On Sat, 4 Aug 2001 17:00:26 +0100, Bob Walkden wrote:
>Hi,
>
>The dof with the 50mm at 1.4 would not be the same as the dof with the
>100mm at 2.8. If you shot them both at f/2.8 then the dof would be the
>same.
Only if the framing was the same. Which would mean that with the 100mm
you would be further away from the subject than with the 50mm to get
the framing the same.
Depth of field is a relationship between the f stop of the lens and the
magnification ratio of the subject to the image on the film. The
magnification ratio is a relationship between the camera to subject
distance and the focal length of the lens.
So, if you frame for a headshot at f2.8 then your depth of field is the
same regardless of what focal length lens you are using (you would be
standing in completely different locations for the shots with the 20mm
lens and the 2000mm lens however). If you stay in the same spot and
increase your focal length then for the same f stop your depth of field
decreases because your magnification increases.
Leon
http://www.bluering.org.au
http://www.bluering.org.au/leon
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