PDAF Struggles with Black?

**BruceBanner** · 06-03-2020, 11:57 PM - **1 Like**

Originally posted by JensE

(Edit: I wrote that without noticing that there's a second page - but the stuff on the Pentax page is still worth knowing.)
Now you're well-prepared to digest How to optimize focusing accuracy with large-aperture lenses / explore, Part 1| RICOH IMAGING, including the difference between cross- and line sensors. The note on the central horizontal f/2.8 line sensor, which detects horizontal changes, i.e. vertical edges, is noteworthy.

Yeah, I read that in the past, I just never really digested the idea of 'lines' being the thing the box picks up LOL

For a long time now I just use Spot AF and not even SEL with a single AF point in the middle as I believe that is actually not as accurate as Spot.

**JensE** · 06-04-2020, 01:21 AM - **1 Like**

Depending on the situation, the 'select' option may even be more accurate or at least better equipped to acquire focus, because it uses a combination of sensors. The 'spot' setting however uses a smaller region.

**BruceBanner** · 06-04-2020, 01:46 AM

My friend who recently acquired a Nikon D500 also told me that with their back button focus you can assign it to have a different AF mode to the front shutter system. Also, I have heard (possibly not D500 related) that when reviewing playback of images taken, when pressing the 'zoom in' button, the zoom will take you not to the centre of the frame but actually to the focus point of the shot, thus assisting massively with negating the necessity of using the D-pad to move around and check the focus of the image (and whether you obtained it or not).
Stuff like this would be so handy, practically making touch screen tech on screens moot, I really would have liked to have seen Pentax roll out more firmware updates (but I guess we're moving off point here a bit).

Wheatfield · 06-04-2020, 08:30 AM

Originally posted by BruceBanner

So, I've been playing with my fairly recently accquired 55-300PLM, it's my first zoom and so I've been shooting birdies with it etc. I've been through Fine Adjutsments and feel as though out of the box it really seems ok and doesn't need excessive tuning. With the shots I have taken this far I am wondering if PDAF struggles more when the target is black rather than white or beige? Is this a PDAF limitation?

I take plenty of shots of my white cat, Kookaburra and other bright birds and at 300mm I can't complain about focus, but if shooing something darker like a crow, it can feel like back focus issues and that the bird is not as sharp as it could me. It feels quite consistent, making me think user error or I need to fine tune my lens but then I am reminded of all the shots of my white cats and brighter subjects and I'm think "hmm... maybe PDAF struggles with locking onto dark objects?

TIA

BB

It’s subject contrast that is the problem. For phase detect autofocus to work, there has to be a phase to detect.

beholder3 · 06-04-2020, 11:08 AM - **2 Likes**

Originally posted by BruceBanner

Shot A

Tactic; Single AF Point, aimed direct on head/eyeball region.

You also seem to have a completely wrong idea of the size of the AF sensor cross. The one center sensor cross extends out to about the size of the spot metering brackets in the viewfinder.

In other words: 60% of the whole bird are focused on in your example. It's an illusion to think you focused on the "eyeball" or "head" technically. That is what you pointed it at, but still the system looks at a whole lot more. And if that "more" contains more sexy contrasty edges then it will focus on that.

**BruceBanner** · 06-04-2020, 03:07 PM

Originally posted by beholder3

You also seem to have a completely wrong idea of the size of the AF sensor cross. The one center sensor cross extends out to about the size of the spot metering brackets in the viewfinder.

In other words: 60% of the whole bird are focused on in your example. It's an illusion to think you focused on the "eyeball" or "head" technically. That is what you pointed it at, but still the system looks at a whole lot more. And if that "more" contains more sexy contrasty edges then it will focus on that.

That's interesting. I have noticed a difference between the size of the 'box' between the KP and K-1, the K-1 appears quite large and the KP really small, thus is it giving the illusion that the sample area (box area) is not as fine/specific as you actually think?

(of course my drawings here are nothing accurate, just me messing around quickly to illustrate where I had to place the 'box' to get better focus).

beholder3 · 06-05-2020, 12:58 AM - **2 Likes**

Originally posted by BruceBanner

That's interesting. I have noticed a difference between the size of the 'box' between the KP and K-1, the K-1 appears quite large and the KP really small, thus is it giving the illusion that the sample area (box area) is not as fine/specific as you actually think?

(of course my drawings here are nothing accurate, just me messing around quickly to illustrate where I had to place the 'box' to get better focus).

That here is what I empirically measured on the K1:

I use this (in png version) as overlay in Lightroom, when in doubt. It helps understand things.

The autofocus will grab the best contrast edge anywhere under the green cross.

Attached Images

**JensE** · 06-05-2020, 01:35 AM

Originally posted by beholder3

The autofocus will grab the best contrast edge anywhere under the green cross.

Did you do this for 'Spot' or 'SEL' mode?

beholder3 · 06-05-2020, 02:29 AM

Originally posted by JensE

Did you do this for 'Spot' or 'SEL' mode?

Spot mode.

For real life application one needs to keep in mind that unless you go through the pain of using a tripod, every human photographer will shake and sway while trying to take a shot. So the focus aim will move around probably 10-25% of the focus coverage in reality. Effectively making the temporary covered area that much larger.

We can not expect that the aiming of the lens when half-button-press activates AF operation is the exact same as when the shutter actually fires.

---------- Post added 5th Jun 2020 at 11:32 ----------

It is actually only one minute work to check the coverage yourself using powerpoint and the multi exposure mode of the camera for 4 shots.

Ian Stuart Forsyth · 06-05-2020, 11:49 PM

Originally posted by BruceBanner

eah. There's no such thing as PDAF Face/Eye Detection is there?

Some DSLR have this feature, it will locate the face a focus on it, when your are closer lets say for head and shoulder portraiture it some will select the nearest eye to the camera

**BruceBanner** · 06-08-2020, 07:16 PM

Originally posted by beholder3

That here is what I empirically measured on the K1:

I use this (in png version) as overlay in Lightroom, when in doubt. It helps understand things.

The autofocus will grab the best contrast edge anywhere under the green cross.

That's really interesting. It feels a little like opening a box of cereal or packet of crisps to only see the contents of the box or bag half full of the actual foodstuff

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06-03-2020, 11:57 PM - 1 Like	#31
BruceBanner Site Supporter Join Date: Dec 2015 Posts: 5,404 Original Poster	Originally posted by JensE (Edit: I wrote that without noticing that there's a second page - but the stuff on the Pentax page is still worth knowing.) Now you're well-prepared to digest How to optimize focusing accuracy with large-aperture lenses / explore, Part 1\| RICOH IMAGING, including the difference between cross- and line sensors. The note on the central horizontal f/2.8 line sensor, which detects horizontal changes, i.e. vertical edges, is noteworthy. Yeah, I read that in the past, I just never really digested the idea of 'lines' being the thing the box picks up LOL For a long time now I just use Spot AF and not even SEL with a single AF point in the middle as I believe that is actually not as accurate as Spot.

06-04-2020, 01:21 AM - 1 Like	#32
JensE Site Supporter Join Date: Dec 2015 Location: Leipzig Photos: Gallery \| Albums Posts: 1,969	Depending on the situation, the 'select' option may even be more accurate or at least better equipped to acquire focus, because it uses a combination of sensors. The 'spot' setting however uses a smaller region.

06-04-2020, 01:46 AM	#33
BruceBanner Site Supporter Join Date: Dec 2015 Posts: 5,404 Original Poster	My friend who recently acquired a Nikon D500 also told me that with their back button focus you can assign it to have a different AF mode to the front shutter system. Also, I have heard (possibly not D500 related) that when reviewing playback of images taken, when pressing the 'zoom in' button, the zoom will take you not to the centre of the frame but actually to the focus point of the shot, thus assisting massively with negating the necessity of using the D-pad to move around and check the focus of the image (and whether you obtained it or not). Stuff like this would be so handy, practically making touch screen tech on screens moot, I really would have liked to have seen Pentax roll out more firmware updates (but I guess we're moving off point here a bit).

06-04-2020, 08:30 AM	#34
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,981	Originally posted by BruceBanner So, I've been playing with my fairly recently accquired 55-300PLM, it's my first zoom and so I've been shooting birdies with it etc. I've been through Fine Adjutsments and feel as though out of the box it really seems ok and doesn't need excessive tuning. With the shots I have taken this far I am wondering if PDAF struggles more when the target is black rather than white or beige? Is this a PDAF limitation? I take plenty of shots of my white cat, Kookaburra and other bright birds and at 300mm I can't complain about focus, but if shooing something darker like a crow, it can feel like back focus issues and that the bird is not as sharp as it could me. It feels quite consistent, making me think user error or I need to fine tune my lens but then I am reminded of all the shots of my white cats and brighter subjects and I'm think "hmm... maybe PDAF struggles with locking onto dark objects? TIA BB It’s subject contrast that is the problem. For phase detect autofocus to work, there has to be a phase to detect.

06-04-2020, 11:08 AM - 2 Likes	#35
beholder3 Pentaxian Join Date: Dec 2011 Posts: 3,112	Originally posted by BruceBanner Shot A Tactic; Single AF Point, aimed direct on head/eyeball region. You also seem to have a completely wrong idea of the size of the AF sensor cross. The one center sensor cross extends out to about the size of the spot metering brackets in the viewfinder. In other words: 60% of the whole bird are focused on in your example. It's an illusion to think you focused on the "eyeball" or "head" technically. That is what you pointed it at, but still the system looks at a whole lot more. And if that "more" contains more sexy contrasty edges then it will focus on that.

06-04-2020, 03:07 PM	#36
BruceBanner Site Supporter Join Date: Dec 2015 Posts: 5,404 Original Poster	Originally posted by beholder3 You also seem to have a completely wrong idea of the size of the AF sensor cross. The one center sensor cross extends out to about the size of the spot metering brackets in the viewfinder. In other words: 60% of the whole bird are focused on in your example. It's an illusion to think you focused on the "eyeball" or "head" technically. That is what you pointed it at, but still the system looks at a whole lot more. And if that "more" contains more sexy contrasty edges then it will focus on that. That's interesting. I have noticed a difference between the size of the 'box' between the KP and K-1, the K-1 appears quite large and the KP really small, thus is it giving the illusion that the sample area (box area) is not as fine/specific as you actually think? (of course my drawings here are nothing accurate, just me messing around quickly to illustrate where I had to place the 'box' to get better focus).

06-05-2020, 12:58 AM - 2 Likes	#37
beholder3 Pentaxian Join Date: Dec 2011 Posts: 3,112	Originally posted by BruceBanner That's interesting. I have noticed a difference between the size of the 'box' between the KP and K-1, the K-1 appears quite large and the KP really small, thus is it giving the illusion that the sample area (box area) is not as fine/specific as you actually think? (of course my drawings here are nothing accurate, just me messing around quickly to illustrate where I had to place the 'box' to get better focus). That here is what I empirically measured on the K1: I use this (in png version) as overlay in Lightroom, when in doubt. It helps understand things. The autofocus will grab the best contrast edge anywhere under the green cross. Attached Images

06-05-2020, 01:35 AM	#38
JensE Site Supporter Join Date: Dec 2015 Location: Leipzig Photos: Gallery \| Albums Posts: 1,969	Originally posted by beholder3 The autofocus will grab the best contrast edge anywhere under the green cross. Did you do this for 'Spot' or 'SEL' mode?

06-05-2020, 02:29 AM	#39
beholder3 Pentaxian Join Date: Dec 2011 Posts: 3,112	Originally posted by JensE Did you do this for 'Spot' or 'SEL' mode? Spot mode. For real life application one needs to keep in mind that unless you go through the pain of using a tripod, every human photographer will shake and sway while trying to take a shot. So the focus aim will move around probably 10-25% of the focus coverage in reality. Effectively making the temporary covered area that much larger. We can not expect that the aiming of the lens when half-button-press activates AF operation is the exact same as when the shutter actually fires. ---------- Post added 5th Jun 2020 at 11:32 ---------- It is actually only one minute work to check the coverage yourself using powerpoint and the multi exposure mode of the camera for 4 shots.

06-05-2020, 11:49 PM	#40
Ian Stuart Forsyth Pentaxian Join Date: May 2009 Location: Somewhere over the rainbow Photos: Gallery Posts: 1,531	Originally posted by BruceBanner eah. There's no such thing as PDAF Face/Eye Detection is there? Some DSLR have this feature, it will locate the face a focus on it, when your are closer lets say for head and shoulder portraiture it some will select the nearest eye to the camera

06-08-2020, 07:16 PM	#41
BruceBanner Site Supporter Join Date: Dec 2015 Posts: 5,404 Original Poster	Originally posted by beholder3 That here is what I empirically measured on the K1: I use this (in png version) as overlay in Lightroom, when in doubt. It helps understand things. The autofocus will grab the best contrast edge anywhere under the green cross. That's really interesting. It feels a little like opening a box of cereal or packet of crisps to only see the contents of the box or bag half full of the actual foodstuff