On Wed, Nov 07, 2007 at 07:12:42AM +0000, Stephen Davies wrote:
> Most Poit and shoot digital cameras have pretty short focal length
> lenses. This means that there is a very good depth of field (when the
> zoom is less that 50mm 35mm film camera equivalent). Using aperture
> priority increases the aperture and gives you an even greater depth of
> field.
> This has to come from somewhere as the aperture, shutter speed and ISO
> Film speed equivalent have a fixed relationship. (See below)
>
> If x = f8 @ 1/125 @ 200iso
>
> then
> x = f4 @ 1/500 @ 200iso
>
> Where x = the amount of light falling on the film/ccd.
> 1/125, 1/500 = shutter speed
> f8, f4 = Aperture
> 200iso = equivalent to a film speed of ISO 200.
>
> As the speed goes up(ie shorter exposure), the aperture goes down.(less
> depth of field)
> As the Aperture increases (more depth of field) the shutter speed slows
> down = more chance of blurry images if there is any movement in the frame.
It might be worth clarifying that when Stephen says an increasing aperture
gives more depth of field, he means an increasing f number gives more depth of
field. The aperture itself is physically smaller.
There are of course many circumstances when a narrow depth of field is
desirable, particularly in portraits.
Tony
