[Roquefort]

Hi,

A while ago I bought two second hand Sigma lenses: a 17-50mm F2.8 EX DC HSM and a 50mm F1.4 EX DG HSM, to be used with my K-3ii. My experience with both has been fairly frustrating, I've had good results when they focus properly (especially with the 50mm), but the AF on both is so unreliable that I've had to throw away the majority of shots taken. When testing them with a DIY lens calibration setup (probably not very precise, but enough to get a general sense of the problem), the AF on both is inconsistent at best. They'll front- or backfocus randomly, so in-body AF adjustment is useless.


For users of Sigma lenses (or third party lenses in general), is this a common problem? I've read about some AF issues with Sigma lenses, but both these lenses still get pretty good reviews here, so did I just get unlucky with bad copies? Maybe they're more useful stopped down, but their main purpose for me is concert photography so I need them at max aperture.


As for alternatives, I could probably do without the 50mm for now, but I need something for that 17-50mm range. I'd prefer silent AF so the Tamron version isn't an option, and all affordable Pentax primes in that range seem to have screwdrive AF as well. So I think my only option is Pentax' own 16-50mm?

[BigMackCam]

I use the Sigma 17-50 f/2.8 on my K-3 and K-3II. I've found that going through the AF cycle twice (i.e. focus once and wait for it to lock, then focus again) is the only way to guarantee accurate focus. with this lens and body combo. It's a little annoying, but for my kind of shooting it's usually not an issue, and the image quality from the lens makes it a worthwhile compromise. I know some other folks in these forums have noticed the same behaviour, so I'd say it's pretty common.

Various Sigma lenses are quite well-known for having some AF quirks on Pentax and other brand cameras. Common wisdom suggests it's at least in part due to their reverse-engineering of Pentax camera and lens protocols...

[AstroDave]

I have the Sigma 17-50 and was generaly impressed with its fast focus (better than the old Pentax 16-50 IMHO) - on my older Pentax cameras K10, 20, and 5. It hasn't been on my K-3 much, since the Pentax 18-135 lives there.

As Big Mack notes, a second (or even third) press of the focus button often helps fine-tune the focus on the Sigma 17-50.

Check that all your contacts are clean, both on the camera body and the lenses.

[ramseybuckeye]

I used to have the 17-50 and as I recall I would take the pictures twice sometimes to make sure of focus, I think that was somewhat subconsciously. Once I bought the Pentax 20-40 I never used the Sigma, the Pentax was so much better I didn’t miss those other focal lengths. The Sigma wasn’t a bad lens, but it was not special either.

[zapp]

Do you own any Pentax lens to confirm that it performs differently from the Sigmas? I had bad AF in Pentax K5 when working in artificial light as well as two Sigma ART35 failing AF-wise on K1ii. With K3ii illumination should not be the problem. Technique may also cause inaccurate focus, touching the shutter release button for readjusted focus, using different AF pattern, … what can I say the new 16-50 should do it. The 20-40 is a decent lens as well. Pricing, but working.

[Oldbayrunner]

I don't own the 17-50mm but I do own both the 17-70mm DC & 50mm F1.4. Neither of those have I had problems with focusing. With that said you mention in body AF adjusting, to my knowledge that only works with Pentax lenses. My 17-70DC can be fine tuned via the Sigma Dock

[BigMackCam]

With that said you mention in body AF adjusting, to my knowledge that only works with Pentax lenses. My 17-70DC can be fine tuned via the Sigma Dock

AF fine adjustment works with all AF lenses from any brand. In fact, it even works with manual focus lenses, changing the range at which the "green dot" is shown for accurate focus confirmation However, the Sigma dock - for the few lenses it works with, of course (and the 17-50 isn't one of them) provides much greater control over focus at different focal lengths and distances. AF fine adjustment is just a single offset per lens... so if a zoom lens requires different adjustment at different focal lengths, you're out of luck...

[C_Jones]

I have the Pentax 16-85 and it provides excellent images. The AF is very reliable. I have it on my Pentax K-3 II.

[Roquefort]

Thanks for the replies everyone!

I use the Sigma 17-50 f/2.8 on my K-3 and K-3II. I've found that going through the AF cycle twice (i.e. focus once and wait for it to lock, then focus again) is the only way to guarantee accurate focus. with this lens and body combo. It's a little annoying, but for my kind of shooting it's usually not an issue, and the image quality from the lens makes it a worthwhile compromise. I know some other folks in these forums have noticed the same behaviour, so I'd say it's pretty common.

Thanks, I'll try this for now and see if there's any improvement. A bit of a workaround, but I could happily live with it if it saves me the cost of a new lens

Do you own any Pentax lens to confirm that it performs differently from the Sigmas? I had bad AF in Pentax K5 when working in artificial light as well as two Sigma ART35 failing AF-wise on K1ii. With K3ii illumination should not be the problem. Technique may also cause inaccurate focus, touching the shutter release button for readjusted focus, using different AF pattern, … what can I say the new 16-50 should do it. The 20-40 is a decent lens as well. Pricing, but working.

I have a few others that I use regularly. For low light situations my usual companion to the Sigma 17-50 is the Pentax 50-135, which I love and gives fairly reliable results. When I was trying to calibrate the Sigmas I also did some tests with my DA 50mm f1.8, which behaved much more consistently. So I do think the problem lies with the Sigma lenses. The body's had some really strange focusing problems in the past, but that looked different than the simple front- or backfocusing I'm seeing with the Sigmas.

I have the Pentax 16-85 and it provides excellent images. The AF is very reliable. I have it on my Pentax K-3 II.

I have the 16-85 and like it, it's usually the only lens I take with me when I travel. But in low light it's just not fast enough.

[mikesbike]

I' ve had this lens for a couple of years- the Sigma 17-50mm f/2.8 EX DC HSM, bought brand new, and have not found any problem at all with AF. It has been fine. No hunting, fast, quiet, and accurate. I even use it frequently under low light conditions, even night shots, where AF is most challenged. Imaging has been excellent. I have used it exclusively on my 2 KP bodies. I have not tried it on my K-5 IIs.

Not long after getting this lens, I acquired the Sigma 10-20mm f/3.5 and have found it to be excellent also. However, with this one the AF does not always lock on right away, or not where I want it. But this occurs mainly at its wider FL settings, and then only when there are no clearly-defined edges or details in the frame of a size for the AF to grab onto. This is one problem when going ultra-wide, as most objects become so much smaller, even though working with single spot AF, the target is smaller than the AF sensor to the degree the sensor has trouble with detection.

I happened to have the Sigma 17-50mm on one of my KP bodies, so I just took it outside for some test shots. A similar story when going wider angle and wide open aperture- AF not locking on well. Then the same shot at a mid WA, mid-FL, or longer then it locks on with accuracy. So my answer is, to get a better result, zoom out to 50mm for the AF to lock on, zoom back to reframe, hold focus or switch to MF, then complete the shot. Also, this lens does not have the "quick shift" feature of many Pentax lenses for instant MF touchup, but it does have the AF/MF switch very handy by the thumb when holding in the shooting position, to use for the same purpose. What I try to do with WA shots using MF, is to find a larger detailed part of the scene I can see well to focus on that is the same distance, then once I get good focus there, reframe where I want it to take my shot. Most of my WA widest shots have not been at f/2.8, and not in dim lighting, so I did not notice AF problems here with this lens before. Now I know, at least with my cameras.

It seems there have been some complaints even with the new Pentax DA* 16-50mm f/2.8 PLM.

[mlag]

I have long used the older, camera driven sigma 17-50 mm f2,8, bit slow focus but without problems,
switched now to the 17-70 mm with HSM motor to have faster autofocus , performs well with K3-II.
No problems in normal daily shooting, except the sigma 17-70 hsm contemporary doesn't work/focus reliable with the 1,4x Teleconvertor - which was an one-time experiment for macro photography of insects (where I concluded other telephoto lenses suites better my purpose, so did not pay too much attention). Combined with 1,4x TC autofocus hunts always , without it’s reliable.

[tibbitts]

I have the 17-50 but for me with any lens I just use CDAF when I need AF to be exactly accurate. Except, surprisingly to me, with a K70 and Sigma 10-20 3.5, where CDAF is way off. The old 4-5.6 is okay, and the 3.5 is okay with any flavor of K5. Well, "okay" being not great at the 10mm end, but probably some of the problem with the 3.5 is just less depth of field wide open, especially at the longer focal lengths. Overall, AF for me was okay before I upgraded from 6 to 16+mp and to faster lenses, but not now.

[Oldbayrunner]

AF fine adjustment works with all AF lenses from any brand. In fact, it even works with manual focus lenses, changing the range at which the "green dot" is shown for accurate focus confirmation However, the Sigma dock - for the few lenses it works with, of course (and the 17-50 isn't one of them) provides much greater control over focus at different focal lengths and distances. AF fine adjustment is just a single offset per lens... so if a zoom lens requires different adjustment at different focal lengths, you're out of luck...

Your right, I miss wrote my reply thinking only of the Sigma lens. I should have stated It doesn't work well if someone has uses multiple Sigma lens because their ID's on some are the read the same therefore if one calibrates one lens then when other Sigma's are used the Pentax camera can read the adjustments the same for the other ones when they are attached which can throw their focus off. I own several Sigma Lens so I don't use my cameras AF fine tune on them.

[BigMackCam]

I should have stated It doesn't work well if someone has uses multiple Sigma lens because their ID's on some are the read the same therefore if one calibrates one lens then when other Sigma's are used the Pentax camera can read the adjustments the same for the other ones when they are attached which can throw their focus off. I own several Sigma Lens so I don't use my cameras AF fine tune on them.

Interesting. I only own four Sigma lenses in K-mount - the 17-50mm f/2.8 HSM, 30mm f/1.4 Art, 50mm f/2.8 EX DG Macro and 18-300 - and I'm reasonably sure the IDs are different than any of the other AF lenses I own... but you've got me wondering now!! I'll have to check that later. Thanks for the heads-up!

[pres589]

I've the Sigma 17-50, 24 EX DG, and 28 EX DG, and none of them are what I would call "accurate focusers" on my K-5 II. Center point AF helps unless the scene in question makes that less that useful but it's still not great. I'm hoping a K-3 III will fix things. I think all three are good lenses (I like the 28 EX DG the most) but the AF situation isn't great. Pretty frustrating actually.