[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.


"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

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