M42 lens - photos incorrectly exposed - HELP!!

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left eye
left eye Senior Member • Posts: 2,599
M42 lens - photos incorrectly exposed - HELP!!

I've just mounted my first M42 lens (Helios) onto a KP using an original Asahi Pentax adaptor.

The resulting photos show exposure error.

As I increasingly stop down and meter (with the aperture at the selected setting) - the results become increasingly overexposed. The KP is somehow overcompensating for the decreasing light it is seeing. Very odd.

I've just found someone else in this forum with the same problem, saying results are only consistent and properly exposed when using Live View...


I tried it, Live View works, but I bought the KP to use an OVF ! - and use vintage manual focus lenses.

Metering the lens through the OVF at f4, matches the Live View results (that is Live View at any aperture). Opening the aperture up from f4 (when using the OVF) results become underexposed, closing the aperture it down from f4, results overexposed.


The odd thing is I found photos from my Sigma 17-50mm f2.8, to be 1 stop overexposed compared to my Pentax lenses. f4.0 to f2.8 is 1 stop, weird.


So in detail...

I always use aperture priority (Av) mode with my modern AF lenses. The M42 lens seemed to be metering fine (in Av) - I set the ISO with the front dial (not Auto ISO) and the shutter speed adapts to available light when I half press the shutter.

I've since read that using an old screw-mount M42 lens is fine with Av.

If I meter with a shutter half-press, with the lens fully open f2, then take a shot the shutter speed is 1/800th.

I stop the lens down to f5.6, meter with a shutter half-press, shutter speed is 1/50th.

From f2 to f5.6 is 4 stops, from 1/800th to 1/50th is 4 stops. All exact and good, and the EXIF proves this. Of course the EXIF doesn't show the aperture, but I know I took these tests at f2 then f5.6 and the metering in the EXIF reflects that perfectly.

BUT, when I look at the shots, the f2 shot looks a little dark and the f5.6 too pale. If I metered at the selected aperture why didn't the meter expose correctly??

So as instructions I found on the net (which it seems I don't actually need to do, but tried anyway!), I set the Green button up for using M42 lenses...

C 4 menu > Using Aperture Ring > Enable

Photo 5 menu > Button customisation > M > Green Button > Tv Shift

Ok so I go into M exp mode, and the exposure is removed from the shutter half-press and is now push-in on the green button. But the photo results are the same, the metering and exposure reads ok, but the photos when stopped down get increasingly overexposed. Odd.

Also trying, C 4 menu > Using Aperture Ring > Disable

the shutter should now not fire without the lens set to A, but the shutter does still fire even though the lens has no A setting and it's mounted on a 'dumb' adaptor. The LED on the VF displays 'F - -' whether 'Using Aperture Ring' is set to Enable or Disable.


I'm totally totally baffled. Why on Earth can't the KP correctly meter the light that is entering through the lens?

I can't keep modifying the EV compensation via a calculation and based on what aperture I fancy.

No one ever mentioned I wouldn't be able to meter a M42 lens using the OVF.

Is my foray into the KP and M42 lenses doomed?


f5.6, so this is four stops from f2, and the shutter speed has automatically compensated to be four stops longer, but look... what went wrong?...

...as I keep stopping down the shutter speed adjusts seemingly correctly, but the results keep getting more overexposed.

And again...



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