Are Meters on my Focus Ring Lying to me?

[cgustav]

Hi there,

I've noticed that if I set the focus to, let's say 3 mt, what it's actually in focus is less than 3 mt away.
I've run the same test on different lenses with the same result.

Should I assume the distance on the focus ring is an approximation or am I missing something?

[calvin83]

Is the infinity position correct? If not, the whole scale will be off.

[cgustav]

Hm... infinity is always a tad off, need to back it up a little.
This applies to all the lenses I have but as far as I know it's supposed to be a normal behavior? (at least on digital cameras?)

[Lightshow]

Are you adapting lenses? if so, the adapter's thickness could be off some, which will change the focus scale.

[cgustav]

[kds315*]

[DConvert]

It's nothing to do with digital. AF lenses have focus travel that goes beyond infinity to prevent damage to the motor/lens that would occur if it bashed up to a hard stop. Longer MF lenses also often travel beyond the infinity focus to allow for temperature changes.

If your using an adapter that's slightly short the focus will be at positions where the focus scale doesn't match the distance. Generally not too much of an issue, but the degree it's out by will vary with focal length. My adapted 10mm lens initially needed the focus set to ~0.27m to focus on infinity - leaving practically no focus movement. I've now added shims so that infinity focus is now before the first distance mark. Not yet ideal but much better.

It seems quite a few adapters can also be slightly too long. A much more inconvenient error as it prevents many lenses from reaching infinity focus...

BTW I assume you mean m (meters) not mt (US abbreviation for metric tonnes).