I do want one of those, you know I think I was operating under the misconception that m42 lenses didn't work with Pentax green button stop-down metering, but I've just seen info that contradicts that.
The 50mm eight-element Super-Takumar is just out of this world. I was able to try one on my brother's K-1 and it's a dream wide-open. The reviewers are not lying.
As long as a Pentax body has a Green Button, stop-down metering is possible with M42 lenses. I use my M42 lenses in Manual and it still zeroes in the exposure after every press. There isn't that aperture sound, though, just like when using manual K-Mount lenses. The M42 lens must have a Manual mode for stopping-down where the OVF will dim-out accordingly.
M42 lenses can be conveniently used with Av, too, that's why they're popular.
Many K-1 owners exclusively use their cameras for M42 lenses, which have the same 45.46mm flange-back distance as the K-Mount. In that case, the adapter is just actually a converter as it stays on the K-Mount when installed.
One the of the reasons I went to a K1 was the green button and focus confirmation beep on legacy glass.
That's how it is with Pentax dSLRs. The K-1s and K-3 III have that focus confirmation light on the whole OVF, whereas with older ones, just like my K-3, there is just the AF point lighting up in the middle. There is the confirmation beep, too.
Catch-In focus is a handy feature for fully-manual lenses, M42 and PK-M, as well. After pressing the Green Button, one can just hold the shutter button even before focusing and the shutter releases as soon as the subject is in focus in the centre AF point. For Catch-In focus to work on modern lenses, the lens must have an AF and Manual button because the bodies use the AF module. Pretty handy.
The Green Button is also very convenient for lenses after PK-A, or when the aperture can be controlled in the body. With other cameras, one must adjust the exposure triangle without shortcuts to zero-in the exposure or at least to adjust it. With the Green Button, even if the light meter shows that the current exposure is 4EV stops overexposed, as soon as the button is pressed, it will just zero-in at 0.0EV. It makes manual exposure lovely and convenient.
The Green Button has more features than that but my favourite is when I use manual lenses and manual exposure.
pentaxrumors.com
