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11-26-2009, 04:09 AM   #1
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M42 on Pentax DSLR (how is expo mesured)

I'm planning to use M42 lens on a Pentax DSLR, and would like to know how exposition is mesured by the camera when coupled with such lens.

11-26-2009, 04:55 AM   #2
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11-26-2009, 01:52 PM   #3
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QuoteOriginally posted by |fred Quote
I'm planning to use M42 lens on a Pentax DSLR, and would like to know how exposition is mesured by the camera when coupled with such lens.
Some M42 lenses have an auto/manual switch. Others do not. In auto mode, an M42 lens aperture is wide open, until the camera pushes the pin to close the aperture to the pre-selected f-stop. In manual mode, the aperture is closed down as the aperture ring is turned. Since no Pentax dslrs have the mechanism to push the aperture pin on an M42 lens, the auto/manual switch is extremely useful.

All Pentax M42 lenses that I know of, have such a switch and are therefore not a problem. Some third-party lenses lack the switch. This means that they can only be used wide-open, unless you modify the lens or find some way to always hold the pin down.

On my K10D, I shoot in Av mode with M42 lenses. With the lens in manual mode, I just compose the shot, focus with the lens wide open, then half-press the shutter to meter the scene while stopping the lens down to the desired aperture. Once the camera has chosen a shutter speed and the SR has stabilized, I press the shutter all the way to complete the shot.

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