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Hi everyone.
thanks to all that contributed to my threads recently re aperture/group portraits e.t.c. I have been looking at a depth of field guide and am wondering what infinity means. Does it mean that anything behind where my camera is focused will be out of focus. Also if i take away the near limit focus from the far limit focus does that give the distance that will be in focus from where i am focusing. thanks james.
Well, according to our own local glossary, infinity is "a distant focusing point". It does not necessarily mean that anything behind the point on which your camera focuses will be out of focus. Even at the widest aperture and with a focusing point that's really close, let's say 1.0 aperture with a 50mm lens, focused at 1.00 metre, you still have just as much in focus behind the focusing point as in front of it. This does change when the variables change, but the area that's in focus does not stop exactly where you've focused, there will always be an area in focus behind it.
Quote: Well, according to our own local glossary, infinity is "a distant focusing point". It does not necessarily mean that anything behind the point on which your camera focuses will be out of focus. Even at the smallest aperture and with a focusing point that's really close, let's say 1.0 aperture with a 50mm lens, focused at 1.00 metre, you still have just as much in focus behind the focusing point as in front of it. This does change when the variables change, but the area that's in focus does not stop exactly where you've focused, there will always be an area in focus behind it.
so am i right in saying that if i subtract the near distance of focus from the far distance of focus does that give me the distance behind where i am focusing that will be in very sharp focus? james
Infinity on a camera lens is a point at which no more adjustment needs to be made for objects that are further away to be in focus. The reason that you can often focus beyond infinity is because this point of focus changes with amongst other things the temperature of the lens.
The amount that is in focus is generally regarded as being twice as much behind the point of focus as in front of it. The distance between these two points is what is known as the depth of field.
this may help or this
They will show that your understanding at present is incorrect.
James - I believe that this link was posted in one of your earlier posts - it's quite helpful, I've got it in my favourites now!
http://www.dofmaster.com/dofjs.html
James, you may have touched on infinity at school. It is the distance from which the light rays that arrive at your lens are effectively parallel instead of diverging, therefore once you reach this point, anything from there on needs the same focus adjustment.
Lenses are sometimes not 100% accurate about the point of infinity and it is therefore not always safe to just stick the focus ring to the end stop.
Ian
Infinity... The difference between what you know, and what you need to know... 
Quote: Infinity... The difference between what you know, and what you need to know...
lol
but a small typo on your part, let me correct
nfinity... The difference between what you know, and what you think you know
It's what you focus on if you wan't to capture a portrait of Buzz Lightyear...
Infinity, for all practical purposes is generally considered as point far enough away to suit the matter in hand.
eg rays from the sun are regarded as parallel. They are not quite but virtually indistinguishable from true parallel rays. So they rays of light from the sun, when passed through a lens, converge to an (almost infinitely) small spot of light at its focal point. ie the 'burning glass'.
Roughly speaking:
When the camera lens is focused at a point, far enough away, where the lens is unable detect any further narrowing of the angle of the beams of light being focused, then that point is infinity as far as the lens is concerned.
The rays of light from that point are indistinguishable from true parallel rays of light by the lens.
If you focus the lens to its longest marked distance on the focusing ring (not EOS and similar lenses!) choose a subject at the end of your street, then rack the lens out to its infinity mark. You will only perceive a minimal increase of sharpness of focus, if any, from the last marked distance to the infinity mark.
A Yashica 28mm ML wide angle lens has12 feet longest marked distance on its focusing ring.
A Yashica 135mm DSB telephoto lens has 50 feet marked.
A Mitakon 200mm MC long telephoto lens 100 feet.
The longer lenses, being designed to work with smaller angles of view, can accordingly focus smaller angles of incident light passing through the lens.
Which I think is obvious!
jas
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