Photographing newspaper text is only a valid test if that is what you plan
to do with the equipment. /jnm
-----Original Message-----
From: owner-olympus@xxxxxxxxxxxxxxx
[mailto:owner-olympus@xxxxxxxxxxxxxxx]On Behalf Of John A. Lind
Sent: Wednesday, January 23, 2002 5:30 AM
To: olympus@xxxxxxxxxxxxxxx
Subject: Re: [OM] OM lens fascination
At 08:48 1/23/02, Thomas wrote:
>Dear Zuikoholics,
>
>well, I am still new in the Zuiko field and I came to it rather by chance.
>I am still not secure about using my Om4Ti and I am not sure what the
>quality and potential use of the lenses is.
>
>So, I did a lens test by photographing a newspaper from tripod with timer
>release to reduce vibrations. And I was a bit disappointed about the
>performance of my lenses at wide aperture with respect to sharpness,
>contrast and vignetting.
[snip]
I second the remarks already made about the very accurate focusing required
at the distances you used with very fast lenses wide open. In addition,
the test you conducted presumes the lens is completely "flat field." If it
has any curvature of field the effects from this will be confounded with
your overall sharpness results. The hint of it may be seen if there seems
to be more softness near the edges than in the center, or vice
versa. Absence of this phenomenon is still not a guarantee the field is
absolutely flat. I would expect a macro lens to have a very flat field as
one of the primary macro applications is copy work of very flat objects
(graphic art and documents). For non-macro lenses and nearly all other
applications, an exceptionally flat field is not as important.
The highest acuity 50/1.4 lens is the MC with S/N greater than 1,000,000;
I've also had no problem with acuity of a very recently made 35/2
[MC]. Most lenses do not perform quite as well wide open as they do
stopped down to about f/5.6 to f/11. This is considered the "sweet spot"
for maximum acuity. Wider apertures exacerbate aberrations related to ray
path convergence across the visible spectrum and flatness of
field. Narrower apertures exacerbate effects of aperture diffraction. In
making lens comparisons, how much degradation is observed between wide open
or stopped down completely and the mid-range "sweet spot" is of interest,
but it is a very, very, very rare lens that has immeasurable differences at
all apertures.
This is a sweeping generalization; a particular lens formulation and even a
specific lens may deviate from the "norm" and perform
differently. Aperture diffraction effects cannot be evaded; it's a matter
of how narrow the aperture must be before it becomes detectable.
-- John
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