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10-05-2009, 04:15 PM   #1
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K20/FA* 200mm with Teleconverter

I picked up a DA 200mm Pentax lens last month to give myself a faster (2.8) telescopic option than my Tamron 70-300 zoom (F5.6 at 300). Really happy with the 200mm in low light situations (i.e. shooting sports in stadium lighting ) and the smoother AF is very nice as well.

But sometimes I yearn for just a little more reach so I thought I’d try the 200 with a 2X teleconverter (knowing full well the sacrifices that entails.) I picked up a Kenko MC7 model based upon some positive reviews I found.

However I ran into an interesting problem and am curious if anyone has any insight:

I can attach the TC with the 200 lens to my *ist and everything works fine. By that I mean the AF works and I can take a clear picture. The AF process seems a bit slower and it does hunt a bit more with the TC on.

When I try the same TC & 200 on my K20, however, I can't get the combination to work. Not only won't AF or Live View work, I can’t even use the lens in MF as the shutter won’t engage. When it's fully connected, the MF displays (despite all the appropriate settings on camera and lens) and blinks. If I try to push the shutter release, the viewfinder just goes black.

I took the TC and lens down to the local camera store and tried them together on a K7 and got the exact same (non-) results as the K20.

However, both my Sigma 17-70 & Tamron 70-300 work (and very nicely) with the K20 & TC. Again, I mean, AF, Live View, and picture taking ability.

My first thoughts were that something isn’t seating correctly. But I’ve repeatedly experimented with it enough to convince myself that isn’t the issue. Since the TC/lens work on the *ist, it’s clear they can play nice together and that the TC-to-lens interface isn’t the issue. Ditto regarding the camera body-to-TC contacts since the K20 and TC play nice with the other lenses.

I’m wondering if there’s something in the K20 (and K7?) firmware that can’t recognize the DA lens when the TC is in the middle.

Just a little disappointing that it works on the *ist but not the others, newer models. Wish I had my old K10 to give it a try but I've already given that away to my nephew.

Anyone have any thoughts on what's going on? Appreciate any thoughts you might have.

Regards,
-tom


Last edited by tpeace; 10-07-2009 at 01:53 PM. Reason: spelling/lens type correction
10-05-2009, 08:00 PM   #2
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A while ago I tried to modify my Kenko MC7 converter to work with SDM lenses on the K20D (either as SDM or screw drive AF, would have been happy with either of them).

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/38514-converti...erter-sdm.html

I made the same observation as you did: it does not engage AF of any sort on the K20D, while it works on a *istDS (screw drive AF). After removing different parts of the converter (SDM/power zoom contacts or screw drive transfer or all A pins or just the 6 first A pins or the 7th data com A pin, or combinations thereoff, and the electronic chip inside the converter), I came to similar conclusion: the K20D (probably all KxxD models) ID this as a SDM lens and forbids the screw drive to engage. It does not help to remove the SDM contacts on the converter. If you try to hide what lens it is by removing the 7th A contact, it will regard the lens as a K mount lens without A contacts or AF of any sort and not engage any AF. My guess is that the engeeneers at Pentax wanted to protect the SDM lenses from any risk that the SDM AF and screwdrive AF engage at the same time, which might hurt the lens. It is however anoying.

The glass of my Kenko 2x MC7 converter is now resting in a box among other spare parts, while the rest of the converter serve as a screwdrive AF capable extension tube (https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/60715-dfa-100m...airy-legs.html).
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