Per
old hand
Reged: 28/11/2005
Posts: 716
Loc: UK Berkshire
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While tidying away one of the first copies of AP I bought (17 Sept 2005), I came across this revelation (to me at least) on page 28, from Geoffrey Crawley...
Infinity...For photographic purposes it is defined as the object distance beyond which a lens of a given focal length images all planes equally sharply, making further focus movement unnecessary...this distance is often taken as 1000x focal length. Thus for a 50mm lens it is 50m.
Can anyone help me out here? Assuming I'm photographing a landscape with a lens of 24mm focal length, should I:
A. Set the hyperfocal distance one third of the way into the scene, say on an object 500m away, and stop down for depth of field
B. Ignore the hyperfocal distance and set the focus distance to approx 24m and stop down (less) as everything further than 24m will be rendered in focus.
C. Set the hyperfocal distance one third of the way to 24m (approx 8m) and stop down (even less), ensuring that 24m is in focus (and therefore infinity) as there is less margin for error than B.
D: Note that A and B are the same. You don't actually focus beyond 24m when you render an object 500m away in focus and that's why the lens doesn't have a distance scale beyond 24m.
Apologies if this is a dumb question, but I hope you can see what I'm driving at 
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There are two things to aim at in life: first, to get what you want; and after
that, to enjoy it. Only the wisest of mankind achieve the second.
Logan Pearsall Smith (1865-1946)
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beejaybee
Marvin
Reged: 18/07/2007
Posts: 4987
Loc: Really Here In Name Only
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Quote:
Infinity...For photographic purposes it is defined as the object distance beyond which a lens of a given focal length images all planes equally sharply, making further focus movement unnecessary...this distance is often taken as 1000x focal length. Thus for a 50mm lens it is 50m.
But the "object distance beyond which a lens of a given focal length images all planes equally sharply" depends on the lens aperture as well as the focal length. There's an avoidable confusion here between "infinity" and hyperfocal distance.
For landscape work - if using a lens with DoF markings, I focus by setting infinity against the mark for the aperture setting one stop larger than the setting I'm using. Without DoF markings, I focus on the most significant object, then stop down until all the other objects I want sharp appear to be so whilst operating the depth of field preview.
Incidentally what's the furthest object you can see without binoculars, telescope or other optical aid? I reckon it's probably the Andromeda Nebula (M31) which is approximately 12 million million million miles away. Still a long way short of infinity....
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AlexMonro
old hand
Reged: 05/06/2006
Posts: 757
Loc: Exeter, Devon (and Somerset so...
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I tend to use a hyperfocal technique for landscapes, similar to that suggested by beejaybee, but mindful of the improvements in film resolution since my Nikon AI lenses were designed and the assumptions made for the circle of confusion, I allow an extra stop, i.e. I set the DoF scale mark for a stop or so wider than I'm actually using against the infinity infinity mark.
So using a 24mm lens at f/11, I'd focus at about 2.3m, and expect everything from just over a metre to be in focus. This is usually adequate to get the foreground sharp. With longer focal lengths and close foregrounds it gets trickier due to the narrower DoF, and I have to think carefully about the composition and what I want to be in focus.
You might find this tutorial and this on-line calculator helpful (especially if your lens doesn't have DoF scales).
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Per
old hand
Reged: 28/11/2005
Posts: 716
Loc: UK Berkshire
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Thanks guys for the assistance and the links.
I'm interested that beejaybee says what I thought:
Quote:
the "object distance beyond which a lens of a given focal length images all planes equally sharply" depends on the lens aperture as well as the focal length
Going back to Mr Crawley again, he says "Note that this figure is irrespective of aperture, the introduction of which as a factor is a question of field depth not of virtual infinity per focal length."
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There are two things to aim at in life: first, to get what you want; and after
that, to enjoy it. Only the wisest of mankind achieve the second.
Logan Pearsall Smith (1865-1946)
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