Where the PDAF sensors are...

Started Dec 15, 2011 | Discussions
Helen
Helen Veteran Member • Posts: 7,485
Where the PDAF sensors are...
1

Thought folks might be interested in this image. I was messing around with the 30-110mm lens last night, and happened to take a shot of the TV screen. When I examined the shot, I saw something interesting. On the attached image, I have (messily) highlighted the ends of the lines to aid visibility, because, even magnified to the largest size that DPReview allows, these do not show as clearly as on the original.

Basically, there are nine evenly-spaced horizontal lines on the image which are symmetrically arranged (if the yellow end markers I added indicate otherwise, it's only due to the difficulty I had seeing the lines in the program I used to edit the image) - and the top and bottom ones are shorter than the others. I am 99.9% sure that these indicate the positions of the phase-detect AF pixel arrays on the J1's sensor (which are not capable of image capture). These lines do not normally show in images, but something about the moving image from my LCD TV caused them to be visible, presumably because that something made it difficult for the image processing in the camera to accurately clone them out of the image as it would normally do - it's mis-guessed the colours to use. I took several shots and the lines were always in exactly the same place on each one - of course, I took normal shots of other subjects (i.e. not LCD screens!) afterwards to verify the lines were not something that would appear in every shot for some reason and in those subsequent shots, the lines were totally invisible, as the designers intended.

Whether this pattern is observable by photographing any LCD screen, I haven't checked. Visibility may vary according to the spec of the screen photographed and the TV system it's displaying - its visibility even varies according to the program you view the image in, but it is there, it's not an artefact. NB - these lines can only be seen if the TV image is photographed close enough (or with a powerful enough telephoto setting) to clearly resolve the screen's pixels - this seems to be what confuses the image processing into leaving the PDAF lines somewhat visible.

Anyway, I just thought it was intriguing to see where the PDAF sensors live...

Nikon 1 J1
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SHood Veteran Member • Posts: 5,694
Re: Where the PDAF sensors are...

Thanks Helen. I had wondered how the Nikon 1 sensor was doing PDAF. I am surprised no review site has picked up on this or at least asked Nikon how it was done.

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SHood Veteran Member • Posts: 5,694
Re: Where the PDAF sensors are...

Helen, I assume that this image was a crop and those lines represent a 1 pixel height. Could you confirm?

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D200_4me
D200_4me Veteran Member • Posts: 4,740
Re: Where the PDAF sensors are...

Interesting. I'll have to shoot my TV later today to see what mine looks like
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Helen
OP Helen Veteran Member • Posts: 7,485
Re: Where the PDAF sensors are...

SHood wrote:

Thanks Helen. I had wondered how the Nikon 1 sensor was doing PDAF. I am surprised no review site has picked up on this or at least asked Nikon how it was done.

I was just having a look around and there's a sort-of mention of this in the DCResource review of the J1 - well, it links to a Hungarian review where they got a line showing a bit when it happened to run at just the right (or wrong!) angle to a horizontal line on a test chart they were shooting. I think this was just a single line that showed in part, whereas I accidentally stumbled upon a way of showing the lot of them.

Pho3NiX Regular Member • Posts: 146
Re: Where the PDAF sensors are...

Nikon 1 state to have 73 autofocus point with the possibility to "choose any of 73 focus points when using the single-point AF mode "

Because there's so many I can guess they are covering about the whole sensor and if anything are more dense near the center as well as the 1/3 rules lines. I also doesn't make sens to allow to choose between points that are at the extreme of frame like that.
http://imaging.nikon.com/lineup/acil/bodies/v1/features02.htm

on another note, if you have a Nikon one... you can probably set it to single AF and loop thru all the point site.

Helen
OP Helen Veteran Member • Posts: 7,485
Re: Where the PDAF sensors are...

SHood wrote:

Helen, I assume that this image was a crop and those lines represent a 1 pixel height. Could you confirm?

It's not a crop, as it happens. It's the whole frame, so we've got quite a large part of it "covered" by PDAF sensitivity. Obviously the image quality in this shot is a bit rubbish due to it being a moving TV image - it's a 37 inch screen shot from about 9 feet away with the 30-110mm lens at (checks EXIF) 57.2mm (154mm equivalent in film terms), 1/40 sec. It is a crop of the TV screen image, because at that zoom level, it more than fills the frame of the shot. But the shot itself isn't cropped.

In case anybody wonders if it's some property of the TV screen that's being shown, I'm pretty sure that it isn't, since the lines occupy the EXACT same pixels on each shot I took, even though I wasn't framing up the TV with any sort of precision (and being a rather slow shutter speed for the focal length, the VR was of course moving the image occasionally, too).

SHood Veteran Member • Posts: 5,694
Re: Where the PDAF sensors are...

In that case these lines are "thick". Can you tell from the original image how many pixels in height are these lines?

Helen wrote:

SHood wrote:

Helen, I assume that this image was a crop and those lines represent a 1 pixel height. Could you confirm?

It's not a crop, as it happens. It's the whole frame, so we've got quite a large part of it "covered" by PDAF sensitivity. Obviously the image quality in this shot is a bit rubbish due to it being a moving TV image - it's a 37 inch screen shot from about 9 feet away with the 30-110mm lens at (checks EXIF) 57.2mm (154mm equivalent in film terms), 1/40 sec. It is a crop of the TV screen image, because at that zoom level, it more than fills the frame of the shot. But the shot itself isn't cropped.

In case anybody wonders if it's some property of the TV screen that's being shown, I'm pretty sure that it isn't, since the lines occupy the EXACT same pixels on each shot I took, even though I wasn't framing up the TV with any sort of precision (and being a rather slow shutter speed for the focal length, the VR was of course moving the image occasionally, too).

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Helen
OP Helen Veteran Member • Posts: 7,485
Re: Where the PDAF sensors are...

SHood wrote:

In that case these lines are "thick". Can you tell from the original image how many pixels in height are these lines?

Well, it's really hard to be sure, but it looks like a maximum of 3 pixels high. In parts though, only 1 pixel height appears discoloured. So the pixel rows above and below might just be some sort of interpolation, like how a single stuck pixel ends up with an array of affected pixels around it forming a larger cross pattern, due to the Bayer interpolation pattern.

Helen wrote:

SHood wrote:

Helen, I assume that this image was a crop and those lines represent a 1 pixel height. Could you confirm?

It's not a crop, as it happens. It's the whole frame, so we've got quite a large part of it "covered" by PDAF sensitivity. Obviously the image quality in this shot is a bit rubbish due to it being a moving TV image - it's a 37 inch screen shot from about 9 feet away with the 30-110mm lens at (checks EXIF) 57.2mm (154mm equivalent in film terms), 1/40 sec. It is a crop of the TV screen image, because at that zoom level, it more than fills the frame of the shot. But the shot itself isn't cropped.

In case anybody wonders if it's some property of the TV screen that's being shown, I'm pretty sure that it isn't, since the lines occupy the EXACT same pixels on each shot I took, even though I wasn't framing up the TV with any sort of precision (and being a rather slow shutter speed for the focal length, the VR was of course moving the image occasionally, too).

Helen
OP Helen Veteran Member • Posts: 7,485
Re: Where the PDAF sensors are...

Pho3NiX wrote:

Nikon 1 state to have 73 autofocus point with the possibility to "choose any of 73 focus points when using the single-point AF mode "

Because there's so many I can guess they are covering about the whole sensor and if anything are more dense near the center as well as the 1/3 rules lines. I also doesn't make sens to allow to choose between points that are at the extreme of frame like that.
http://imaging.nikon.com/lineup/acil/bodies/v1/features02.htm

on another note, if you have a Nikon one... you can probably set it to single AF and loop thru all the point site.

Yes, you can. I read that page - it's interesting. So, 73 (maximum) AF targets in PDAF. The manual only mentions that there are 135 total focus areas in single-point AF, and 41 in auto-area. These totals will include the CDAF points no doubt, which are smaller (I suspect) and inherently more "mobile". The figures are the same for the V1 and J1, by the way. The cameras seamlessly switch between the two systems according to the light and contrast levels, so it's difficult to be certain about how it acts - except that the PDAF variety is pretty much instant and clearly in use in bright light.

SHood Veteran Member • Posts: 5,694
Re: Where the PDAF sensors are...

The interpolation would likely only apply to a JPEG image. A RAW image would tell us exactly how many pixels in height are impacted from the PDAF sensors.

Helen wrote:

SHood wrote:

In that case these lines are "thick". Can you tell from the original image how many pixels in height are these lines?

Well, it's really hard to be sure, but it looks like a maximum of 3 pixels high. In parts though, only 1 pixel height appears discoloured. So the pixel rows above and below might just be some sort of interpolation, like how a single stuck pixel ends up with an array of affected pixels around it forming a larger cross pattern, due to the Bayer interpolation pattern.

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micksh6
micksh6 Senior Member • Posts: 2,613
Re: Where the PDAF sensors are...

This may be an effect of PDAF sensors but there can be some other explanation.

It could be a result of some weird interference between LCD refresh rate and exposure time and aliasing (pixelation) could make it more pronounced.

Is your TV 60Hz or 120Hz? 1/40s exposure time could be problematic - it may expose interference artifacts. And incandescent light too - it is also 60Hz in US, as well as LCD backlight refresh rate. LED vs CCFL backlight may matter here too.

Did you try different exposure? Would be interesting to see 1/25, 1/50, 1/100 and 1/200, for example.

I understand these lines are always on the same position on the sensor, right? Can you look at RAW image in software that doesn't apply distortion correction? I don't know such software for Nikon 1, perhaps Picasa or RawTherapee? And if you shoot LCD when camera is rotated?

It these lines belong to sensor it should be possible to find another subject that would "confuse" image processing and would also reveal the lines. For example, if you make a photo of some another high resolution very contrast photo with lots of details the image processing should fail at least in some places. You know the location of the lines so you can inspect them more carefully.

And even if these belong to sensor it could be a sensor defect. I'm sure sensor specs allow several dead lines and software compensates for that. You might have found a case when it doesn't work well. Perhaps others can reproduce it.

PS. I don't have Nikon 1. I'm just curious of possible PDAF effect on mirrorless cameras.

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Billx08 Forum Pro • Posts: 11,373
Re: Where the PDAF sensors are...

Helen wrote:
. . .

Helen, I assume that this image was a crop and those lines represent a 1 pixel height. Could you confirm?

It's not a crop, as it happens. It's the whole frame, so we've got quite a large part of it "covered" by PDAF sensitivity. Obviously the image quality in this shot is a bit rubbish due to it being a moving TV image - it's a 37 inch screen shot from about 9 feet away with the 30-110mm lens at (checks EXIF) 57.2mm (154mm equivalent in film terms), 1/40 sec. It is a crop of the TV screen image, because at that zoom level, it more than fills the frame of the shot. But the shot itself isn't cropped.

In case anybody wonders if it's some property of the TV screen that's being shown, I'm pretty sure that it isn't, since the lines occupy the EXACT same pixels on each shot I took, even though I wasn't framing up the TV with any sort of precision (and being a rather slow shutter speed for the focal length, the VR was of course moving the image occasionally, too).

Helen, if you edit your DPR profile, there's a setting at the bottom of the "Privacy and safety" tab that will allow us to view and download your original, full size photos for closer examination.

Helen
OP Helen Veteran Member • Posts: 7,485
Re: Where the PDAF sensors are...

micksh6 wrote:

This may be an effect of PDAF sensors but there can be some other explanation.

Yes, I initially thought that.

It could be a result of some weird interference between LCD refresh rate and exposure time and aliasing (pixelation) could make it more pronounced.

And I suppose it still could, even if it does result in in showing the position of the PDAF arrays - after all, something about the images of my TV screen causes the camera to fail to hide the lines as thoroughly as usual.

Is your TV 60Hz or 120Hz? 1/40s exposure time could be problematic - it may expose interference artifacts. And incandescent light too - it is also 60Hz in US, as well as LCD backlight refresh rate. LED vs CCFL backlight may matter here too.

It's neither - it's 50Hz or 100Hz, because I am in the UK and that's our rate (electricity supply is 50Hz and it's the PAL TV system). The reason I'm not sure of the figure is that the specs of the TV are a bit hard to decipher, due to marketing-speak. There is a close equivalent TV on the US market, though, I believe. My TV is a Samsung UE37D6530; an LED backlit model from this year's UK range. I believe the panel in it is a 100Hz one though when the TV's accepting input from my PVR (though this picture and others I will upload was straight from the TV's tuner) it labels it as 50Hz. The set has the motion processing (CMR) set to "Clear" and the flickering backlight is not enabled - I think if it was, that would fulfil the marketing-speak that allows them to claim it can act at the equivalent of 200Hz (mentioned on the packaging!).

Did you try different exposure? Would be interesting to see 1/25, 1/50, 1/100 and 1/200, for example.

I've examples taken at 1/20, 1/25 and 1/60 as well, which I shall upload in due course. Unfortunately I've got to do some other family duties before I can get onto that.

I understand these lines are always on the same position on the sensor, right?

I think so, unless I've looked at them too quickly and jumped to conclusions.

Can you look at RAW image in software that doesn't apply distortion correction? I don't know such software for Nikon 1, perhaps Picasa or RawTherapee? And if you shoot LCD when camera is rotated?

So far I've only shot them as jpgs. I'm not a big fan of RAW, so the only thing I have to hand I think is the latest version of ACR - don't know whether that applies distortion correction and I've only just had the software it's in (trial version!). I'll try and shoot some in RAW later and have a look.

It these lines belong to sensor it should be possible to find another subject that would "confuse" image processing and would also reveal the lines. For example, if you make a photo of some another high resolution very contrast photo with lots of details the image processing should fail at least in some places. You know the location of the lines so you can inspect them more carefully.

And even if these belong to sensor it could be a sensor defect. I'm sure sensor specs allow several dead lines and software compensates for that. You might have found a case when it doesn't work well. Perhaps others can reproduce it.

I hope not! I will be interested (and relieved, now you've raised the ghost of a malfunction) if others find them in their own cameras. Since these Nikon 1 cameras do have PDAF detection integrated onto the imaging sensor itself (and I remember reading that the portions which carry out the function cannot participate in image capture), I am hopeful that this is what they are, especially given their coverage pattern.

PS. I don't have Nikon 1. I'm just curious of possible PDAF effect on mirrorless cameras.

Helen
OP Helen Veteran Member • Posts: 7,485
Re: Where the PDAF sensors are...

Billx08 wrote:

Helen wrote:
. . .

Helen, I assume that this image was a crop and those lines represent a 1 pixel height. Could you confirm?

It's not a crop, as it happens. It's the whole frame, so we've got quite a large part of it "covered" by PDAF sensitivity. Obviously the image quality in this shot is a bit rubbish due to it being a moving TV image - it's a 37 inch screen shot from about 9 feet away with the 30-110mm lens at (checks EXIF) 57.2mm (154mm equivalent in film terms), 1/40 sec. It is a crop of the TV screen image, because at that zoom level, it more than fills the frame of the shot. But the shot itself isn't cropped.

In case anybody wonders if it's some property of the TV screen that's being shown, I'm pretty sure that it isn't, since the lines occupy the EXACT same pixels on each shot I took, even though I wasn't framing up the TV with any sort of precision (and being a rather slow shutter speed for the focal length, the VR was of course moving the image occasionally, too).

Helen, if you edit your DPR profile, there's a setting at the bottom of the "Privacy and safety" tab that will allow us to view and download your original, full size photos for closer examination.

Thanks, I'll have a go - though it may be a few hours till I can.

OleThorsen Veteran Member • Posts: 3,111
No auto distortion correction

micksh6 wrote:

I understand these lines are always on the same position on the sensor, right? Can you look at RAW image in software that doesn't apply distortion correction? I don't know such software for Nikon 1, perhaps Picasa or RawTherapee? And if you shoot LCD when camera is rotated?

No auto distortion correction is made on N1 RAWs in any of the converters I know of - ACR, Nikon ViewNX2, Nikon Capture NX2, RAW Therapee unless you positively instruct them to do distortion correction. In camera JPGs and RAWs are not changed with any auto distortion correction either.

The N1 lenses doesn't need any auto distortion correction like the most M43 lenses do. Not that N1 lenses are distortion free, but they have pretty normal distortion and comparable to Nikon DSLR lenses.

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micksh6
micksh6 Senior Member • Posts: 2,613
Re: Where the PDAF sensors are...

Yes, I saw image with 1/60s exposure and others. I also saw full resolution JPEGs.

The lines always start at pixel Y=328 on full resolution. Then next line is at around Y=566, then 806, 1046 and so on. About 240 pixel difference between the lines. The last line is at Y=2592 or 344 pixels from the bottom.

Take closer look at DSC0475.jpg in your gallery. Note how lines are invisible on pure red color.

From what I see it looks like BGBG lines have been taken out from Bayer matrix. And yes, these lines seem to be in good place for PDAF sensors.

I wonder why nobody noticed it before? I doubt LCD has much to do with revealing it.

I'm sure this should be visible on many bright, sharp, high contrast pictures with lots of green color (green channel seems to be most affected).

So, I would still try longer exposure (something like 1/3s, better with tripod to preserve details), just to be sure light flickering doesn't contribute to that.
Also try to rotate and shift camera against TV. It still could be your TV.

And surely check RAW files. Could be as simple as in-camera JPEG compression artifacts. Download free RawTherapee, it should support this camera.

Try another SD card for the sake of eliminating all extra variables from the equation.

It's interesting to see if anyone else can reproduce this. If not, maybe your camera is defective?

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Bob Meyer Veteran Member • Posts: 5,375
Re: Where the PDAF sensors are...

SHood wrote:

The interpolation would likely only apply to a JPEG image. A RAW image would tell us exactly how many pixels in height are impacted from the PDAF sensors.

I doubt it. We know that Nikon is partially "cooking" the raw files (applying NR at ISO 800 and up, for example) and I suspect any interpolation that's being done to mask these rows is done to the raw, too. People would be pretty ticked off if they found horizontal lines in their raw files.

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Bob Meyer Veteran Member • Posts: 5,375
Re: Where the PDAF sensors are...

Pho3NiX wrote:

I also doesn't make sens to allow to choose between points that are at the extreme of frame like that.

Why not? On some Panny's you can put the focus point at the very edge of the frame. I might not do it often, but I can certainly imagine wanting to focus on a person or other object at the very edge of the frame.

Helen wrote:

The cameras seamlessly switch between the two systems according to the light and contrast levels, so it's difficult to be certain about how it acts - except that the PDAF variety is pretty much instant and clearly in use in bright light.

According to sansmirror.com, the J1, at least, seems to use the CDAF even when you're using PDAF, to "fine tune" the AF.

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Bob Meyer Veteran Member • Posts: 5,375
Re: No auto distortion correction

OleThorsen wrote:
er.

The N1 lenses doesn't need any auto distortion correction like the most M43 lenses do. Not that N1 lenses are distortion free, but they have pretty normal distortion and comparable to Nikon DSLR lenses.

Which goes a long way to explaining why the lenses are so huge compared to the body and sensor size.

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