Re: [Rollei] bokeh

[Richard Knoppow <dickburk >]

At 08:29 PM 09/18/2001 +0200, you wrote:
>Now, I hope this doesn't start a flame war as it used to do on the LUG - but
>I am REALLY enjoying the out of focus areas of my xenotar when used at wider
>appetures. It is inspiring me to doa whole sweries of stuff wide open. Do
>all 'flex lenses have this characteristic? I notice that there are 5 blades
>in the xenotar, does the planar have more?
>
>Julian
>-----------------------------------
>Julian Thomas
>Bruc 168-6-1
>08037 Barcelona
>
>http://www.borderless-photos.de/jthomas/jthomas-01.html
>
>tfno 679676321
  The iris is part of the shutter. Since both Planar and Xenotar Rollei's
use the same shutter they should have the same type of iris. The same
five-bladed iris is found on other lenses mounted in Synchro-Compur
shutters, like the Schneider Super Angulons.
  The shape of the iris opening does affact the shape of blur spots in the
image. The Japanese use the word bokeh to describe the character of the out
of focus areas of the image. One can argue interminably about this.
  The older version of the Synchro-Compur, used in the Rollei MX and
Rolleicord IV, has a ten blade iris and a nearly perfectly round hole. EVS
cameras use a different shutter and may have the five blade iris. 
  I think the use of fewer iris blades may have to do with a different
method of driving them. You will notice that the five blade iris is found
on EVS cameras and that the f/stop scale is linear. The older shutters have
a non-linear scale, compressed at the small stop end. For EVS the linear
scale is necessary. I suspect that the diameter of the iris mechanism may
get too large when more blades are used. 
  The lenses with the closest to circular iris openings seem to be process
lenses. Here, the shape was important when using the lens for half-tone
work. However, very often Waterhouse stops were used with special shapes
(square or eliptical were common) because of advantages of the special
shapes. Some process lenses have twenty blade iris diaphragms. 
- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk  

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