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10-15-2016, 10:30 AM   #1
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Sigma 50-500 OS HSM and Pentax HD DA 1.4X TC focus dance

I have searched and searched and didn't find anything related to this topic.
I already had a DA 1.4X TC and then got the Bigma. I tried it right away with the TC but I thought it might not be compatible.
The phenomenon is the following: Natively the Bigma focuses fast and accurate with very little hesitation. Putting on the Pentax TC focusing is just as fast as before but it only hunts around the focus plane running over the focus then quickly turning back over and over again giving up after a few cycles and reporting no AF lock (little green hex in the viewfinder blinking). The hunting doesn't go out to focus limits, stays near the focus, seems like the AF sensor might catch up the data but the focus is too fast for it.
Then I heard about the Sigma mount replacements for the K-1, K-70 etc... and also the related firmware updates.
My Bigma also has a bit annoying behaviour, it doesn't tell the camera exactly the right lens code. In Lightroom I have to select manually the lens profile.
Now, I think this might be connected, having no perfect communication from the lens chip might result in the TC not recognizing it and screwing up the AF.

Can someone else confirm this with the same combination? Does it work with the Sigma 1.4X TC?
Anyone with a rechipped/firmware updated Sigma HSM lens can tell if it behaved similarly with the Pentax TC?

I bought the Bigma from SRS Microsystems, I will also contact them about this issue after this little survey.
Thanks in advance for your replies.

10-15-2016, 10:33 AM   #2
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I get exactly this behaviour with the Sigma 17-5/2.8 + HD 1.4x on my K-3...
10-15-2016, 01:17 PM   #3
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QuoteOriginally posted by BigMackCam Quote
I get exactly this behaviour with the Sigma 17-5/2.8 + HD 1.4x on my K-3...
Is it the same on the K-3II?
10-15-2016, 02:18 PM   #4
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With the 1.4 TC when I had the Sigma 150-500 it would work, but only with a high contrast target in decent lighting. Tonight if it is clear go out and see if it will focus on the moon for you. I bet it will. I also bet your aperture and focal length will be incorrect though.

10-15-2016, 03:16 PM   #5
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QuoteOriginally posted by Sterby Quote
Is it the same on the K-3II?
I haven't tried it, but if I get a chance I'll test it tomorrow
10-16-2016, 01:25 AM   #6
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The HD 1.4 doesn't support Sigma HSM, though, surely?

10-16-2016, 01:35 AM   #7
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QuoteOriginally posted by clackers Quote
The HD 1.4 doesn't support Sigma HSM, though, surely?
Well, the HSM motor operates - but the lens hunts and fails to reach focus...
10-16-2016, 02:29 AM   #8
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I thought the DA 1.4X AF compatibility is recommended for lens with F4 or faster aperture?

(Copied from our review page)

AF compatibility:

The lens must be F4 or faster. Slower lenses can be used, but phase detect autofocus may be unreliable and contrast detect AF (live view) is recommended instead. Curiously, the smc Pentax-DA 17-70mm F4 may not acquire focus in contrast detect mode (live view), here phase detect (viewfinder) should be used.

10-16-2016, 02:48 AM   #9
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QuoteOriginally posted by bigfoxkiB Quote
I thought the DA 1.4X AF compatibility is recommended for lens with F4 or faster aperture?
...
The lens must be F4 or faster. Slower lenses can be used, but phase detect autofocus may be unreliable and contrast detect AF (live view) is recommended instead.
"May" is the key word here. I use the HD 55-300 + 1.4x converter in decent light without any problems.
10-16-2016, 02:59 AM   #10
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QuoteOriginally posted by BigMackCam Quote
"May" is the key word here. I use the HD 55-300 + 1.4x converter in decent light without any problems.
Nice, was about to ask about the 55-300 with the 1.4X too. Want to bring my peeping Tom level up.
Did you notice more focus hunting in lower light situation?

And I am wondering about the 17-50 from Sigma too, since it was in one of my wanted list before I decide to go for HD DA16-85.
It seems the problem with the 1.4x is more on Sigma?
10-16-2016, 07:52 AM   #11
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QuoteOriginally posted by bigfoxkiB Quote
It seems the problem with the 1.4x is more on Sigma?
This is what I am thinking too. If it was a slower-than-F4 problem then it would lock on in liveview but the AF just drives from one end to another, only locks on when I pull back the focus by hand at an appropriate moment (kind of manually altering the PID loop of the AF drive). In OVF phase detect the focus is just too quick to step in like that.
My theory is the TC doesn't slow down the AF which it should and does on all my other lenses. All my other lenses are screwdrive though. Again, natively it works just fine, quick and snappy both in OVF and liveview.

I bet if the lens communication was okay the TC would step in and slow the AF accordingly.
10-16-2016, 08:08 AM   #12
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I have had very little success with the 1.4 TC on ƒ6.3 lenses.

As a result, my last two lens purchases have been ƒ2.8 lenses.

I keep telling folks, if you buy a 150-600 or 50-500 or any ƒ6.3 lens you can't reliably use a TC but do people listen?

I know this is tough for the low end users, but, if you are going to use a TC, your best options are ƒ2.8 glass, with ƒ4 glass being the second choice. Your AF system needs light to work, and ƒ6.3 with a TC wide open just isn't enough light. And there's nothing you can do with about it.

I whined and cried about it for years before i accepted that, it's not fair, it's not negotiable, it just is what it is.
10-16-2016, 08:39 AM   #13
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It does the same thing at f/4.5 at 50mm. I'm fairly sure it is not the amount of light. Also my 50-200/4-5.6 works okay in lower light on the long end with the TC as the others say the 55-300 does as well.
And it should focus in liveview CDAF at f/8 or lower since it doesn't rely on the phase detect AF sensor.
So far the Sigma 50-500 and 17-50 does the same and have the common problem of not reporting the lens code properly to the camera. This cannot be coincidence.
10-16-2016, 09:34 AM   #14
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I said 6.3 won't work with a TC, 5.6 works fine.

As for the TC messing up the lens info, ya, it happens. But all my lenses with messed up lens info still focus correctly. My F 1.7x adapter messes up almost all lens info and I use it quite a bit with no problems.

When I bought my Tamron 300 ƒ2.8, I bought it because I very successfully use my DA* 200 with stacked TCs, producing 2.4x magnification.I thought I'd be able to do the same with the Tamron. The Tamron info says the 300 ƒ2.8 works poorly with stacked TCs, and it does. SO clearly there are predictable factors in the design of the lens that make it suitable or unsuitable for use with TCs, or stacked TCs. So my guess is both things are happening. The 50-500 is not a design tha plays nicely with TCs, and an ƒ6.3 lens is only partially useable with a TC.One does not cancel out the other. But it is possible you are being affected by both things.
10-16-2016, 11:30 AM   #15
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I wouldn't dismiss it so easily, today's AF sensors can work in -3EV, f/11 shouldn't be a problem (though reduced to only center point) let alone near f/8. Also the Sigma 17-50 was reported above having the same problem and that is an f/2.8 lens.
My observations indicate the AF picks up the focus since it dances around the focal point closely but seems like the AF is too fast, something having to do with the AF motor control having bad PID settings, probably too much linear (P) value for the incoming signal.
This is no TC problem because if it was the TC being faulty the same problem or something similar should occur with some of my other lenses which is clearly not the case. So far it looks Sigma related.
I wonder if Sigma had an actual license for the KAF3 mount or reverse engineered it. The latter would explain how this problem occurs in some of its lenses.
But until this problem is solved (if it is ever solved...) there is still manual focusing and focus confirmation, which works and the lens gives a good image with the HD 1.4X TC.
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