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10-23-2017, 11:13 AM   #1
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Sigma 28 DG AF issues & what to do next

Does anyone have a handy guide for figuring out if the AF performance of my newly purchased Sigma 28 EX DG refurb is sub-par? I've had the lens about a week now and I've had a lot of missed shots or generally oddball AF misfocusing on my K-5 II that no other AF lens seems to have. I've adjusted the camera to correct for a mild backfocus issue with this lens specifically (went -3 to correct) and I'm happy with that aspect. When it hits focus it's sharp and things are great. Often when I lift the camera, half-press the shutter to activate autofocusing, the lens will hunt, and many times would settle on something that was clearly not in-focus.

This could happen at max aperture or something like f8, makes no difference. Center point AF used probably 90% of the time, never with Live View, until I tried playing with it a bit last night. Live View doesn't seem any better, just different. Multiple AF points used also seems worse. At times the lens would hit focus but maybe 50% of the time it wouldn't. Over the weekend normally lit areas as well as darker rooms all had this issue.

The lens was sold to me as a refurb item that Sigma is selling through Adorama. I would like to do a comparison between this and my DA 35 f2.4 as that is about the closest lens I have as far as focal length and max aperture to ensure that this is an issue with the Sigma specifically.

I'm excited about this lens, in theory, but in practice I was really let down over the weekend by coming home to so many unfocused shots. In short, I'm looking for a walk-through or test procedure to follow to determine if this is a salvageable combination or not.

10-23-2017, 05:21 PM   #2
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I've had the Sigma 24mm f/1.8 EX DG for many years and on many cameras, including the K-5 and K-5 IIs, with no AF issues whatever. Even when shooting fast-action indoor roller hockey on numerous occasions. I cannot imagine that your 28mm Sigma, designed and released during the same time period, would have a different AF technology!
10-23-2017, 06:02 PM   #3
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I took it out this evening for a walk around in lower light situations and it performed pretty well. A couple shots weren't well focused straight off, and it will just stay there, but if I aim the AF at something different then back to the subject, it found focus. I'll keep playing with it and seeing what I can get out of it. I guess the world's supply of Pentax F 28's probably isn't changing that fast, if I decide I want to try something else. That's probably the lens I would try next, but I really like having something faster than f2.8.

I did take this with focusing via the AF system. Taken at f1.8. Really smooth out of focus, a bit of green and purple fringing;

10-26-2017, 04:05 AM   #4
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So I've done some low light testing, not entirely scientific, where I would walk around the house in the evening and take ten shots of something with text on it to see how well the K-5 II and Sigma focus. 9 of the ten shots were in focus, clearly, and the last one was somewhat questionable. I'm getting comparable results with my DA 50 1.8. Center point AF seems to be the way to go; "auto" multipoint ends up being a focus hunt fest.

I'm wondering if there was something on a contact on the lens mount or what was going on the one day I had real issues with this lens. Right now I'm really liking the lens, everything about it save for the size/weight. I'm trying to hone my lens collection down five or six lenses total and this is the physically largest of all of them. In any case I think I'll consider the issue closed for the time being.

10-26-2017, 05:01 AM   #5
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I have this same lens, but have only ever used single/center-point focus on my K-3/K-50 with it and have had above-average focus rates with it.... in my review of the lens, the only negatives I could find were its size/weight and the kludge that is its manual/autofocus clutch...
10-26-2017, 05:31 AM   #6
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I wouldn't mind the AF/MF slide clutch setup if I didn't also have to swing the AF switch on the camera to actually manually focus the thing. It's odd, I can swing the AF/MF switch on my K-5 to AF from MF without changing how I am holding the camera. I never seem to do it the other way, so it's an away from face, play with the switch, then compose again setup.

I wonder what this lens is like to focus on a K-1 or KP, if it's any faster or more accurate, etc.

As for the size, I don't know... I may have been smarter to buy an F 28 f2.8 but they're about the same price in excellent condition, and I really wanted that shallow depth of field that f1.8 offers. I picked up my Kalt 28mm after taking the Sigma off the camera and it was like, wow this lens feels tiny!
10-26-2017, 06:14 AM   #7
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QuoteOriginally posted by pres589 Quote
I wouldn't mind the AF/MF slide clutch setup if I didn't also have to swing the AF switch on the camera to actually manually focus the thing. It's odd, I can swing the AF/MF switch on my K-5 to AF from MF without changing how I am holding the camera. I never seem to do it the other way, so it's an away from face, play with the switch, then compose again setup.

I wonder what this lens is like to focus on a K-1 or KP, if it's any faster or more accurate, etc.

As for the size, I don't know... I may have been smarter to buy an F 28 f2.8 but they're about the same price in excellent condition, and I really wanted that shallow depth of field that f1.8 offers. I picked up my Kalt 28mm after taking the Sigma off the camera and it was like, wow this lens feels tiny!

agreed - the size can be onerous, especially if you are used to more 'normal-sized' primes...
10-26-2017, 06:37 AM   #8
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I actually set my core kit out on the desk last night.
DA15 for UWA duties
DA50 1.8 for portraits
DA18-135 for do-all and poor weather shooting
And then the Sigma 28 EX sitting there in the back for normal field of view and wider portraits. Making all the others look small to tiny.

I may pick up an F or FA 28 at some point assuming something else doesn't come along, and just see if I really like using one of those more than the Sigma, all due to size and weight. The other lens I've become much more interested in lately is the DA 21. And aside from a couple old oddballs, that will probably be every lens I own, eventually.

10-26-2017, 10:33 AM   #9
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Regarding the bee photo you present as an example- in this case, focus would be unusually difficult with any lens compared with a more common use. It is a closeup (this Sigma WA lens series, including my 24mm, has unusual closeup capabilities) so right there depth of field becomes much more shallow, then shooting at f/1.8 it becomes even more so. Focus would go in and out, off and on, upon your slightest movement as well as that of the subject. To try out this closeup capability of your lens more fairly, it would be best to put the camera on a tripod with a still subject. And fire the shutter by remote control or use the 2-sec timer, which also has mirror lockup.

The Sigma 28mm you have is a very good one for low light use, and its wide aperture capability is the reason for its larger size. Together with the other fine lenses you list, you have a very good kit of lenses indeed! The Pentax FA 28mm f/2.8 is optically a very fine lens, but does not have the exceptional aperture range of your Sigma, nor the closeup capability.

Last edited by mikesbike; 10-26-2017 at 11:23 AM.
10-26-2017, 06:24 PM   #10
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No, the bee picture was not something I would call "shot optimally", done freehand quickly. I think the fur on the bee shows that it's got decent sharpness up close and wide open.

If I can get used to the bulk, I think the Sigma is definitely good at holding up it's end of the range of my lenses. If a similar lens comes from Pentax at some point I'll probably have to buy it new, just to put my money where my mouth is, but so far this is looking good. Definitely a different design ethos is at work at Sigma.

And I could see a spot for a Helios 44-2 or one of the old 55mm f1.2's making its way up onto the shelf as well. Or the DA 70. Or...
10-27-2017, 06:38 AM   #11
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QuoteOriginally posted by pres589 Quote
And I could see a spot for a Helios 44-2 or one of the old 55mm f1.2's making its way up onto the shelf as well. Or the DA 70. Or...


cranking up the LBA machine?
10-27-2017, 06:47 AM   #12
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I've always thought that a prime lens a bit longer than 50mm would be useful for an alternative portrait lens. The DA 50 1.8 is a good all-around on my crop body but there's been a few times already that something a bit more narrow would have been nice. I'm in no rush to pull this trigger.

The DA 70 probably makes the most sense, but, swirly bokeh might win. Or the dark horse, a lens with a depth of field about as wide as a gnat if I want it. This probably won't happen for a year at least.
10-27-2017, 06:49 AM   #13
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QuoteOriginally posted by pres589 Quote
I've always thought that a prime lens a bit longer than 50mm would be useful for an alternative portrait lens. The DA 50 1.8 is a good all-around on my crop body but there's been a few times already that something a bit more narrow would have been nice. I'm in no rush to pull this trigger.

The DA 70 probably makes the most sense, but, swirly bokeh might win. Or the dark horse, a lens with a depth of field about as wide as a gnat if I want it. This probably won't happen for a year at least.
get all three... the DA70 Limited is a special, pixie-dusted lens...

the Helios 44K-4 is amazing in its own right, and much better stopped down that people realize...

but I absolutely love my Porst 55mm f1.2 lens...
10-27-2017, 10:54 AM   #14
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The DA 70mm f/2.4 Limited is certainly a fine lens. Amazingly small. In actual testing it came out at around 65mm as Recall. This is quite ordinary, since tele lenses are very often rated slightly more than actual. The FA 77mm f/1.8 Limited, on the other hand, came in right on at 77mm, hence the odd FL designation, which was a reflection of Pentax precision in their FA Limited series.

Since you have a very good portrait lens in your DA 50mm, you might consider saving up a bit longer and going for the FA 77mm LTD. It would provide an even more meaningful addition to your 50mm. It is great for portrait use, is still very small for its FL and aperture speed. You don't have to get as close to your subject with its longer FL, which is especially good for nabbing candid shots without drawing attention. Bokeh is absolutely wonderful. I've had mine for many years, having bought it for my film bodies.
10-27-2017, 12:22 PM   #15
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77 might be too long on crop, I don't know, would rent the 70 first. Again, nothing I'm doing for a year or so, unless I suddenly start getting real work as a portrait photog (or any kind of paying photog work really).
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