On 7/14/2019 3:02 PM, christrask@xxxxxxxxxxxxx wrote:
I was sorting out some unfiled papers yesterday evening and came across a
lens that I had bought a few years ago. It's a Tele-Vivitar 200/3.5. It has a
T-mount, and I just happen to have a T to OM adapter. The focusing control
covers a full 330 degrees, but it only focuses down to 3.5 m (11 ft).
Interestingly, this lens has two aperture rings. The first ring controls
the iris, and the second ring acts as a stop. Seems that you adjust the stop
ring to your desired f-stop, then open up the aperture ring for focusing. Then
you adjust the aperture ring to the stop ring setting so you don't have to look
away from the viewfinder.
In addition, the aperture dues not seem to have any leaves. No matter
what the f-stop setting is, the aperture remains round. So any bokeh will come
out round. I wonder how they do that?
Both of these characteristics are common to lenses made before auto-aperture
camera bodies.
The first is called preset aperture, or aperture preset.
Second, take a flashlight and look at the aperture. You will find that it is made of many blades, about 16 in a '50s
Carl Zeiss Jena Tessar 2.5/50, for example.
Auto aperture required a diaphragm that closed quickly, and with minimal force. That led to 6-8 blade diaphragms. And
there was no longer need for the preset mechanism.
Iris Moose
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What if the Hokey Pokey *IS* what it's all about?
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