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03-21-2011, 07:01 PM   #1
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Manual M, K, and AF lens question

I have been researching some lenses and came across this link The Pentax Camera Lens Compatibility Chart which has a chart for Pentax lens compatibility. I see a lot of older Pentax-M lenses for sale everywhere. The lens compatibility chart says that the M, K, and AF lenses are compatible with current dslr cameras but there are limitations in metering. The yellow check mark on the bottom left of the chart is where I am looking. For all those that have these lens types, what are the limitations that they speak of. Is it the stop down in metering that they say, and what exactly does that mean? Thanks for the help.

03-21-2011, 07:43 PM   #2
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Hi Ratm:

M and K lenses are pretty much the same, as I believe the K lenses came out right after Pentax switched from the M42 screw mount. AF is as it seems, auto focus. However if it the KA lenses, these are different.

On your Kx, the AF lenses should be perfectly compatible, except with the understanding that the Kx has a smaller than 35mm sensor and there is a multiplying factor of 1.5 to determine the effect the lens would have if comparing it to the 35mm frame (e.g., a 100mm AF lens would have an effective focal length of 150mm on the Kx, K7, K5, Kr).

The M and K lenses can be used on the Kx in the Av and M modes, but are manual focus. To determine the proper exposure, make certain the camera is set to allow for the use of the aperture ring ( see your manual, possibly in the appendix). Set the aperture you want to use and then press the GREEN button. The lens will stop down and a shutter speed will be chosen by the camera. You can change things from there. The KA lenses can be used as a more modern lens by keeping the lens set to "A". Of course on these lenses, focus is done manually.

If you get real ambitious you can use older screw mount lenses with an adapter (get the actual Pentax one).
03-21-2011, 08:59 PM   #3
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Pentax K-Mount Lenses Explained: The differences between various Pentax lens series

The limitation is that you can't use the aperture ring the way it was meant to be used, since the camera has been stripped of the contacts necessary to detect what the aperture ring is set to.

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03-22-2011, 11:15 AM   #4
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Big Dave has an error of sorts

K and M lenses when used in Av mode shoot at maximum aperture all the time. In manual mode you can press the green button ( on cameras that have one) or the AE lock on ones that don't to stop the lens down momentarily and set shutter to match, in both axes you can also stop the lens down with DOF preview and meter without changing settings

KA lenses work just like present lenses

The othr big issue is flash. Except for the *istD cameras which support both TTL and P-TTL you cannot do flash with K and M lenses and have the camera control flash intensity

03-22-2011, 07:52 PM   #5
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Thanks for the clarification on the K & M lenses for me. I am working mostly in manual with the K7, so I have the best of both worlds in this area. And with the flash, this makes sense (just started doing more work with a fully functioning PTTL flash).


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