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Say_Cheese

 
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Member Since: Feb 17, 2006
Location: High Wycombe, Just outside London, England
Posts: 385
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Mac Specs: 20" iMac 2.0Ghz, 2Gb RAM Early 2006, 30Gb iPod 5th Gen. 15" MacBookPro, 2.33GHz

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An f stop is a reference to the diameter of the aperture (hole) through which the light passes before it hits the recording medium (film or digital chip).

The fstop is referred to as a number and they go in the following order.
1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, etc.......

The lower the number the wider the aperture thus the more light that passes through the hole. The more light that passes through the hole the brighter the image. However you may then need to adjust the shutter speed accordingly to get a correctly exposed image.

How does this relate to the blurry background?

Well the larger the aperture (smaller the fstop number) the less that you get in focus in front and behind the point of focus. For example should you focus on a person and set an fstop of 1.4 you might get about 20cm's in front and behind the person in focus before the rest falls into an unfocused blur.
However if you focus on the same person and set an fstop of 22 then you might get 5 metres in front and behind the person in focus before the rest falls into an unfocused blur.

Exactly how far you get in focus in front and behind is all down to another much more complicated calculation that you really don't need to know about.

I would say have a play and experiment. Just remember that the more in front and behind your subject you want to be in focus the higher an fstop you need to set.
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