Hi RTogog,
I have searched hard for a comprehensive guide on this forum for your question but couldn't find it, so here are some part-answers.
RTogog said: Quote: So, the other option is to use Screw mount w m42 mount. I compared both Screw mount m42 on Pentax or Pentacon have "needle" to manage the aperture when set on AUTO mode. Does this needle have function on DSLR?
No, it does not have any function on any DSLR. RTogog said: Quote: What is the setting mode for acquire correct exposure (w m42 mount lens) on DSLR?
It is either AV-mode and around +1.5 EV depending on the lens or M-mode.
You gotta dig into the menues to get the M42 going. Here is the do-or-die setting first,
K100D (Super) Pictorial guide to using manual lenses [imgs]: Pentax SLR Talk Forum: Digital Photography Review
Note: The rest of the above link talks about "manual lenses" which is PK-mount lenses. That's a different thing to M42!
Here's some more about it,
https://www.pentaxforums.com/forums/pentax-dslr-discussion/5423-m42-lenses-k100d.html
You gotta find a PK-M42 adapter that sits flush on the PK-mount. If you get an adapter with a collar/flange, you will not get infinity.
The M42 lens should have an "A-M"-switch on it (read: Auto-Manual-switch). This allows the lens to be stopped down on a DSLR. You can check it on any M42 lens by turning the aperture ring to say f/8. If the lens is switched to "M", the aperture will close down synchronous to your turning of the ring. If not, the aperture will only close down when you push down the needle you have talked about (read: aperture pin).
Hope you are not discouraged by all this, I am a big fan of those M42 and for me it did help me pick up the basics of photography faster.
Long lenses for cheapskates: M42 MC Enna/Revuenon 300/5.6 on K100D
[IMGWIDEleft]http://farm4.static.flickr.com/3243/2895197126_875cdbcf15_b.jpg[/IMGWIDEleft]
Cheers, Georg (the other)