I'm getting tired of this thread so here's a suggestion, (not to flip I hope).
Anyone who wants to play with differing focal lengths and f settings effect on
DOF should take the time to download FCalc. It's a neat little program with
versions that run under a number of OS's. Best of all you don't need to know
any math what so ever to use it. (This should be a great help to those of
us who are
recent graduates of US public schools). I won't tell anyone the answers to
the myriad questions this thread has raised, not the least of which is "do
we really
know what DOF is? (I do). However playing with FCalc will answer the
question asked
in the original post and probably put a bit of egg on a number of peoples
faces.
The URL is http://www.tangentsoft.net
Now the disclaimer. I have no connection with Tangentsoft nor do I get
anything
for shilling for them. My only connection is that every time I play with
this
little utility I wish I had thought of it first.
At 09:25 PM 8/5/2001 +1000, you wrote:
>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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