[ZombieArmy]

Hi, I have a Sigma 70-210 that I bought a while back. Lens performs "ok" but it isn't particularly useful because this thing does absolutely nothing but back focus. I hear that back focusing shouldn't be a thing on CDAF but this lens most certain exhibits it, and manual focus is laughable at best on this lens. Any solution?

[Na Horuk]

Is it definitely back focus? Or is it maybe focus shift? Lens decentering?
The reason why CD AF doesn't have front/backfocusing is because it doesn't even look at "focus", it only looks for "contrast" and it maximizes contrast. This is why some really low contrast lenses don't focus well in live view. Or lenses with high contrast and tremendous DoF might misfocus because the camera overestimates its DoF. Maybe you could post a sample photo?

[ZombieArmy]

Is it definitely back focus? Or is it maybe focus shift? Lens decentering?
The reason why CD AF doesn't have front/backfocusing is because it doesn't even look at "focus", it only looks for "contrast" and it maximizes contrast. This is why some really low contrast lenses don't focus well in live view. Or lenses with high contrast and tremendous DoF might misfocus because the camera overestimates its DoF. Maybe you could post a sample photo?

Oh yeah it's def back focus, I've manual focused and gotten what I consider decent pictures (given the lens) while the auto focus is just awful

It has NEVER gotten focus right at 210, and it's hit and miss at 70.

[Giklab]

I hear some older lenses simply have AF issues in live view, because *reasons*... Sorry, can't help you there, you may want to use MF with peaking until you get a lens that works properly.

[Adam]

One solution might be to try a smaller focus area in SEL mode. But yes, there are some older lenses that don't work reliably in live view even through traditional FF/BF isn't the cause.

[GUB]

But yes, there are some older lenses that don't work reliably in live view even through traditional FF/BF isn't the cause.

Is there other discussions about this Adam? I am experiencing it with my 8 element super tak 50 1.4 . At 2 -3 metres away I seem to be focusing sort of 200mm too close. Figure it is operator error and more practise needed.

[Adam]

Is there other discussions about this Adam? I am experiencing it with my 8 element super tak 50 1.4 . At 2 -3 metres away I seem to be focusing sort of 200mm too close. Figure it is operator error and more practise needed.

Well that's something entirely different, and possibly related to a flanged adapter?

The issue with the select Sigma lenses is that in CDAF mode, they simply can't autofocus correctly. I believe the 50-150mm also requires a firmware update for CDAF to work properly.

Adam
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[GUB]

Ahhh silly me - "older lenses" is a relative term - of course you are talking AF. Na my problem is idiot error - of course if it saying infocus on the sensor the sensor is going to be infocus. The only exception I can think of is focus shift with aperture change (I have one lens with it) but the tak is in manual mode.

[aaacb]

It could be that the lens has such an imprecise focusing mechanism (gears etc) that the camera can't get to a better result when it's raking focus quickly. Using a tripod and with patience, I've been able to focus manually better than the lv af with my k3ii and DA 35/2.4... which I think is one of the least consistent lenses I have, af-wise. The difference was very slight though, not what you're describing, but also my lens is much newer so the gears have less wear.

[THoog]

If it's the Sigma 70-210 UC or UC II, those lenses are only f/4 at the short end, and f/5.6 at the long end. That's not really bright enough for the K-01's CDAF system. There have been lots of threads over the years with people complaining about the K-01's AF on variable aperture long zooms. Unless you are shooting a high-contrast subject in bright "sunny f/16" conditions, it's going to struggle to focus correctly. (and if you ARE shooting in bright conditions, you may not be able to see the screen well enough to see what it's focusing on, unless you are using some kind of hood on the LCD).

[ZombieArmy]

If it's the Sigma 70-210 UC or UC II, those lenses are only f/4 at the short end, and f/5.6 at the long end. That's not really bright enough for the K-01's CDAF system. There have been lots of threads over the years with people complaining about the K-01's AF on variable aperture long zooms. Unless you are shooting a high-contrast subject in bright "sunny f/16" conditions, it's going to struggle to focus correctly. (and if you ARE shooting in bright conditions, you may not be able to see the screen well enough to see what it's focusing on, unless you are using some kind of hood on the LCD).

I shoot my K-01 in the bright state of Florida so I don't think light is an issue.
Also a more elegant solution to the LCD problem is just to wear a hat, been manual focusing my K-01 in harsh light for a while without a loupe with no problems.

[THoog]

I shoot my K-01 in the bright state of Florida so I don't think light is an issue.

You might be surprised. We really need to see some sample shots to give a better answer. I remember one case where a K-01 was always focusing behind the subject, but the shooter hadn't noticed the black picture frame on a white wall in the background. The K-01 can't resist contrast like that.

[ZombieArmy]

You might be surprised. We really need to see some sample shots to give a better answer. I remember one case where a K-01 was always focusing behind the subject, but the shooter hadn't noticed the black picture frame on a white wall in the background. The K-01 can't resist contrast like that.

Yeah I know that kinda stuff, it's why shooting stuff indoors is so frustrating sometimes. I'll try to get a sample shot for you once it's day out.