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donut

donut

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what point ( distance wise ) does infinity begin ?
 
Infinity......Well on earth and for millions of years I would have thought it would be what animals can't see past the curvative earth at ground level...what's that erm about 3 to 21 miles? or where the sky meets the earth
 
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That's actually a very interesting question - and has a very interesting answer.

Quite simply, it's when light rays are parallel, or at least when a lens focuses on light rays that are parallel.

Modern lenses can focus beyond infinity to account for autofocus and also for expansion/contraction in the materials used.
 
kendo1 said:
Quite simply, it's when light rays are parallel, or at least when a lens focuses on light rays that are parallel.

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Click to expand...

@donut Well there you go, just keep your eyes open for parallel light rays ......I'm damned if I can see 'em, I ask my lenses to search 'em out!! :ROFLMAO:
 
For most practical (photographic) purposes, the horizon is a decent approximation. After that, next stop Luna!
 
trust me to get the chocolate teapot lot to answer !
ok ,ive got to calibrate a couple of rangefinders ,,,so for the purpose of infinity for camera focusing what distance would you be looking at ? a couple of hundred feet ????
 
A period of time that seems very long, especially on account of being tedious or annoying :)

A couple of hundred feet should do it, though a couple of thousand would be better.
 
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thank you all for your replies ,,,,and a special mention for Ken who actually answered :)
 
I can go away and verify this if you like, but the standard optical answer as far as I recall is 600 times the focal length of the lens - so it depends (as you'd logically expect) on the lens. I think it's in Arthur Cox's Photographic Optics if nowhere else.
 
Addendum. I just checked Photographic Optics (p34 in my copy) which give the statement that 1000 yards is effectively infinity for lenses normally used in photography. He then goes on to give the calculated error for a specific lens, showing it to be quite small. I assume that if you want an arbitrary level of accuracy, you can calculate it in terms of the fraction of a millimeter that you want.
 
And finally (if not infinitely) the Dictionary of Photography, 1961 edition states inter alia that to find infinity when setting a lens, focus on an object at a distance not less than 2000 times (two thousand) the focal length of the lens divided by the f number of the largest aperture. That seems to come out at around 50 yards for a 50mm f/2 lens if my fingers worked the calculator correctly.

Edit to add: I strongly suspect that there may be as many different formulae for this as there are to calculate the optimum size of a pinhole for photography.

This revised formula seems to be about the same as my first recollection, assuming a lens of about f/2.8.
 
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thanks Stephen ,,,,,50 yards is good enough for me .
 
I think I remember the much revered/reviled Ken Rockwell reviewing the Mamiya 6, saying it was important to check the range finder infinity on very distant objects... 500 yards plus?

On my Pentax lenses I reckon a reasonable guess at infinity is 50 yards plus... but for most apertures depth of field takes care of any error there. Rangefinders are different!
 
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