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Re: [OM] Birds, Bokeh, and DZes

Subject: Re: [OM] Birds, Bokeh, and DZes
From: WayneS <om3ti@xxxxxxxx>
Date: Tue, 06 Jan 2009 21:19:51 -0500
Very good analysis Ken and what I had always wondered.
Perhaps you can resurrect some comments you made in the
past on overly corrected zoom lenses and bokeh.

My take away from what you wrote some time ago about
highly corrected lenses was that these lenses produce sharp focus
at the plane of focus, but diverge the light differently when out of
focus, for instance the light entering the perimeter may be slightly
different angle than closer to the center. The bokeh then is very 
dependent on aperture and relative distance of the out of focus
parts of the scene.

The effect can seem as if a point out of focus will be non-uniform.

Also textured backgrounds are sometimes worse. Here is a relatively
unprocessed image, full resolution, no sharping image shot with
Can*n 70-200/2.8 lens. The tree branch moves off into the back
ground, so the relative out of focus areas vary.  large1.5m image:
http://www.zuik.net/OM-4t/beast_MG_9170.jpg

Another one, and this is a really really bad shot, but the bark creates
funky out-of-focus texture. (Can*n 24-105/4 lens) 1.2m image:
http://www.zuik.net/OM-4t/ugly_MG_1410.jpg

Of course the other issue is that both these lenses were using
IS and hand-held. IS can introduce another dimension to the
bokeh phenomenon.

So, I wonder, even the best of lenses, whether the out-of-focus
area is pleasing or not will depend on what the actual
background is? Add to that sharpening effects for even more variation.

WayneS


At 01:55 PM 1/6/2009, you wrote:
>I'm reminded of a lesson learned with the Vivitar 2X macro extender. This
>device sat between the lens and the camera body and not only provided
>variable extension but included the 2X multiplier optics.  The rule of thumb
>is this:
>
>The multiplier sitting behind the extension provides magnification of the
>image being transmitted through it.  However, if the multiplier is sitting
>in front of the extension, the image will have the characteristic of the
>multiplied focal length of the lens used. The resulting images are
>completely different.
>
>Inotherwords, if you placed a 2X multiplier behind a 50mm lens, you
>generally got a similar image to that from a 100mm lens.  And then you just
>extend this lens-combination to get the close-focusing you want. However, if
>you place the extension between the 50mm lens and the multiplier, you got
>the image characteristics of a cropped 50mm lens image. This had a
>tremendous bearing on the resulting bokeh.
>
>I suspect that many lens designs today (especially zooms) are essentially a
>short focal-length followed by a multiplier (or variable multiplier in the
>case of zooms).  Since most lenses now are IF (internal focusing), the gap
>between the front group and the back group is being extended during
>focusing.  This would explain why many zoom lenses give the image results of
>a crop of a lesser focal length instead of the bokeh of a fixed prime lens
>of say a classic Tessar design.
>
>AG
>
>You want beautiful bokeh?  Get yourself a classic Zuiko 35/2.8.

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