Interestingly, my manual focus Sigma 14mm f3.5 worked fine with this TC, as did my Tokina 400mm f5.6 and Kiron 105 f2.8 macro (that's all the A compatible non-Pentax lenses I have.) You'd think if Sigma could make it work right with their A compatible lenses, they could get it right for their AF lenses as well.
As you no doubt know, this TC does translate aperture setting to the effective aperture when coupled with an A or higher lens. Apparently this does not work so well with the new Sigmas. I don't know what that does for exposure /aperture settings - the other Pentax TC's do not convert aperture data to the effective aperture, and I've used them without problem in the past.
- MCC
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Mark Cassino Photography
Kalamazoo, MI
www.markcassino.com
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----- Original Message ----- From: "Jostein" <[EMAIL PROTECTED]>
To: <pentax-discuss@pdml.net>
Sent: Sunday, April 17, 2005 5:46 AM
Subject: Sigma lens oddity
A couple of days ago I experimented with the AF 1.7TC, the autofocus teleconverter, on the *istD. My Pentax lenses worked fine, but my Sigma EX 70-200/2.8 did not work well with the TC. On full opening, the aperture reported was f/2.8. When stopped down, however, I could select f/40. That's 2/3 stops more than the lens supports on it's own.
Today took readings with MZ-S, and the same happens there.
Also, exposure was way off at some apertures It overexposed badly on full opening, then worked fine at f/4 to f/16, and then gradually began to overexpose again. At f/32 it was at least as bad as at f/2.8, and the "f/40" was useless.
Anyone else had the same experiences?
Jostein
