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RE: [OM] Depth of Field with Long Teles / Need advice on long tel e-lens

Subject: RE: [OM] Depth of Field with Long Teles / Need advice on long tel e-lens for OM4
From: "Windrim, Brian" <brian@xxxxxxxxx>
Date: Mon, 17 Jan 2000 17:24:06 -0000
Cc: "'IRBWalters@xxxxxxxxxxxxx'" <IRBWalters@xxxxxxxxxxxxx>
Buddy Walters wrote:

>> I was wondering if lens design ie. "internal" focus / Tamron or Tokina
300mm
>> vs "helicoid" / standard Zuiko 300mm could make a difference in the depth
of
>> field if all else is equal.
>> To put my question another way.  Will the depth of field be exactly the
same
>> at 20 feet and at f5.6 with both types of lenses?  Could the design of
the
>> lens increase or decrease this depth?
>> I understand that the minimum focus could be and usually is different.
>> Any ideas?

Unfortunately, depth-of-field is determined geometrically
by subject distance and the effective diameter of the lens'
entrance pupil. No "implementation tricks" can alter this.

Therefore, for a given focal length the depth-of-field will
always be the same between different lenses at the same
aperture (provided that the apertures are accurate).

There *is* a difference between conventional and internal
focussing lenses, in that the latter focus by changing their
focal length. Hence an IF lens will give a less magnified image
than a non-IF lens at close focussing distances, even if both
are the same at infinity. This effect has been noted previously
on the list with regard to macro lenses.

I believe the above to be correct, hope it helps.

-Brian



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