Disabling auto-diaphragm in M42 screw mount lens

Started Nov 9, 2004 | Discussions thread
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Zaldidun
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Disabling auto-diaphragm in M42 screw mount lens
Nov 9, 2004

As per some tips by Richard Allen found here in these forums, I've been converting some two dozen prime lenses from M42 Pentax Universal screwmount to K-mount by means of an adapter with its retaining spring removed and a the filing of a small groove to catch the retaining pin on the camera body K mount, so that the adapter remains permanently on the lens not in the body mount.

( http://forums.dpreview.com/forums/read.asp?forum=1036&message=10585399 )

Photo ©2004 Richard Allen (Thank you!):

So far I have converted some 14 lenses.

As forewarned, because of the smaller diameter barrel of some M42 lenses, this is only possible by means of the use of a different type of adapter (see illustration below) which then places the lens a few millimeters foward, preventing it from focusing to infinity. This still gives me, e. g., a very nice, fast Asahi Super Takumar 105mm f/2.8 workable prime lens for use at relatively shorter ranges. Perfect for people shots where you want the background to disappear. The lenses are still freely usable all the way to infinity by using unmodified Pentax adapters as designed, of course.

Now I have hit a stumbling block in the case of a particularly sharp and fast 35mm Japan-made Hanimex wide angle lens that does not have a manual-auto diaphragm switch, so that it is effectively always on auto-diaphragm. It's beautifully built. The problem is that, on the *istD, it can only shoot wide open at F/2.8, because the K-mount diaphragm closing mechanism does not active the one on the M42 lens. I'm wondering if anyone can suggest a means of disabling the auto-diaphragm feature short of disassembling the lens and taking out the auto mechanism. Any tip will be appreciated.
Thanks in advance.

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