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01-12-2010, 07:03 AM   #1
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Issue with stop down metering - 50mm SMC Pentax-M with K-X

I have an 50mm 1.7 SMC Pentax-M (it is an Asahi if it makes any difference) that I have been using with my K-X.

I have followed all of the instructions to use this non-A lens and it functions as expected in most conditions. However, when using a small aperture in a low lit area, the max shutter speed will only be set by the camera at 1.6". I can use the e-dial to go beyond 1.6", but the camera won't automatically take me there. Any ideas?

I have followed all of the instructions on using non-A lenses. I have set the focal length correctly. I have tried both the exp. comp. button and green button to set the shutter speed.

The only other mention of this on the forum that I can find is in this somewhat unrelated post, but the suggested fix does not work:

Post #8 -

01-12-2010, 07:10 AM   #2
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In all the pentax cameras there are limits in manual mode to the slowest shutter speed that can be metered.

If you change the ISO this minimum shutter speed will change but it all relates back to the minimum EV (exposure value) limit of the camera.
01-12-2010, 01:47 PM   #3
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To expand on what Lowell said...
There is a limit to meter sensitivity
The meter in your camera has a lower limit of EV 1 (presumably @ ISO 200). That means the amount of light striking the meter cell for 1 second at f/1.0. This is different than the ISO sensitivity of the sensor. Consider the following case:
  • Available light to the lens EV (200) = 1
  • Selected aperture = f/5.6
While this combination works fine for open aperture metering with an "A" series or higher lens, there are problems using green button/stop-down metering. When stopped down to the selected aperture for reading, the amount of light at the meter is only EV(200) -4 (1/10,000th the light it requires)! The meter is not linear at this light level, if indeed, it responds at all.

For shooting in dim light using non-"A" contact lenses, using M mode with a sensitive hand-held meter might be the best solution.


Last edited by stevebrot; 01-12-2010 at 02:02 PM.
01-14-2010, 10:33 PM   #4
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Thank you!

Thank you very much! This makes sense!


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