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11-16-2009, 07:03 AM   #1
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Does anyone know what the default f-stop is when you place an old manual teleconverter or close-up ring between a pentax dslr and a newer digital lens? There doesn't appear to be any way to adjust f-stop when this is done. Also, does anyone have experience with a teleconverter such as the Kenko 2x DG and a lens that is just under 100mmm long, such as the Tamron 90mm macro?

11-16-2009, 07:06 AM   #2
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When using the old non "A" converters, you have to set the aperture manually with the aperture ring on the lens.
11-16-2009, 07:12 AM   #3
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There is no aperture ring as such with the newer auto focus digital only lenses.
11-16-2009, 07:18 AM   #4
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It will shoot wide open all the time. The camera has no way detecting anything without contacts on the converter.

11-16-2009, 10:54 AM   #5
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I'm moving this to the DSLR forum, since that seems more appropriate. I suppose I could have moved it to the lens forum, but this seems perhaps a DSLR-specific question.

Anyhow, I'd agree the combination of a completely manual TC (one with no aperture contacts) and lens without an aperture ring is not a good one - you'll have no way to control aperture at all. It will either be wide open or fully closed, depending on whether the TC is pushing the aperture lever or not, I suppose. I'd expect a K-mount TC to push the lever. I suppose if for macro photography you preferred the aperture fully stopped down, you could modify the TRC to do that.

I haven't used the Kenko 2X TC, but given that you lose two stops of light with it, I'd expect that combining it with a 90/2.8 lens would yield a 180/5.6 lens that was in most respects noticeably worse than the DA50-200 or any other consumer zoom lens at that focal length. But you would of course keep the minimum focus of the macro lens, so you'd now have more magnification.

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