CV 12mm M Mount

[ho_co]

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I don't understand what you mean when you say "a lens hits infinity at 3 meters."

 

Normally, a lens is designed for best performance at 'infinity,' which is usually defined as, what, 1000 times focal length?

 

But optical design is one thing, focus another. A lens hits 3 meters at 3 meters. A rangefinder-coupled lens should "hit infinity" on the rangefinder at a distance a lot further than 30 meters.

 

IIRC, with a standard linear helicoid (the way most focusing mounts are made), each doubling of the focused distance will require half the mount rotation needed for the previous doubling. That is, focusing from 0.5 m to 1 m will take a certain rotation of the mount. Going from 1 m to 2 m will take half that rotation, from 2 m to 4 m half again, etc.

[tecumseh]

The infinity symbol is engraved on the lens next to three metres which I understand to mean everything three metres and beyond will be in focus. Regardless of the scientific explanation I assume lens manufacturers engrave ∞ on their lenses to make it easier for us to scale focus. Although it may well be technically incorrect to say "infinity at three metres" I assume that Joe Average with little knowledge of the complex world of optics will be able to quickly comprehend the concept.

 

Is this correct "even things that are real close are in focus so you don't need to worry so much about a coupled rangefinder"?

[ho_co]

tec, I differ completely with your first paragraph.

 

As for the second, there have been a lot of threads on depth of field and a lot of warnings that traditional film ideas of depth of field don't translate directly to digital. You can find a quickie basic reference on that in Gary Ferguson's LuLa article Digital Focusing.

 

A shorter lens certainly has more depth of field than a longer one. But when Sean Reid tested several superwides on the M8, he commented that the idea of 'just letting depth of field cover it' didn't work as well as one might expect. That is, focusing on a license plate at 3 m reproduced it noticeably sharper than simply putting it within depth of field. He may have been using hyperfocal distance for the comparison; I'm not sure.

 

That's the same as Enrico said just above, and makes sense to me.

 

But as long as it works for you, then you're doing it right and no amount of technically "proving" something is necessary or would even be helpful.

 

 

Thanks for the explanation of how 3 m and infinity are placed on the lens; I understand now.