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03-30-2020, 12:45 AM   #1
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Anyone ever used a Sigma 1.4 35mm Art on a P30?

Lately I was wondering, on which of my analog cameras my Sigma 35mm/1.4 should work best.

As no autofocus camera from the film area will work with the ultrasonic drive of the sigma, I figured out a manual focusing camera able to work with "A" lenses should do the job.
Which is supported by the compability chart .The Pentax Camera Lens Compatibility Chart
Having only a working P30 available, I mounted the sigma onto it and tryed without a film, how the P30 worked with it in P mode.
To my surprise the lens, as far as I could see, never closed the aperture.
An A 50mm/1.7 worked as expectd on the P30, i.e. stoping down to f-stop 8 shooting directly into the light.
The Sigma 35 would stop down on my K-70 as well, but not on the P30.

Has anyone an explanation?
Where my expactations wrong and the P30 and the Sigma should not work together at all?
Did I just not see the lens stoping down?
Does the Sigma tell the P30 it's smallest aperture is 1.4?

While writing this an other test came to my mind. I should control the time setting the P30 is choosing. By using an exposure meter, I should be able to deduct the f-stop the P-30 is calculating with.

One funny thing I noted, while operating the Sigma on my K-70. The sigma first stops down to the needed aperture and then instantly closes down to its smallest one.
I have to try this with my other lenses too.

Thanks for your help!

03-30-2020, 09:59 AM - 1 Like   #2
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Ok, riddle solved. Sigma did not extend the aperture simulator as far as needed. It is only a short slit. Thus the aperture simulator on the P30 remains at aperture 1.4. Wether the camera can not close the aperture further, or it will not, because the lens thus indicates 1.4 as smallest aperture I can not say. But obiously the P30 can only use aperture 1.4 on the Sigma. What a shame.
Metering however is fine.
03-31-2020, 07:36 AM   #3
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I just had a look at my Sigma 70mm F2.8 EX DG macro. The aperture simulator looked like to be expected (longer version).

Mounted on the P30, the lens behaved like it should in program mode, i.e closed the aperture.

So, Sigma knows, how the KAF mount should look like. I wonder why they did not make the 35 mm Art backwards compatible?

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35mm art, aperture, camera, f-stop, film, k-70, lens, p30, sigma, sigma 1.4 35mm
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