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10-14-2008, 09:19 AM   #1
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How slow is the AF on a Sigma 70-200 F2.8 using the 2X APO Teleconverter?

I was thinking of picking up a TC for my Siggy DG EX (non-macro) for when I go on vacation towards the end of the month. I'm hitting a state park and the local zoo during that time-frame, and I was thinking the extra reach would be nice to have, but I'm worried that the extra glass would slow down my K20d's autofocus performance-that lens ain't exactly zippy to being with. I initally thought of snapping on my Tamron 1.4X AF TC on it, but I'm not sure if it'd handle a lens that heavy.

Now I know the 2X TC would make my shots a little softer (especially wide-open) and possibly introduce some CA, but I'll be shooting in RAW (I'm finally comfortable with that idea), so those issues shouldn't be particularly troublesome. I'm also planning on shooting using +2 exp compensation to counteract the light loss & I'm bringing at least a monopod with me for this lens and using Shutter-Priority to compensate for the extra length (Pentax, fer cryin' out loud, give us the ability to tell the SR system that we're using TC's!).

If it turns out that the 2X TC would slow down the AF to glacial-speeds (I'd have no way to test this unless I buy the TC 1st), I'd just as soon use my Tamron 19AH with matching 2X TC that I have for it-my Tammy's nice & sharp wide-open, and the 2X TC for it doesn't really have that much of an degrading effect on it, but I still have some problems nailing manual-focus. Whaddaya guys think? Thanks in advance.

10-14-2008, 11:43 AM   #2
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QuoteOriginally posted by Grimlock Quote
I was thinking of picking up a TC for my Siggy DG EX (non-macro) .....

I'm also planning on shooting using +2 exp compensation to counteract the light loss ....
The statement in bold makes no sense. Care to clarify?
10-14-2008, 12:32 PM   #3
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Well, since a 2X TC reduces the amount of light hitting the imaging surface by 2 f-stops, wouldn't increasing the exposure compensation by the same amount counteract that?
10-14-2008, 12:36 PM   #4
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Nope, the meter will see that it suddenly got 2 stops dimmer and automatically compensate.

That's the beauty of TTL metering

Same thing if you put a Circular Polarizer on your lens; or a Neutral Density filter, or turned off the lights

10-14-2008, 12:41 PM   #5
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I have found from a lot of experience that except in deep shade (perhaps a forest where the trees block out a lot of the light) the AF performance is not an issue with the 2X TC on my Sigma APO 70-200 F2.8 EX (origonal - non macro non DG rear element coated).

It works just fine! as does the 1.4X TC. I have both sigma TCs.

A couple of things to look out for are specifically with respect to metering.

I have found on my K10D that the addition of a TC modifies the metering and the K10D tends to over expose with a TC attached. WIth the 2X TC I use -1.3 stop (i.e. under expose) on the compensation to get back to correct metering.

The metering issue is a long story on the K10D, related to maximum apature and focusing screen. WHat ever you do, test your camera against a uniform target with and with out the TC on the lens and make sure you know how it behaves before you go out shooting.

As for image quality, see below.

https://www.pentaxforums.com/forums/187646-post19.html

https://www.pentaxforums.com/forums/208249-post1.html
10-14-2008, 12:49 PM   #6
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QuoteOriginally posted by egordon99 Quote
Nope, the meter will see that it suddenly got 2 stops dimmer and automatically compensate.

That's the beauty of TTL metering
except on the K10D, and perhaps the K20D since it uses the same focusing screen, there is a problem with metering as a function of actual F stop, and adding the TC introduces an error not related to the 2 stops of light lost, but related to the fact that the K10D metering is not linear with F stop

see below

https://www.pentaxforums.com/forums/241716-post69.html
10-14-2008, 04:32 PM   #7
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If the conditions are brightish it shouldn't be too bad. I have done rugby with the combo, and as long as the sun was shining all was okay (not great, but okay). I would think with zoo and park as targets you will be fine.
10-14-2008, 08:00 PM   #8
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Thanks for the info, guys.

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