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08-28-2015, 03:00 PM   #1
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Sigma Zoom 100-300mm 1;4.5-6.7 UC. AF or not?

So I bought this lens on eBay and it arrived today but I'm trying to figure out exactly what I have. Cosmetically and optically, it's great but the seller advertised it as being an AF lens and my three Pentax cameras don't recognize it as such. As soon as I put it on the camera, it gives me a screen to input the focal length. Depressing the shutter button does not activate any autofocus mechanism; however the camera does accurately set the exposure in any of the automated exposure modes. The way I see it, there are two possibilities here. Either it's a manual function lens or the auto focus is totally non-functional. I'd like to know which it is.

The lens is a modern, mostly plastic design and looks like it came off the same workbench as my Sigma-designed Quantaray 28-90mm Aspherical zoom. I have another Sigma UC zoom lens, a 'Sigma UC Zoom 70-210mm F4-5.6 across the front, although that lens is a push/pull zoom and the 100-300 I just is a two-ring design. The markings on the front of the lens are as they appear in the subject line with 'UC' at the end instead of between 'Sigma' and 'Zoom'. The contacts on the base are also a little different. There are six contacts, all spaced evenly, on the 100-300; whereas there are seven contacts on my 70-210 with the second and fourth from the left either blanked out or grounded to the lens base. The other five contacts appear to be surrounded by plastic insulators, as are all six of the contacts on the 100-300. The contact configuration on the 70-210 is also consistent with that of my other Pentax and Tamron AF lenses.

So is this a manual focus lens or an autofocus with that function broken or disabled? Knowing exactly what it is will help me decide to either keep it or sell it.

08-28-2015, 03:04 PM   #2
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Does the lens have an AF shaft? That would be the easiest way to tell. Sounds like it's a KA-type lens, though.

Adam
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08-28-2015, 03:06 PM   #3
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QuoteOriginally posted by E-man Quote
As soon as I put it on the camera, it gives me a screen to input the focal length.
Hmmm...that should be your first clue. K-mount AF lenses all have a data pin that supplies the focal length to the camera. Does the mount on your lens look like the diagram below?




FWIW, a little Google work indicates that the lens AF, though there are a few that reference a MF version.

Steve

Last edited by stevebrot; 08-28-2015 at 03:13 PM.
08-28-2015, 03:09 PM   #4
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That lens is AF, unless there are two versions. Put the aperture ring in 'A' position.

08-28-2015, 03:19 PM   #5
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QuoteOriginally posted by vonBaloney Quote
Put the aperture ring in 'A' position.
This is highly probable
08-28-2015, 03:45 PM   #6
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This MFer is an MF

Thanks for your quick replies. Mystery solved. It is indeed a (perhaps rare?) KA-type manual lens. Comparing the base of my lens to your diagram, there is no AF transmission shaft and the contacts are different. All six of the contacts on mine are evenly spaced, whereas your diagram shows a gap between pins 5 and 6 (going from left to right). And yes, the aperture ring was locked in the 'A' position when I took my test shots.

So here's another related question. Since it's a manual focus lens, what should I enter as a focal length? All three of my camera bodies seem to assume that all manual lenses are primes. Should I enter 100? 300? or split the difference at 200. Or does it even matter? I am assuming the camera uses focal length data to help it calculate exposures.


QuoteOriginally posted by stevebrot Quote
Hmmm...that should be your first clue. K-mount AF lenses all have a data pin that supplies the focal length to the camera. Does the mount on your lens look like the diagram below?




FWIW, a little Google work indicates that the lens AF, though there are a few that reference a MF version.

Steve

Last edited by E-man; 08-28-2015 at 03:51 PM. Reason: additional info.
08-28-2015, 04:43 PM   #7
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It uses the focal length for the shake reduction, nothing more (and it records it in the EXIF image data). The particular pin arrangement is different for each lens, so it would necessarily be the same. The thing you're looking for is the screw-drive slot, which is not a pin but like a recessed flat-head screw head -- the screw drive from the camera fits in that slot to drive the autofocus. Does the lens per chance have a little AF/MF switch on it? (Or a push-pull mechanism? Do you see "MF" or "AF" anywhere on it?) I really think that is an autofocus lens...
08-28-2015, 05:50 PM   #8
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QuoteOriginally posted by vonBaloney Quote
Put the aperture ring in 'A' position.
QuoteOriginally posted by LeDave Quote
This is highly probable
Should not be. The prompt for focal length means that the camera cannot detect the data pin. AF should still work regardless of the aperture ring position.


Steve

08-28-2015, 05:59 PM   #9
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QuoteOriginally posted by vonBaloney Quote
The particular pin arrangement is different for each lens, so it would necessarily be the same.
So true. It is also possible for the lens to have the seventh (data) contact and not be an AF lens (KA2 mount), though in that case, it should not prompt for the focal length. In any case, ALL (save one) Pentax AF lenses will have the seventh (data) contact. That being said, the lens in question should also have the drive pawl if it is AF*. The lack of which should be proof enough.


Steve

* The 100-300 uc is a bargain lens and does not have its own drive motor.
08-28-2015, 07:22 PM   #10
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With the information gathered in this thread, there is no doubt in my mind that this is indeed a manual focus lens. The flathead screw drive pawl is most definitely not present on this lens. It has no AF/MF switch and there are no markings whatsoever on it to indicate AF or MF, which is what was confusing to me.

It sounds like '300' would be the better number to enter if the camera just wants it for shake reduction purposes.

It's interesting that my two Sigma UC lenses are so radically different from one another. This 100-300mm lens is a 'two-touch' lens with separate zoom and focus rings. My other Sigma UC lens, the 70-210mm, is autofocus with push-pull zoom and a separate focusing ring.

I've pretty much decided not to ask the seller for a refund on the lens, even though being a manual lens, I think paid on the high side of what it's worth at $35 including shipping. I'm going to play with it some, see how I like it, then decide whether to keep or sell it. Heck, it's such an oddball lens I may just keep it as a novelty. That and the test shots I took with it this afternoon weren't half bad and would be better if I stopped the lens down more. In Program mode, the camera seems to want to set the exposure around as wide an aperture as possible.
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08-28-2015, 08:31 PM   #11
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QuoteOriginally posted by E-man Quote
It sounds like '300' would be the better number to enter if the camera just wants it for shake reduction purposes.
Set to close to where you're actually going to shoot or somewhere in the middle (feel free to turn camera off/on to change it anytime) -- if you set at 300 and shoot at 100, the SR itself will overcompensate and give you blurry images. Whereas if you set it at 100 and shoot at 300, it won't really hurt, it just won't help that much either.
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