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How Far is Infinity?

Travis McGee , Aug 03, 2010; 10:55 a.m.

As you know, if the subject is a flat plane or so far away that the lens will focus at infinity, you should use your sharpest aperture. So how far away is infinity? A hundred yards? A half mile? Do you look at the barrel of your lens to see where it focused or do you have a rule of thumb? Thanks!

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Daniel Flather , Aug 03, 2010; 11:08 a.m.

How wet is wet?

Bob Boudreau , Aug 03, 2010; 11:11 a.m.

How high is up?

Luis G , Aug 03, 2010; 11:12 a.m.

Infinity is just as far as two infinities.

Frank Dzambic , Aug 03, 2010; 11:13 a.m.

A good zen question to ponder when you're set up and waiting for the light to change.

Maybe it's all just varying degrees of infinity?

Todd Roseman , Aug 03, 2010; 11:14 a.m.

you need a DOF calculator thingy, to see where the CoC is acceptably sharp at the closest distance, when focus is set to infinity

JDM von Weinberg , Aug 03, 2010; 11:19 a.m.

The more meaningful question in relation to lenses is not how "far" infinity is (that, not surprisingly, is infinity), but how "close" infinity on the lens is so that the image on the film/sensor plane is close enough to focus to fool the human eye.

The answer depends on the lens and on the f/stop the lens is set to. What I am talking about is "hyperfocal distance" (link).

On my Zeiss Tessar 50mm f/2.8 at f/2.8 everything from roughly 10 meters to infinity will be in focus (more or less). At f/16, the Tessar is in focus from less than 2 meters to infinity. Or to put it as you did, if the lens is set to infinity mark, then at f/2.8 depth of field will cover to 20 m or so, at f/16 to 3.5 m.

Older, prime lenses used to have markings showing all this, but these became difficult with zoom lens (hence all the curving lines on the barrel), and seem to have mostly and literally dropped off the scale with AF lenses.

Wayne Campbell , Aug 03, 2010; 11:42 a.m.

Infinity is somewhere beyond 25-30 feet from the camera. A better question is what if something is beyond infinity? Will it still be in focus?

Ed V. , Aug 03, 2010; 12:01 p.m.

...what if something is beyond infinity? Will it still be in focus?

Yes, but you can't see it because it would take forever for the light to reach you.

Tom Harvey , Aug 03, 2010; 12:04 p.m.

...ad infinitum?


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