focal point calibration concerns

Wired · 03-06-2015, 07:47 AM

Wondering if anyone else is seeing this problem...

For a while now I've been getting frustrated with the apparent softness of my K3. It's tack sharp most of the time, but every so often I get a slightly OOF image. At first it seemed random, so I started to determine it as user error. However, I noticed something on my last two shoots, it only seems to be when I'm using the furthest Focal points from center. The ones far left and far right.

anytime I use the center focal points the images are tack sharp. I use the outside ones though it appears to have some back focus. When I calibrate the center points they are dead accurate. But I haven't tried the side points.

its frustrating because all points should be calibrated the same no?

JohnX · 03-06-2015, 08:04 AM

Most lenses will be softer at the edges than the centre, so if that's where your subject is, and I presume it is because you're using the outer focal points, then it could be lens, not body?

Wired · 03-06-2015, 08:19 AM

Originally posted by JohnX

Most lenses will be softer at the edges than the centre, so if that's where your subject is, and I presume it is because you're using the outer focal points, then it could be lens, not body?

At f8?

08amczb · 03-06-2015, 08:20 AM

The 25 inner focus points are cross type, so they are sensitive to vertical and horizontal edges too. The 2 outer are vertical only (I think). Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points.

Wired · 03-06-2015, 08:29 AM

Originally posted by 08amczb

The 25 inner focus points are cross type, so they are sensitive to vertical and horizontal edges too. The 2 outer are vertical only (I think). Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points.

figured as much as that would be the cause.

Just didn't think with good light it would be that bad. They are pretty much useless for me right now.

this was the outermost to the right FP set on her eyebrow.

stevebrot · 03-06-2015, 09:16 AM

Originally posted by 08amczb

Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points.

More precise...meaning they are more likely to be accurate.

The other points have a focus sensitivity of f/5.6 which should be adequate at f/8. That being said, even the f/2.8 sensors will fail to satisfy if pixel peeping at a 24Mpx image. After all, there is only one true point of focus and DOF means little when viewing at full resolution.

Steve

Wired · 03-06-2015, 09:47 AM

Originally posted by stevebrot

More precise...meaning they are more likely to be accurate.

The other points have a focus sensitivity of f/5.6 which should be adequate at f/8. That being said, even the f/2.8 sensors will fail to satisfy if pixel peeping at a 24Mpx image. After all, there is only one true point of focus and DOF means little when viewing at full resolution.

Steve

true that. I print a lot, and often at 14x16 on matte or metallic. But I haven't seen how these images look at that print size because I haven't decided to waste money on that. But I'll print the above image and see if it meets my standards.

For me, if I'm doing tear sheets and headshots for paying clients that are using these images to get work on their own, I want that eye to be as sharp as possible. I could care less about the skin, I smooth that anyways!

JohnX · 03-06-2015, 09:51 AM

Originally posted by Wired

At f8?

It's inherent in the optical design, not the focussing.

Have to say, however, that I'm not seeing an issue with the photo you posted.

Wired · 03-06-2015, 09:54 AM

Originally posted by JohnX

It's inherent in the optical design, not the focussing.

Have to say, however, that I'm not seeing an issue with the photo you posted.

Eyes are not sharp. Should have posted the original. But I applied pretty aggressive post processing sharpening masks to simulate sharpness.

JinDesu · 03-06-2015, 10:05 AM

Could it possibly be the lens performance? I am not very well versed in the separation of signals for phase detect AF, but how does the light transmission perform on an off center part of the lens to the AF sensor?

Wired · 03-06-2015, 10:18 AM

Originally posted by JinDesu

Could it possibly be the lens performance? I am not very well versed in the separation of signals for phase detect AF, but how does the light transmission perform on an off center part of the lens to the AF sensor?

great question. But it does not seem to matter on which lens. I primarily only use the FA 31, FA 43, FA 77 over the past 6-7 months however.

JinDesu · 03-06-2015, 11:30 AM

Originally posted by Wired

great question. But it does not seem to matter on which lens. I primarily only use the FA 31, FA 43, FA 77 over the past 6-7 months however.

Is it a consistent back focus no matter which end of the far focal points you are using?

rawr · 03-06-2015, 11:32 AM

You should probably be focus-and-recomposing, rather than just selecting one edge focus point, the since the eyes are going to be close to outside of the AF frame. Pic crudely related:

Wired · 03-06-2015, 11:36 AM

Originally posted by JinDesu

Is it a consistent back focus no matter which end of the far focal points you are using?

correct.

rawr · 03-06-2015, 11:40 AM - **1 Like**

Live View AF would probably work better than the optical viewfinder for photo elements at the very periphery of the frame. You could also then, of course, zoom in right on the eyes to confirm focus before you take the shot.

03-06-2015, 07:47 AM	#1
Wired Veteran Member Join Date: Aug 2009 Location: Edmonton, AB Photos: Gallery \| Albums Posts: 2,519	focal point calibration concerns Wondering if anyone else is seeing this problem... For a while now I've been getting frustrated with the apparent softness of my K3. It's tack sharp most of the time, but every so often I get a slightly OOF image. At first it seemed random, so I started to determine it as user error. However, I noticed something on my last two shoots, it only seems to be when I'm using the furthest Focal points from center. The ones far left and far right. anytime I use the center focal points the images are tack sharp. I use the outside ones though it appears to have some back focus. When I calibrate the center points they are dead accurate. But I haven't tried the side points. its frustrating because all points should be calibrated the same no?

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03-06-2015, 08:04 AM	#2
JohnX Pentaxian Join Date: Jul 2009 Location: London Posts: 1,116	Most lenses will be softer at the edges than the centre, so if that's where your subject is, and I presume it is because you're using the outer focal points, then it could be lens, not body?

03-06-2015, 08:19 AM	#3
Wired Veteran Member Join Date: Aug 2009 Location: Edmonton, AB Photos: Gallery \| Albums Posts: 2,519 Original Poster	Originally posted by JohnX Most lenses will be softer at the edges than the centre, so if that's where your subject is, and I presume it is because you're using the outer focal points, then it could be lens, not body? At f8?

03-06-2015, 08:20 AM	#4
08amczb Veteran Member Join Date: Dec 2013 Posts: 796	The 25 inner focus points are cross type, so they are sensitive to vertical and horizontal edges too. The 2 outer are vertical only (I think). Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points.

03-06-2015, 08:29 AM	#5
Wired Veteran Member Join Date: Aug 2009 Location: Edmonton, AB Photos: Gallery \| Albums Posts: 2,519 Original Poster	Originally posted by 08amczb The 25 inner focus points are cross type, so they are sensitive to vertical and horizontal edges too. The 2 outer are vertical only (I think). Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points. figured as much as that would be the cause. Just didn't think with good light it would be that bad. They are pretty much useless for me right now. this was the outermost to the right FP set on her eyebrow.

03-06-2015, 09:16 AM	#6
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by 08amczb Furthermore the center and one above and one below are F2.8 sensitive, so they are more accurate then any other focus points. More precise...meaning they are more likely to be accurate. The other points have a focus sensitivity of f/5.6 which should be adequate at f/8. That being said, even the f/2.8 sensors will fail to satisfy if pixel peeping at a 24Mpx image. After all, there is only one true point of focus and DOF means little when viewing at full resolution. Steve

03-06-2015, 09:47 AM	#7
Wired Veteran Member Join Date: Aug 2009 Location: Edmonton, AB Photos: Gallery \| Albums Posts: 2,519 Original Poster	Originally posted by stevebrot More precise...meaning they are more likely to be accurate. The other points have a focus sensitivity of f/5.6 which should be adequate at f/8. That being said, even the f/2.8 sensors will fail to satisfy if pixel peeping at a 24Mpx image. After all, there is only one true point of focus and DOF means little when viewing at full resolution. Steve true that. I print a lot, and often at 14x16 on matte or metallic. But I haven't seen how these images look at that print size because I haven't decided to waste money on that. But I'll print the above image and see if it meets my standards. For me, if I'm doing tear sheets and headshots for paying clients that are using these images to get work on their own, I want that eye to be as sharp as possible. I could care less about the skin, I smooth that anyways!

03-06-2015, 09:51 AM	#8
JohnX Pentaxian Join Date: Jul 2009 Location: London Posts: 1,116	Originally posted by Wired At f8? It's inherent in the optical design, not the focussing. Have to say, however, that I'm not seeing an issue with the photo you posted.

03-06-2015, 09:54 AM	#9
Wired Veteran Member Join Date: Aug 2009 Location: Edmonton, AB Photos: Gallery \| Albums Posts: 2,519 Original Poster	Originally posted by JohnX It's inherent in the optical design, not the focussing. Have to say, however, that I'm not seeing an issue with the photo you posted. Eyes are not sharp. Should have posted the original. But I applied pretty aggressive post processing sharpening masks to simulate sharpness.

03-06-2015, 10:05 AM	#10
JinDesu Veteran Member Join Date: Jun 2011 Location: New York City Photos: Gallery Posts: 5,638	Could it possibly be the lens performance? I am not very well versed in the separation of signals for phase detect AF, but how does the light transmission perform on an off center part of the lens to the AF sensor?

03-06-2015, 10:18 AM	#11
Wired Veteran Member Join Date: Aug 2009 Location: Edmonton, AB Photos: Gallery \| Albums Posts: 2,519 Original Poster	Originally posted by JinDesu Could it possibly be the lens performance? I am not very well versed in the separation of signals for phase detect AF, but how does the light transmission perform on an off center part of the lens to the AF sensor? great question. But it does not seem to matter on which lens. I primarily only use the FA 31, FA 43, FA 77 over the past 6-7 months however.

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03-06-2015, 11:30 AM	#12
JinDesu Veteran Member Join Date: Jun 2011 Location: New York City Photos: Gallery Posts: 5,638	Originally posted by Wired great question. But it does not seem to matter on which lens. I primarily only use the FA 31, FA 43, FA 77 over the past 6-7 months however. Is it a consistent back focus no matter which end of the far focal points you are using?

03-06-2015, 11:32 AM	#13
rawr Veteran Member Join Date: Jun 2009 Posts: 11,913	You should probably be focus-and-recomposing, rather than just selecting one edge focus point, the since the eyes are going to be close to outside of the AF frame. Pic crudely related:

03-06-2015, 11:36 AM	#14
Wired Veteran Member Join Date: Aug 2009 Location: Edmonton, AB Photos: Gallery \| Albums Posts: 2,519 Original Poster	Originally posted by JinDesu Is it a consistent back focus no matter which end of the far focal points you are using? correct.

03-06-2015, 11:40 AM - 1 Like	#15
rawr Veteran Member Join Date: Jun 2009 Posts: 11,913	Live View AF would probably work better than the optical viewfinder for photo elements at the very periphery of the frame. You could also then, of course, zoom in right on the eyes to confirm focus before you take the shot.