[rollei_list] Re: 3.5 Tessar sharpness
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sat, 24 Dec 2005 15:02:32 -0800
----- Original Message -----
From: "J Patric Dahlén" <jenspatricdahlen@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Saturday, December 24, 2005 2:19 PM
Subject: [rollei_list] Re: 3.5 Tessar sharpness
>From: Eric Goldstein <egoldste@xxxxxxxxx>
I always learned that most lenses are best at
mid-aperture. A lot of people confuse the increased DOF
with better lens performance...
There are a few lenses that are certainly not best 2-3
stops from max. The first gneration Leitz Summacron 50/2
comes to mind (best at f/4) as does the Cosina/Voigtlander
50/3.5 Heliar (best wide open!)
The 3,5 six element Planar on my 3,5F is best already at
5,6. Since razor sharpness sometimes can destroy the mood
in pictures, I recommend the Rolleisoft filters. They are
great! I have them for all bayonets (except IV since I
don't own a Wide) and also the push-ons for the early
Rolleis.
/Patric
"Best" depends on where you are looking. Some
aberrations are proportional to image angle, sometimes
called image height. These are apparent at the margins and
corners of the image but not near the center. So, some
lenses can be very sharp at the center when wide open but
will not have good sharpness at any distance from the center
until stopped down. The Heliar mentioned may be one of these
lenses but the Heliar type in general will not be at its
sharpest at the corners wide open. Also, most lenses have
some mechanical vignetting from the mount when wide open.
Generally, this vignetting does not disappear until the lens
is down two stops. The way to tell is to examine the back of
the lens from the corner of the film gate. If any of the
iris is obscured by the mount the lens is vignetted.
The f/2 Summicron you mention _is_ two stops down at f/4.
Delibrately leaving some softness in a lens by opening it
up is perfectly legitimate but is subjective. The idea of
"optimum stop" is based pretty much on objective measurement
(or calculation) of the overall correction.
An intesting lens this way is the Goerz Dagor. When wide
open Dagors have a significant amount of zonal spherical
aberration, which gives them a slight soft focus effect,
which can be quite flattering for some portraits. Spherical
is pretty much uniform over the field but varies with the
stop. At a "normal" coverage angle of around 60degrees, the
Dagor becomes sharp all over at around f/16. However, there
is still some other residual aberration which affects the
margins, so, to get the maximum coverage of nearly 90
degrees, the lens must be stopped down to f/45. At this stop
the center of the image shows some blurring from
diffraction. So, "optimum" for this lens for best overall
sharpness can vary from around f/11 to f/45 depending on the
image angle its expected to cover, plus, at maximum angle
the overall sharpness is compromised.
I wonder how much softness your Planar gives you. These
are exceptionally well corrected lenses although margin
sharpness and vignetting will still get a little better when
stopping down.
I went through a long period of learing how to get sharp
images. It took me awhile after that to appreciate the value
of not-so-sharp images for some subjects.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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