UK K30 Repair/Replace conondrum 2021

Started 4 months ago | Questions thread
Futax Senior Member • Posts: 1,257
Re: UK K30 Repair/Replace conondrum 2021

VictorDB wrote:

Wow, on my Nikon D750, for non-CPU lenses, I just plugged in the focal length and max aperture of all my manual Nikkor lenses and have a function button and wheel select combo that calls up each lens when I use them and it gives me open aperture (no stopdown required) spot, average, matrix and i-TTL (flash) metering in both manual and aperture priority mode along with basic EXIF data (think it just misses out zoom data but I have no manual zooms).

Is that similar for your K-50 method?

In a word - no.

Here's the background:  the original K-mount (~1970s) had a mechanical coupling from lens to body, supplying relative-aperture info in order for the body to be able to meter with the lens wide open, and calculate the required exposure.

The introduction of the KA mount in the 1980s allowed the camera body to set aperture via the stop-down lever, so the relative aperture coupling wasn't used (though min and max aperture were communicated to the body via electrical contacts).

When autofocus came along, the KAF mount on the camera bodies omitted the mechanical aperture coupling (the so-called crippled K-mount).

The upshot is that modern Pentax dSLRs, without aperture info, can't calculate correct exposure if the lens sets the aperture.  But you can get round this by implying to the camera the correct relative aperture info.  You do this by Exposure Compensation.  So, for an f2 lens which you've set to f4, you would add +2 stops of compensation.  This emulates what the aperture coupling would have done.

As this apes what the "normal" mechanism does, it provides nice and accurate exposures.  However, it's only possible with the K-50 (and K-30 ??) which have the aperture problem.  I don't know why this is, but try it on a sound K-50, and the camera will fail to stop down the lens at all (and this is the "normal" behaviour for a modern Pentax).

I find this is preferable to the stop-down metering with the green button, but it's only really viable for lenses with fixed maximum aperture.

Anyway, you did ask! 

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- Keith.

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