focus. Similarly, the 50 Summilux is dead accurate at minimum focus distance. All lenses show focus at exactly infinity on the scale when set to infinity.

I am curious if this is a common phenomena or "feature", being able to focus a little bit closer than 0.7 meter on some lenses? Clearly there is no fault with the lenses as they all focus perfectly.

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Posted (edited)

The story is that the marked minimum focus-scale distance is what Leica guarantees will focus correctly, on any camera back to the first cameras of the 1960s with 0.7m RFs (M2s?).

But yes, there is also often "unofficially" calibrated leeway below that, to about 6-point-something.

However, it is not unheard of for some lenses to go a bit below 0.7 - and the RF stops moving/doing anything useful. Just depends on lens model, era, and calibration (mostly pre-1990).

(Except of course for longer focal lengths like 90 (~.9 to ~1 meters) or 135 (~1.5 m) - and the newest-of-the-new lenses that now focus to 0.3m or similar with an EVF (e.g. 35mm APO-Summicron-M ASPH).)

But Leica has to end the lens scale engraving somewhere, so they use the nominal round-number 0.7m, rather than going to the expense of adding "  0.65  " (which may not for certain be calibrated) to the focus ring.

Edited by adan
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In many cases the engraved numbers are close approximations...just keep that in mind. As mentioned previously, several things can affect variations from the engraved numbers...the sensor distance or film plane as well as the calibration of the camera body. With EVF the engraved numbers are irrelevant, but with just the rangefinder a number of different issues can come into play. I as well as students of mine, always ran a "dress rehearsal" with each new lens and/or body, just so we knew exactly when and where variances might be encountered. A sacrificial roll of film was a cheap price to pay.

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3 hours ago, adan said:

The story is that the marked minimum focus-scale distance is what Leica guarantees will focus correctly, on any camera back to the first cameras of the 1960s with 0.7m RFs (M2s?).

But yes, there is also often "unofficially" calibrated leeway below that, to about 6-point-something.

However, it is not unheard of for some lenses to go a bit below 0.7 - and the RF stops moving/doing anything useful. Just depends on lens model, era, and calibration (mostly pre-1990).

(Except of course for longer focal lengths like 90 (~.9 to ~1 meters) or 135 (~1.5 m) - and the newest-of-the-new lenses that now focus to 0.3m or similar with an EVF (e.g. 35mm APO-Summicron-M ASPH).)

But Leica has to end the lens scale engraving somewhere, so they use the nominal round-number 0.7m, rather than going to the expense of adding "  0.65  " (which may not for certain be calibrated) to the focus ring.

Very interesting. Thanks for the explanation. Makes sense. I guess it is nice that we can sometimes, depending on the lens, focus a touch closer if we like. 

2 hours ago, spydrxx said:

In many cases the engraved numbers are close approximations...just keep that in mind. As mentioned previously, several things can affect variations from the engraved numbers...the sensor distance or film plane as well as the calibration of the camera body. With EVF the engraved numbers are irrelevant, but with just the rangefinder a number of different issues can come into play. I as well as students of mine, always ran a "dress rehearsal" with each new lens and/or body, just so we knew exactly when and where variances might be encountered. A sacrificial roll of film was a cheap price to pay.

Thank you. 

2 hours ago, Al Brown said:

Engraved focus scale is just an approximation, just like most depth of field scales or focus peaking. When it gets critical use visual WYSIWYG approach.

Thanks. WSYIWYG is always a good option. It is either in focus or it isnt :-). 

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5 hours ago, adan said:

(Except of course for longer focal lengths like 90 (~.9 to ~1 meters) or 135 (~1.5 m) - and the newest-of-the-new lenses that now focus to 0.3m or similar with an EVF (e.g. 35mm APO-Summicron-M ASPH).)

Should have noted (because someone will) that there have been other min. focus oddities in the Leica M history:

The dual-range 50 Summicron when its "goggles" are installed (?m) - the M3 goggled version of the original 35mm f/1.4 (0.65m?) - the 21mm Super-Angulons (which could go to 0.4m, but lost the RF capability below 6-point-something) - the original 15mm Zeiss Hologon "für M" - the very first batch of 21mm Elmarits (1980) which could do the same.

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It was more important for lens distance markings to be accurate in the Barnack days when Leitz still sold models for general use that had VF but not RF (like the Standard). Users would check distance (separate RF or other means) and set lens scale to same. That's why the model II was called "autofocus" - as when the camera's RF was aligned the lens was in focus.

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