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Macro DOF (was Re: [OM] 200mm f4, good or not??)

Subject: Macro DOF (was Re: [OM] 200mm f4, good or not??)
From: "Paul D. Farrar" <farrar@xxxxxxxxxxxx>
Date: Tue, 10 Mar 1998 22:18:58 -0600
Cc: farrar@xxxxxxxxxxxxxxxxx
At 07:10 PM 3/10/98, Tatu Laitinen <tatul@xxxxxxx> wrote:
>Hi everybody,
>Thanks for your answers
>
>I know that 200mm lens is not a macro lens and that it doesn´t focus close
enough for flower close-ups. I was thinking about using it with a little
extension so that I could go to 1:2-1:5 scale. 
>
>DOF is the same in all focal lenghts, its the magnification that counts.
But because Im shooting with extensions the DOF is going to be narrower than
normally. (or is it????) 
>
>The reason why Im interested of the 200mm is its narrower angle. I think
that its much easier to get out of focus backgrounds with 200mm than 90mm
lens. using a narrow angle lens,  I could close down the aperture and sweep
the background for a nice and smooth spot. So the 200mm might be handy in
places where there is lots of vegetation behind the subject!!  

Funny you should mention it, but I was just thinking about that subject
today. The old bromide is "For equal magnification and equal f, DOF is the
same regardless of focal length." But working through the formula and
thinking about extreme cases today, I realized that that is just an
approximation that works for some cases. For ordinary closeup-macro, it
works almost exactly, but for other cases, the approximation breaks down,
and one of the cases it doesn't work for is distant macro backgrounds.
Consider this case: a 20mm lens and a 400mm at zero magnification, i.e.
infinity focus, and the same f stop. Do they have the same depth of field?

Using the formula from Brandt's _Photographic Optics_, where x is focus
distance, tf is the distance beyond x where the circle of confusion is c, F
is focal length, and f is f-stop.

tf = x**2/(F**2/(c*f) - x)

For the case of a 50mm and a 100mm set at 1/4 magnification at f/8, and a c
of 1/30mm (a good value for 35mm full frames). tf will be about 7.5mm for
both lenses: the rule works. But a point 30m behind the object will have a c
of about 1.24mm for the 50mm, and 2.45mm for the 100mm. Even though they
have the same DOF, the background will be more out of focus for the 100mm!
One way of looking at it is that the background will have different
magnifications for the two lenses, even though the subject doesn't.

Besides that, with the 100mm, you can use the narrower angle to pick the
part of the background you want to use, as mentioned.

Paul Farrar



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