Telephoto lenses & blurred backgrounds

futureal33

futureal33

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Hi all,

Just a quick question (hopefully).
Been playing with my 55-250 lens a lot recently and Im loving the way it completely throws the background OOF when focusing on a subject.

Its similar to a low aperture lens, such as using my 30mm @ 1.4, the backgrounds get thrown out just the same.

Im just wondering technically, why does this happen?
Even when using narrow apertures such as F10 the background will still be miles OOF.

Thanks for any clarification!
 
Part of it is down to the perspective effect. If you frame an object the same way with different focal lengths then the background elements will be bigger at the longer focal lengths - which can make it appear more out of focus than it actually is.

A good way of seeing this is to place an object a couple of metres in front of a patterned background. Take a shot at a shorter focal length in which the object fills the frame and then do the same with a longer focal length - you'll find that you see less of the pattern with the longer focal length.
 
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You have to remember that depth of field isn't an absolute thing, it's simply a matter of how far in front and behind the point of focus appears to be in focus. Only the actual point of focus is truly in focus; everything else is blurred to some degree, but over a certain range the blurring is too slight to notice.

When you use a long focus lens you are in effect using a magnifier. Everything is bigger than it is with a shorter focus lens, including the blurring of out of focus elements. For this reason, the blurring becomes apparent to the eye at a shorter distance from the true focus point than it does with a short lens, i.e. the DOF is less.
 
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