Pentax AF points - what am I missing?

mike lix · May 6, 2018

Hi guys,

I have a question - what focus points do you use? And with what success?

To explain my question. I have a KP with mostly FA 77 attached to it. And I am talking about AF-S now. I shoot mostly at f/2.2 - f/2.8. When I use the center AF point, it is almost always precise, very good indeed. But I would usually like to use the corner ones (out of the 25 cross points, I am not talking about the side linear ones). But those are almost unusable - they miss most of the time.

I read the center point (plus the one above and the one under it) are precise enough for f/2.8, the others are precise enough for f/5.6. When I tried using the corner AF points at f/5.6, the results were all OK. But f/5.6 is definitely not what I need with a fast portrait lens.

I know I can use the center point and recompose. Well, I don´t find the results as sharp as without recomposing. So I try to avoid recomposing if possible.

I know I can use LV. Well, I use it quite a lot when I need something off center to be sharp (which is pretty often). But I just don´t like LV. It is slow, heavy-footed, impractical and lacks the joy of taking photos with an optical viewfinder. I am using a dslr, what is the point of using LV? If I did´t want to use an optical viewfinder, I could have bought a Sony instead...

Eventually, I know I can use the center AF point and shoot from bigger distance and crop later but that is very makeshift solution, I try to avoid it as much as I can.

If the corner AF points were as precise as the center point, I would be as happy as I can get. The great optical viewfinder experience in connection with reliable focusing points would be amazing. I don´t even miss any wider AF coverage, the 25 AF points cover as much as I need.

But I am just stuck with the center point or LV. Am I missing something? Or can you share some experience with the AF points that you use?

mike

Holger Bargen · May 6, 2018

If I use AF at all I either use the centre AF point - but I will activate the AF system by means of pressing the buttion at the back of the camera and do not link it to the release button. This way I can clearly split AF measuring from releasing. I find it important as most of the time those parts I am interested in are not in the centre.

The second way I use it is the selction of a "swarm of AF points" in the region I intend to have the focus. This works fine to me - much better than using all AF points at the same time as I often made the experience that the camera selects points I would not like to set the focus at - and it is almost impossible to keep the camera from selecting these points reliably.

In LV I hace the option that in sharp regions the edges are highlighted activated. This works fine for me and I like to work MF with it.

If you have portraits you can activate the face detection. Otherwise guide the AF field to your region of interest and let the camera find the focus.

Best regards

Holger

Tatouzou · May 6, 2018

mike lix said:
Hi guys,

I have a question - what focus points do you use? And with what success?

To explain my question. I have a KP with mostly FA 77 attached to it. And I am talking about AF-S now. I shoot mostly at f/2.2 - f/2.8. When I use the center AF point, it is almost always precise, very good indeed. But I would usually like to use the corner ones (out of the 25 cross points, I am not talking about the side linear ones). But those are almost unusable - they miss most of the time.

I read the center point (plus the one above and the one under it) are precise enough for f/2.8, the others are precise enough for f/5.6. When I tried using the corner AF points at f/5.6, the results were all OK. But f/5.6 is definitely not what I need with a fast portrait lens.

I know I can use the center point and recompose. Well, I don´t find the results as sharp as without recomposing. So I try to avoid recomposing if possible.

I know I can use LV. Well, I use it quite a lot when I need something off center to be sharp (which is pretty often). But I just don´t like LV. It is slow, heavy-footed, impractical and lacks the joy of taking photos with an optical viewfinder. I am using a dslr, what is the point of using LV? If I did´t want to use an optical viewfinder, I could have bought a Sony instead...

Eventually, I know I can use the center AF point and shoot from bigger distance and crop later but that is very makeshift solution, I try to avoid it as much as I can.

If the corner AF points were as precise as the center point, I would be as happy as I can get. The great optical viewfinder experience in connection with reliable focusing points would be amazing. I don´t even miss any wider AF coverage, the 25 AF points cover as much as I need.

But I am just stuck with the center point or LV. Am I missing something? Or can you share some experience with the AF points that you use?

mike

You should get in-focus image with DA 70 using off center AF points.

I do it on K3, which has the same AF engine as KP, when selecting 1 AF point only through the 4-ways controller.

If you dont get in-focus shots with your DA 70 at f2.4/2.8 this way, maybe you need fine AF adjustment of your DA70: compare OVF PDAF with live view CDAF, if CDAF is sharper, then you do need AF fine tuning.

My DA70 required such adjustment.

I use back button AF, to avoid any uncontrolled refocus by the camera when I press the shutter release. I use it mostly in AF-C, even for still scenes, as the camera wont change focus once I release the back button.

mike lix · May 6, 2018

Tatouzou said:
mike lix said:

Hi guys,

I have a question - what focus points do you use? And with what success?

To explain my question. I have a KP with mostly FA 77 attached to it. And I am talking about AF-S now. I shoot mostly at f/2.2 - f/2.8. When I use the center AF point, it is almost always precise, very good indeed. But I would usually like to use the corner ones (out of the 25 cross points, I am not talking about the side linear ones). But those are almost unusable - they miss most of the time.

I read the center point (plus the one above and the one under it) are precise enough for f/2.8, the others are precise enough for f/5.6. When I tried using the corner AF points at f/5.6, the results were all OK. But f/5.6 is definitely not what I need with a fast portrait lens.

I know I can use the center point and recompose. Well, I don´t find the results as sharp as without recomposing. So I try to avoid recomposing if possible.

I know I can use LV. Well, I use it quite a lot when I need something off center to be sharp (which is pretty often). But I just don´t like LV. It is slow, heavy-footed, impractical and lacks the joy of taking photos with an optical viewfinder. I am using a dslr, what is the point of using LV? If I did´t want to use an optical viewfinder, I could have bought a Sony instead...

Eventually, I know I can use the center AF point and shoot from bigger distance and crop later but that is very makeshift solution, I try to avoid it as much as I can.

If the corner AF points were as precise as the center point, I would be as happy as I can get. The great optical viewfinder experience in connection with reliable focusing points would be amazing. I don´t even miss any wider AF coverage, the 25 AF points cover as much as I need.

But I am just stuck with the center point or LV. Am I missing something? Or can you share some experience with the AF points that you use?

mike

Click to expand...

You should get in-focus image with DA 70 using off center AF points.

I do it on K3, which has the same AF engine as KP, when selecting 1 AF point only through the 4-ways controller.

If you dont get in-focus shots with your DA 70 at f2.4/2.8 this way, maybe you need fine AF adjustment of your DA70: compare OVF PDAF with live view CDAF, if CDAF is sharper, then you do need AF fine tuning.

My DA70 required such adjustment.

I use back button AF, to avoid any uncontrolled refocus by the camera when I press the shutter release. I use it mostly in AF-C, even for still scenes, as the camera wont change focus once I release the back button.

Holger Bargen · May 7, 2018

mike lix said:
Tatouzou said:

mike lix said:

Hi guys,

I have a question - what focus points do you use? And with what success?

To explain my question. I have a KP with mostly FA 77 attached to it. And I am talking about AF-S now. I shoot mostly at f/2.2 - f/2.8. When I use the center AF point, it is almost always precise, very good indeed. But I would usually like to use the corner ones (out of the 25 cross points, I am not talking about the side linear ones). But those are almost unusable - they miss most of the time.

I read the center point (plus the one above and the one under it) are precise enough for f/2.8, the others are precise enough for f/5.6. When I tried using the corner AF points at f/5.6, the results were all OK. But f/5.6 is definitely not what I need with a fast portrait lens.

I know I can use the center point and recompose. Well, I don´t find the results as sharp as without recomposing. So I try to avoid recomposing if possible.

I know I can use LV. Well, I use it quite a lot when I need something off center to be sharp (which is pretty often). But I just don´t like LV. It is slow, heavy-footed, impractical and lacks the joy of taking photos with an optical viewfinder. I am using a dslr, what is the point of using LV? If I did´t want to use an optical viewfinder, I could have bought a Sony instead...

Eventually, I know I can use the center AF point and shoot from bigger distance and crop later but that is very makeshift solution, I try to avoid it as much as I can.

If the corner AF points were as precise as the center point, I would be as happy as I can get. The great optical viewfinder experience in connection with reliable focusing points would be amazing. I don´t even miss any wider AF coverage, the 25 AF points cover as much as I need.

But I am just stuck with the center point or LV. Am I missing something? Or can you share some experience with the AF points that you use?

mike

Click to expand...

You should get in-focus image with DA 70 using off center AF points.

I do it on K3, which has the same AF engine as KP, when selecting 1 AF point only through the 4-ways controller.

If you dont get in-focus shots with your DA 70 at f2.4/2.8 this way, maybe you need fine AF adjustment of your DA70: compare OVF PDAF with live view CDAF, if CDAF is sharper, then you do need AF fine tuning.

My DA70 required such adjustment.

I use back button AF, to avoid any uncontrolled refocus by the camera when I press the shutter release. I use it mostly in AF-C, even for still scenes, as the camera wont change focus once I release the back button.

Click to expand...

CarlosRG · May 7, 2018

According to Pentax, the AF points just above and below the center one are also 2.8 capable.

altis · May 7, 2018

AF works by comparing the images coming from the left and right (or top and bottom) halves of the lens.

How Phase Detection Autofocus Works

photographylife.com

This works best in the centre of the frame. Think of a fin in the middle of the sensor casting a shadow away from the direction the light is coming.

As you move further away from the centre then the shadows will become uneven - or the fin has to be tipped over.

Lyff · May 7, 2018

mike lix said:
Hi guys,

I have a question - what focus points do you use? And with what success?

To explain my question. I have a KP with mostly FA 77 attached to it. And I am talking about AF-S now. I shoot mostly at f/2.2 - f/2.8. When I use the center AF point, it is almost always precise, very good indeed. But I would usually like to use the corner ones (out of the 25 cross points, I am not talking about the side linear ones). But those are almost unusable - they miss most of the time.

Maybe a FA 77 issue, I use a lot the other cross points on my K-3 and they works very good for a DSLR (came from D810/D3200 that had mostly linear ones and so was doing focus and recompose most of the time).

mike lix said:
I read the center point (plus the one above and the one under it) are precise enough for f/2.8, the others are precise enough for f/5.6. When I tried using the corner AF points at f/5.6, the results were all OK. But f/5.6 is definitely not what I need with a fast portrait lens.

You don't need to stop down because the focusing is always done at its widest aperture on a dslr (this way the AF system gets all the light it needs), it only stop down to your selected aperture once the focus is done.

Pentax AF f/5.6 points are quite good even used with an f/2.8 or faster lens, the issue seems more when you're shooting very low constrast subjects (but that's expected from such AF points).

mike lix said:
I know I can use the center point and recompose. Well, I don´t find the results as sharp as without recomposing. So I try to avoid recomposing if possible.

That depend of the lens, lenses with strong field curvature are an issue with this kind of shooting but others do usually fine as long as your subject is still.

mike lix said:
I know I can use LV. Well, I use it quite a lot when I need something off center to be sharp (which is pretty often). But I just don´t like LV. It is slow, heavy-footed, impractical and lacks the joy of taking photos with an optical viewfinder. I am using a dslr, what is the point of using LV? If I did´t want to use an optical viewfinder, I could have bought a Sony instead...

Eventually, I know I can use the center AF point and shoot from bigger distance and crop later but that is very makeshift solution, I try to avoid it as much as I can.

If the corner AF points were as precise as the center point, I would be as happy as I can get. The great optical viewfinder experience in connection with reliable focusing points would be amazing. I don´t even miss any wider AF coverage, the 25 AF points cover as much as I need.

But I am just stuck with the center point or LV. Am I missing something? Or can you share some experience with the AF points that you use?

mike

I think it's an issue with your lense or camera, outside of very dark areas the outside AF points usually do fine. Maybe your side af points are miscalibrated, not sure if it's possible.

The only lense that produces unreliable results in AF-S with my K-3 was the sigma 30mm f/1.4, but that happens both with the center points or the others, not sure if it's because the lens is too fast or malfunctioning but I used f/1.4 lenses on a D810 before and didn't get much issues. The f/1.8 and f/2 lenses I tries worked fine though.

mike lix · May 7, 2018

Thank you guys for your answers, I read everything carefully. And after reading your answers, I decided to test it very properly. I found out that the problem is probably in the lens. I think there is some play in it (i can hear something wobble inside when i shake it). I also tried my Sigma 70-200 at 70mm f/2.8 and both the center point and corner AF points worked consistently well (1 missed shot out of 25, all of the 24 others usable without problems) but the microadjustment needed for the FA 77 differed wildly. So I will contact Pentax service to have the problem fixed.

mike

Tatouzou · May 9, 2018

mike lix said:
Thank you guys for your answers, I read everything carefully. And after reading your answers, I decided to test it very properly. I found out that the problem is probably in the lens. I think there is some play in it (i can hear something wobble inside when i shake it). I also tried my Sigma 70-200 at 70mm f/2.8 and both the center point and corner AF points worked consistently well (1 missed shot out of 25, all of the 24 others usable without problems) but the microadjustment needed for the FA 77 differed wildly. So I will contact Pentax service to have the problem fixed.

mike

Just for your information before you contact Pentax service center: my son has K3 and FA77 is his most used lens.

He often shoots wide open or f2/2.5 for subject separation, always using off-center points to get the focus where he wants it in his composition, and it works fine event in AF-C when panning the camera to track moving targets.

Thus it may probably be that your AF77 needs some AF calibration that exceeds what you can do at home.

My own experience is also that my DA limited lenses need AF adjustment more often than other large aperture lenses.

The only lens me and my son both have which is tricky to AF on off-center points is DA21: though it has some noticeable field curvature, it is easier with this lens to use center aF and recompose.

mike lix · May 12, 2018

Thank you for sharing your experience!

I found out the problem is in the body, not in the lens. I did vast testing on two bodies (KP and K-50), even at different appertures. I tested all cross points and the result is that every AF point is calibrated differently, some are same as the center one, some are backfocussing, some are frontfocusing, others differ wildly shot from shot - and there is not one side better than others, really each AF point is different. On the K-50 I sold this week, the tolerances between AF points were much tighter, tolerable.

I can´t contact the service center now since I will need my camera for a few occasions, but in July I will start solving it.

With this experience, I know that next time I want to buy a new body, the first thing I do is extremely precisely test all AF points and if there is any inaccuracy, I will return it immediately. It seems there are big tolerances (quality variations) in AF modules of different bodies, not just lenses.

mike

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Pentax AF points - what am I missing?

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How Phase Detection Autofocus Works

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