Archived posting to the Leica Users Group, 2003/05/07

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Subject: Re: [Leica] Re: Wow, dig that crazy 90/AA!
From: Bob Adler <rgacpa@pacbell.net>
Date: Wed, 7 May 2003 13:55:57 -0700 (PDT)

Once again, nicely done Mr. Brick!
Bob Adler
Palo Alto, CA
- --- Jim Brick <jim@brick.org> wrote:
> All 90mm lenses, basically, produce the identical
> depth of field. If one 
> 90mm lens has more contrast and higher resolution
> numbers, and the ability 
> of recording fine detail better than the others, it
> will "appear" to have 
> LESS DOF than the other lenses. This is because of
> the visual sharpness 
> difference between the sharp and unsharp areas.
> There is an abrupt change 
> between the very sharp and the less sharp areas.
> This is also why lens 
> manufacturers say that to fully utilize the DOF
> scales on modern lenses, 
> use it as if you were shooting two stops wider, ie;
> shoot at f/16, read the 
> scale for f/8. The scales were computed back when
> lenses were not so sharp 
> and film was not nearly as good as today's lenses
> and film.
> 
> If your lens exhibits lower contrast, less fine
> detail recorded, etc, then 
> the difference between the sharp and unsharp areas
> is less apparent thus 
> giving the "impression" of greater DOF. It is a
> smoother transition between 
> the sharp image parts and the less sharp image
> parts.
> 
> This is a very well known fact and is why many
> people think that old lenses 
> have greater DOF than new lenses. It's not true,
> it's simply the visual 
> difference (sharpness contrast) between the sharp
> and unsharp areas.
> 
> Jim
> 
> PS... Depth Of Field is what you see in the
> photograph. Depth Of Focus is 
> at the film plane and, even though (like depth of
> field) is dependent upon 
> the aperture, is a different subject.
> 
> 
> At 09:06 AM 5/7/2003 -0700, Jerry Lehrer wrote:
> 
> >Don
> >
> >In this regard, I wish that someone could explain
> why and HOW
> >different lenses with the same aperture and focal
> length can produce
> >different depths of focus.  How can the laws of
> optics be ignored?
> >
> >Get as technical as you need to.  I can handle it,
> but don't resort to
> >mysticism, please.
> >
> >Jerry
> 
> --
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=====
Bob Adler
Palo Alto, CA
rgacpa@pacbell.net
- --
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