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06-01-2020, 05:36 PM   #1
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PDAF Struggles with Black?

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

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06-01-2020, 06:18 PM - 5 Likes   #2
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Bruce, you understand that PDAF systems do not focus on objects or colours?

They focus on lines and edges.
06-01-2020, 06:26 PM - 3 Likes   #3
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That's possible. With darker objects, the lines/edges may be less distinguishable than with something lighter in color. I do actually find that CiF isn't as good with black bears...
06-01-2020, 07:39 PM - 1 Like   #4
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QuoteOriginally posted by clackers Quote
Bruce, you understand that PDAF systems do not focus on objects or colours?

They focus on lines and edges.
I never really thought about it like that! That's good and perhaps armed with this knowledge I can better improve focus accuracy by placing the focus point (square) on a place where the object line exists rather than on the object itself? Example;

MS Paint time!

Black crow with a dark eye, placing the AF point on that area, perhaps the lines are not that 'obvious' to PDAF and the result is a soft shot.

If placing the AF point on the edge of the birdy, or maybe something with stronger lines like the beak etc, it can thus better improve AF accuracy? It might be the eye of the crow is still soft as where the eye is located falls out of the focus plan captured, but at this time I can work with that, I'm just a bit curious if perhaps the lines the PDAF struggles to lock on exists more with black hairy/feathery things vs brighter things like white moggy's and swans?

Furthermore, if PDAF fails on certain scenes where clear lines are harder for it to pick up on (crow example again), would the system err on being more front or back focusing (does it have a bias/preference)?

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06-01-2020, 07:44 PM - 8 Likes   #5
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QuoteOriginally posted by bertwert Quote
That's possible. With darker objects, the lines/edges may be less distinguishable than with something lighter in color. I do actually find that CiF isn't as good with black bears...
Yes, and the last three guys I sent out to take macro pictures of bears haven't reported back to me. Don't know why.
06-01-2020, 07:52 PM - 4 Likes   #6
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QuoteOriginally posted by BruceBanner Quote
I never really thought about it like that! That's good and perhaps armed with this knowledge I can better improve focus accuracy by placing the focus point (square) on a place where the object line exists rather than on the object itself?
Can't believe you've taken this long to figure it out!

People who accuse their system of backfocusing in situations like backlighting don't realize the camera has focused on the only edge it can find in the vicinity, which is the outside of the head or whatever.

In a wedding with dim lighting, you're better off focusing on the groom's collar maybe, depending on your angle - that edge with the white shirt. You'll work out what the 'proxy' focus point is. Unlikely to be their ears. For me and football players, it'll be the 'V' on their uniform front or whatever.

With a CDAF focus system like the Fuji medium formats or the new Panasonic full frame (or even a Pentax in Live View), it may never find focus in time for the shot.

The closer the subject is, the harder this gets. Quite a few of the bird photographers (there's a Melbourne Nikon guy named Jan Wegener) will stop down to f8 when taking pictures even if they have an expensive f4 prime, because they will prefocus on the branch, and they want to get everything from the tail to the eyes in focus, no guessing.

Last edited by clackers; 06-01-2020 at 08:00 PM.
06-01-2020, 07:57 PM - 1 Like   #7
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QuoteOriginally posted by BruceBanner Quote
I never really thought about it like that!
Absolutely! I always focus on edges, never on slabs of featureless colour. This is a carry-over from my old days shooting an OM1 with split screen focussing, where broken edges needed to be aligned. The strategy still works.
06-01-2020, 08:18 PM - 1 Like   #8
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QuoteOriginally posted by clackers Quote
Can't believe you've taken this long to figure it out!

People who accuse their system of backfocusing in situations like backlighting don't realize the camera has focused on the only edge it can find in the vicinity, which is the outside of the head or whatever.

In a wedding with dim lighting, you're better off focusing on the groom's collar maybe, depending on your angle - that edge with the white shirt. You'll work out what the 'proxy' focus point is. Unlikely to be their ears. For me and football players, it'll be the 'V' on their uniform front or whatever.

With a CDAF focus system like the Fuji medium formats or the new Panasonic full frame (or even a Pentax in Live View), it may never find focus in time for the shot.

The closer the subject is, the harder this gets. Quite a few of the bird photographers (there's a Melbourne Nikon guy named Jan Wegener) will stop down to f8 when taking pictures even if they have an expensive f4 prime, because they will prefocus on the branch, and they want to get everything from the tail to the eyes in focus, no guessing.
I know! I am a mystery to the photographic world! How can this Bruce/Eddy guy be so god damn gifted in photography but continue to be a clueless git!

Nah thanks heaps, it is helpful. I think you know I'm pretty nontechnical when it comes to photography. I'm quite proficient with the camera and menu system and what menu items do what etc, but some of the basics in photography are completely oblivious to me. It's like Photoshop, I dived right in and know how to do some trickier stuff such as FS/DB or quick efficient stray hair removal with Surface Blur techniques... but I've never used the Pen tool in my life (I'm actually only now just making my way through a bought for Photoshop Tutorial courtesy of PRO-EDU hehe).

Looking back, a lot of my portrait work has been done with CDAF and Face Detection, I used PDAF for a lot of the 'walking down the aisle' stuff and other long distance work. It's only now I am using the 55-300PLM and shooting stuff further away that feels a lot better to use PDAF than CDAF (even just for steadying for the shot), and I don't really do wildlife so this is kinda new genre to me. I wasn't sure if I was being unlucky or seeing a pattern form whereby if I was shooting black cats/birds then the focus felt quite a lot softer than shooting my own white moggy or kookaburras/cockies etc.

06-01-2020, 08:25 PM   #9
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QuoteOriginally posted by BruceBanner Quote
Looking back, a lot of my portrait work has been done with CDAF and Face Detection
I do a certain amount of that myself. But if there isn't direct light, there can be not enough contrast between the eyes and the rest of the face, and I find that it really struggles.

Same with a Sony A7 IIIR I shot with too, BTW. It clearly relies on CDAF for its Face/Eye Detection. It annoyingly switched over to face detection again and again, because it had no luck with the eyes unless close.

Here's a screenshot of a Canon R series fullframe mirrorless failing to find the eyes for the same reason. Don't know why people get so excited by features that make errors a human never would.
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06-01-2020, 08:41 PM   #10
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QuoteOriginally posted by clackers Quote
I do a certain amount of that myself. But if there isn't direct light, there can be not enough contrast between the eyes and the rest of the face, and I find that it really struggles.

Same with a Sony A7 IIIR I shot with too, BTW. It clearly relies on CDAF for its Face/Eye Detection. It annoyingly switched over to face detection again and again, because it had no luck with the eyes unless close.

Here's a screenshot of a Canon R series fullframe mirrorless failing to find the eyes for the same reason. Don't know why people get so excited by features that make errors a human never would.
Yeah. There's no such thing as PDAF Face/Eye Detection is there? In these past discussions about Pentax possibly doing a Hybrid Viewfinder, the ones that shot it down were still saying you can do Eye Detection through the OVF without EVF, I never really understood how though. I mean I've seen the lame tracking we currently have with Pentax via the PDAF/OVF but it ain't great, has any company done Eye/Face Detection through the eyepiece without using CDAF?
06-01-2020, 09:31 PM   #11
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QuoteOriginally posted by BruceBanner Quote
I mean I've seen the lame tracking we currently have with Pentax via the PDAF/OVF but it ain't great,
It does what it's supposed to do. See this Youtube video, works for me. Note it's the left edge of the headlight it's tracking.



QuoteOriginally posted by BruceBanner Quote
has any company done Eye/Face Detection through the eyepiece without using CDAF?
No, because they all run primitive shape recognition systems. Look at this still from a Sony allegedly with eye tracking of the basketballer with white top. What is it focussing on?

Hype, because again, no human who knew what he was doing would make that tracking error. You would pan and follow the target.
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Last edited by Sandy Hancock; 06-01-2020 at 10:48 PM.
06-01-2020, 09:44 PM - 2 Likes   #12
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QuoteOriginally posted by BruceBanner Quote
has any company done Eye/Face Detection through the eyepiece without using CDAF
Yes, Nikon I believe. D500, D810, D850 and others. It uses the RGB metering and scene recognition sensor.
06-01-2020, 09:54 PM   #13
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QuoteOriginally posted by caliscouser Quote
Yes, Nikon I believe. D500, D810, D850 and others. It uses the RGB metering and scene recognition sensor.
I do have a screen grab of a Nikon D500 with its 3D tracking of a dog (I know, not a face recognition example). Its algorithm pushed the point out in front and it's now happily attempting to focus on the background trees, it doesn't seem to know any better.
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Last edited by clackers; 06-02-2020 at 04:51 PM. Reason: Naughty word
06-01-2020, 11:21 PM - 1 Like   #14
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I shoot lot of dance battles. And always curse when the dancers have lot of black clothes.
If there is enough light it is ok. But if the lights go down, I don't even try anymore. As it is almost useless.
06-01-2020, 11:48 PM - 2 Likes   #15
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QuoteOriginally posted by clackers Quote
It does what it's supposed to do. See this Youtube video, works for me. Note it's the left edge of the headlight it's tracking.

PENTAX K-3 II "Auto tracking" - YouTube




No, because they all run primitive shape recognition systems. Look at this still from a Sony allegedly with eye tracking of the basketballer with white top. What is it focussing on?

Hype, because again, no human who knew what he was doing would make that tracking error. You would pan and follow the target.
Yeah, I've not had any real success with that kinda tracking on my KP/K-1, I might give it another go however. I hope even something like this is improved a good 50% on the new K-new, that would be great!

Anyway, stop swearing and using naughty words, Sandy's onto us

Just after I came back from the school pick up I decided to try and do the MS paint image type tactic with a willing subject on my telegraph pole outside my joint. What a cooperative fellow.

KP+55-300PLM, minimal editing, some exposure correction and sharpening/texture/clarity boost to help see the difference.

Shot A

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



Crop+AF Point approximation



Note that the metal power cable thingy majig is more infocus in front of the birdy.


Shot B

Tactic; Single AF Point, aimed on the outline of the birdy head region.



Crop+AF Point approximation



Note that the metal power cable thingy majig is now out of focus, and the head/eye/beak is better focused.

Better late than never to learn how to AF I guess
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