[outsider]

My sigma 85mm 1.4 is front focusing like H*ll on the K-5 and I can't understand why. This only happens when I'm shooting closeup faces. Half of the shots are unuseable and the other half are just good enough to sharpen and keep. A few are just perfect, but not many. It has no problem with the brick wall test or just about anything else. It's as though it gets confused, stops, beep confirmation and most times I can't see well enough to tell if it's actually spot on until viewing the pictures on a monitor. I have no problems while using the k-x, it's a dream. I've tried everything except focus adjustment, but I don't think that's the problem. It seems as though the point focus is significantly larger area on the k-5 than the k-x. Many times the focus will stop at the frame of someone using glasses, rather than the eye itself. Very frustrating. I've noticed that the sigma focuses much more surely on the k-x than k-5, even in low light. Sometimes the sigma just stops in low light on the k-5 without catching focus and no beep confirmation. Maybe this is just an adverse occurance with these two. Any suggestions?

[Smeggypants]

Calibrate the AF with that lens before evaluating any further.

[outsider]

Calibrate the AF with that lens before evaluating any further.

Honorable Mr. Smeggypants. I'll be happy to do that, but I honestly have no idea how I can take pictures, but that exceeds my specific technical capability Is this the same as fine tuning the focus adjustment?

[Artifarty]

To AF fine adjustment please see

PENTAX DSLRs: PART-1. Free Autofocus Adjustment charts for front and back focusing problems. Good for Pentax, Canon and Nikon.

[rvannatta]

being mindful of various mis focusing complaints of the K5 in low light, I did a quick and dirty test on mine with a lately acquired 100mm macro. My target will re recognized as a name tag on the side of a Pentax lens sitting on a table top, and I sat in a chair beside the table putting me about arm's length away and set about a 30 degree angle to the tag.

Lighting was so dark that I couldn't read the tag with the naked eye. The exposure called for 1/10th of a second, ISO 12800, and f/2.8. I took both photos hand held with single point focusing, and single point exposure, and kept my fingers out of the focus assist light path.

Lighting was a single incandescant bare bulb overhead and a bit to the back shadowing the bottom of the tag a little more.

In one photo I put the red indicator on the top lines of text and in the other on the bottom lines of text. You won't have a problem telling which is which.



My conclusion: at least with my K5 and my recently acquired FA 100 macro I'm getting a reasonable exposure, and it focuses where I point it even with crummy lighting

[outsider]

being mindful of various mis focusing complaints of the K5 in low light, I did a quick and dirty test on mine with a lately acquired 100mm macro. My target will re recognized as a name tag on the side of a Pentax lens sitting on a table top, and I sat in a chair beside the table putting me about arm's length away and set about a 30 degree angle to the tag.

Lighting was so dark that I couldn't read the tag with the naked eye. The exposure called for 1/10th of a second, ISO 12800, and f/2.8. I took both photos hand held with single point focusing, and single point exposure, and kept my fingers out of the focus assist light path.

Lighting was a single incandescant bare bulb overhead and a bit to the back shadowing the bottom of the tag a little more.

In one photo I put the red indicator on the top lines of text and in the other on the bottom lines of text. You won't have a problem telling which is which.



My conclusion: at least with my K5 and my recently acquired FA 100 macro I'm getting a reasonable exposure, and it focuses where I point it even with crummy lighting

Thanks so much for your work and advice. To be honest, I'm not having much trouble either in similar scenarios. With relatively simple geometric, 3D objects, it performs quite well. I only seem to have the problem when photographing faces (so far only with the 85mm 1.5 sigma), standing about 1.5 arms length away. It's ridiculous how many times it front focuses or stops just before the eye ball itself. This is painfully noticeable shooting at 1.4-2.0, which of course is one reason I got the sigma in the first place. The way this sigma is focusing on anything else, gives my no reason whatsoever to have to do a lens adjustment. I almost feel like maybe pentax should have made the point focus smaller on the k-5. I can stand back almost 10 feet with the k-x/sigma 85mm combo, shoot in aperture 1.4, aiming for the eye (which is important for obvious reasons), and nail focus 7 of 10 times. The k-5, not so. Sometimes stops just at the tip of the nose, even close up (close enough for the entire face to fill the viewfinder). If I turn, and point it at a wall, a car, my old high school report card, it will nail focus everytime, just as people praise the k-5 for. This is a pretty damn specific problem here. Probably not many other people are using the k5/sigma 85mm in the same way for the same purpose, but I wouldn't be surprised if they ran into the same issue.

[jbinpg]

Just wondering, would there be any difference between AF.C and AF.S in this case? Yes, focus and recompose is impossible with AF.C but it may point to a different algorithm used between the different modes.

Jack

[Class A]

Any suggestions?

You might be hit by the K-5's front-focus problems?
They seem to occur not only in very low light but also when the camera is confused by light temperature.

Have a look at this "More about low light AF" thread. Someone could take a sharp picture of their wife who seemed to have the wrong skin colour for the K-5 to work properly. He used a kit lens which is pretty slow and hence could easily provoke the K-5 FF issue in relatively good light. In principle, you should be fine with your very fast Sigma, but it still sounds a bit like a camera-induced FF problem to me.

Given that brick wall shots work, I doubt that the problem is with the lens, but I am not a 100% sure what the source of your problem is.

If you want to try AF adjustment (easy to do with K-5 menus) you may want to check out my AF adjustment hints.

[creampuff]

Don't be too quick to point the problem at the camera. Sigma's fast prime lenses are often in need of focus calibration. This is a known issue and does crop up with other camera mounts, not just Pentax and sending it to Sigma for re-calibration is the best option. While there might be mis-focusing with the K-5 in low light conditions, this shouldn't be an issue in bright daylight. I would rather get Sigma to re-calibrate the lens rather than use the in-camera AF fine adjustment as it is a proven solution, based on friends of mine who have owned the Sigma 30mm, 50mm and 85mm in Pentax and Canikon mounts and who have encountered fine focus errors themselves.

[Class A]

Sigma's fast prime lenses are often in need of focus calibration.

How do you explain that the OP's shots of brick walls and other 3D objects are fine?

If Sigma checks the lens with a standard focus target, chances are that it will perform as intended.

I concede that I'm not a 100% sure where the cause of the problem is -- it might even just be a focusing technique problem, i.e., no technical issue -- but the fact that lens focuses fine on many targets makes it more plausible to me that it is the AF system that gets confused by certain subjects.

Outsider, can you tell us a bit more about the lighting conditions (Tungsten?) and face (dark coloured?) when the AF problems appear? What features are you trying to focus on? Some recommend the "bridge", I always go for the eyes.

Have you done your AF tests with other objects at the same subject distance?

[outsider]

How do you explain that the OP's shots of brick walls and other 3D objects are fine?

If Sigma checks the lens with a standard focus target, chances are that it will perform as intended.

I concede that I'm not a 100% sure where the cause of the problem is -- it might even just be a focusing technique problem, i.e., no technical issue -- but the fact that lens focuses fine on many targets makes it more plausible to me that it is the AF system that gets confused by certain subjects.

Outsider, can you tell us a bit more about the lighting conditions (Tungsten?) and face (dark coloured?) when the AF problems appear? What features are you trying to focus on? Some recommend the "bridge", I always go for the eyes.

Have you done your AF tests with other objects at the same subject distance?

The shots of the OP's and bricks are fine, because they're fine. I mean, they're in focus in the right areas. There's nothing out of the ordinary for them. I have a feeling that it may be the relationship between the lens and camera body? I always shoot for the eyes, even closeup this k5/sigma duo still can't nail it 60% of the time in wide apertures. I assume it does the same stopped down, tho it's may just be harder to tell since everything is more in focus. I always use auto white balance. I've heard about the tungsten thing, but haven't tried it. Like you mentioned, it may be just a weird thing that spooks the focus whenever I take pictures of people. I haven't done any tests yet. It may be better to just leave the 1.4 off of the k-5, since it purple fringes so badly at wide apertures.

[simico]

With relatively simple geometric, 3D objects, it performs quite well. I only seem to have the problem when photographing faces (so far only with the 85mm 1.5 sigma), standing about 1.5 arms length away. It's ridiculous how many times it front focuses or stops just before the eye ball itself. ... Sometimes stops just at the tip of the nose, even close up (close enough for the entire face to fill the viewfinder).

I guess part of the nose, eyelashes, etc, are also within the oversized AF area, therefore the camera can legitimately lock focus on those instead of the eye since they are closer to the camera than the eye. The thin dof caused by close distance and large aperture makes misfocus very obvious - stopped down and/or shooting from a distance would hide it with the larger dof (eye would still appear to be in focus).

I wish Pentax made smaller AF sensor areas... that's why having 30+ and 50+ focus sensors in Nikon is good, the sensors are very small therefore only your intended target (e.g. eye) will cover the sensor area, allowing you to pinpoint focus on exactly what you want.

[outsider]

I guess part of the nose, eyelashes, etc, are also within the oversized AF area, therefore the camera can legitimately lock focus on those instead of the eye since they are closer to the camera than the eye. The thin dof caused by close distance and large aperture makes misfocus very obvious - stopped down and/or shooting from a distance would hide it with the larger dof (eye would still appear to be in focus).

I wish Pentax made smaller AF sensor areas... that's why having 30+ and 50+ focus sensors in Nikon is good, the sensors are very small therefore only your intended target (e.g. eye) will cover the sensor area, allowing you to pinpoint focus on exactly what you want.

You have a point. the k-5 seems to more readily grab at anything that is close by the center point (or what seems to be the center point). This is not good, disasterous after a hard days out shooting. The D90 has 2 different size center focus point sizes to choose from!! Nikons have their issues, but boy do I love the center point/focusing on the D90!! The focusing is fast, sure AND obedient and i'm a happy guy when I get home and see the focus areas after shooting at wide apertures. The k-5/sigma has broken my heart so many times with front focus, blurry eyes and sharp noses, that I even considered getting rid of my k-5. cheap. but something tells me that i'm better off keeping it. after all, it really is a fantastic little machine, and pentax has a comendable relationship with it's customers. The D90, for example, one of the most popular slr's of this age, has had 1 firmware release since the camera's release several years ago. pathetic. It would be nice if pentax looked into center focus point issue. I'm going to use a few other lenses to more accurately pinpoint the source of the problem.

[Winder]

My sigma 85mm 1.4 is front focusing like H*ll on the K-5 and I can't understand why. This only happens when I'm shooting closeup faces. Half of the shots are unuseable and the other half are just good enough to sharpen and keep. A few are just perfect, but not many. It has no problem with the brick wall test or just about anything else. It's as though it gets confused, stops, beep confirmation and most times I can't see well enough to tell if it's actually spot on until viewing the pictures on a monitor. I have no problems while using the k-x, it's a dream. I've tried everything except focus adjustment, but I don't think that's the problem. It seems as though the point focus is significantly larger area on the k-5 than the k-x. Many times the focus will stop at the frame of someone using glasses, rather than the eye itself. Very frustrating. I've noticed that the sigma focuses much more surely on the k-x than k-5, even in low light. Sometimes the sigma just stops in low light on the k-5 without catching focus and no beep confirmation. Maybe this is just an adverse occurance with these two. Any suggestions?

Pentax has come a long way with AF, but they still have a few things that they need to improve. Smaller AF points is definitely one of them. When working with really fast glass like the 85mm f/1.4 you only have to be off by a little bit to really ruin an image. Hopefully we will see smaller AF points that allow for more precise AF work.

[creampuff]

How do you explain that the OP's shots of brick walls and other 3D objects are fine?

If Sigma checks the lens with a standard focus target, chances are that it will perform as intended.

I concede that I'm not a 100% sure where the cause of the problem is -- it might even just be a focusing technique problem, i.e., no technical issue -- but the fact that lens focuses fine on many targets makes it more plausible to me that it is the AF system that gets confused by certain subjects.

Outsider, can you tell us a bit more about the lighting conditions (Tungsten?) and face (dark coloured?) when the AF problems appear? What features are you trying to focus on? Some recommend the "bridge", I always go for the eyes.

Have you done your AF tests with other objects at the same subject distance?

Sorry but before you go off and attempt to hypothesize, do you even have a fast Sigma prime mated to a K-5 to begin with?
I may not have the 85 yet but here's a shot taken with a K-5 and a Sigma 50 that's been calibrated at max aperture focused at the subject's right eye.
Friends using other makes have also had great results after sending their lenses to the local Sigma service center for AF re-calibration.