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06-14-2010, 04:30 AM   #16
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QuoteOriginally posted by dlacouture Quote
I don't really understand what prevented the old mechanism from doing "live" adjustements
It's continuous during live view too and it may even matter more: because the metering isn't done with a predetermined lens aperture it's LV image does better reflect the DoF properties of the final image.

In LV, the K-7 tries to bring the aperture close to the final value while the K-x seems to stay in some compromise aperture which looks like f/4. Both if aperture isn't preset by the user.

As for the old mechanism, read:
  1. Features and Operation of the Original K-Mount
  2. Features and Operation of the Ka Mount
Now, with this understanding, look at the standard body side implementation of the "lens' diaphragm actuator" (this is the lever which is moved by an exact distance which is proportional exactly to the number of stops to close down the lens):

This standard implementation is based on a spring and a stop: The camera electronics computes the number of stops to close down and moves a mechanical limiter accordingly (which happens to be force free). Then, when the image is taken, the spring which keeps the aperture open is released which causes the lens to stop down until the body side of the lens diaphragm actuator hits its mechanical limiter. After the image is taken, the spring which keeps the aperture open is engaded again. And the limiter becomes force-free again ready to be set to another value.

This doesn't work if the aperture isn't kept fully open while the aperture is changed. Any change towards opening the aperture requires work against the aperture blade spring. The standard implementation simply requires to wind-up a spring which is probably part of winding up shutter and mirror.

Indeed, the K-x quickly first opens up the aperture before stopping it down when taking an image out of LV. But indeed, it first closes its shutter too.

The K-7 however, actually does move the lens diaphragm actuator working against the aperture blade spring force, at any time and with high precision. Which means that what was a spring did have to become a separate motor. I've even heard that it needs to autocalibrate

I really thought all this is well known since the K-7 came out a year ago

06-14-2010, 05:54 AM   #17
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Well, I do seem to remember the K7 also have a colour temp sensor (same as advertised for the 645D), but I simply cannot confirm this information again... No matter what review or manual I read, this is now part of my Pentax "urban legend" list...
Accurate models descriptions are not Pentax strongest points, IMO... Worst advertized Pentax feature being the interchangeable screen, as it's even officially denied on the Kx!

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