PXW-Z90 - Aliasing Issues?
 

Hello. I recently purchased a Z90 and Doug Jensen's excellent Video series on the Z90/NX80.
Has anyone seen Aliasing issues in 1080p?
Doug commented on an another post I created and stated, like many others, they have not had any issues aliasing and it might be my workflow.

It is a awesome camera. I even tried a HX80 just to double check. Same issues.I saw the problems viewing on my computer (VLC, Premiere pro, my 55" Plasma TV) No grading, no effects applied, just straight out of the camera. I viewed all the clips a 1:1 zoom ratio's to make sure there was no scaling. I shot with and without Clear Image Zoom, Digital Extender etc. Look at the Black Drawers on the tool box. They "shimmer" when the clip is played.

I have posted short clip (few seconds) so you could see it for yourself. Look at the Black Drawers on the tool box. They "shimmer" when the clip is played.
Here is the link:
https://drive.google.com/open?id=15i...jMDmkCsfJXzkLw

Download the clip. It is an unedited clip trimmed with Avidemux so no processing was done to the clip. Download the original.

I also have a Sony A7Sii. I shoot 1080p only in Super35 mode since if I shoot 1080p from the full sensor, I get the same issue I am seeing with the Z90/NX80

Re: PXW-Z90 - Aliasing Issues?
 

The download link doesn't work.

Re: PXW-Z90 - Aliasing Issues?
 

I fixed the link. Thanks.

Re: PXW-Z90 - Aliasing Issues?
 

That’s light reflected off the white vehicle on to the toolbox.

Re: PXW-Z90 - Aliasing Issues?
 

Are you looking at the JPG or did you download the video?
Also, look at the trim around the grill of the Suburban.

Re: PXW-Z90 - Aliasing Issues?
 

Hmmm...
I'm not sure if I am seeing aliasing.
It would be interesting to see the same shot locked down on a tripod.

Re: PXW-Z90 - Aliasing Issues?
 

Well, you have a very narrow white or silver line against a black background, and that line is a) just slightly off horizontal and b) not much more than 1-2 pixels in width, with camera moving all about. Seems perfectly normal to me, just the perfect storm to create aliasing/flickering with any camera perhaps. Not seen anywhere else in image. I zoomed in 200% in Premiere to check it out.

If you have say a 1-pixel-width line, with high contrast, slightly off horizontal, it must by nature have stair-steps in it at the pixel level, and then when the camera moves, so do those steps, like an escalator in motion.

Might be able to play with sharpness controls or something to help a bit?

Thanks

Re: PXW-Z90 - Aliasing Issues?
 

This blow-up from Photoshop illustrates the issue. When watching the video, I don't see the wider aluminum handles shimmering, but just the very narrow "stripe" just above each handle. PS shows that the stripe is indeed only a couple of pixels wide at best.

Same with grille, you have literally a single pixel width of bright reflection from chrome strip, with dark background. You are going from a black pixel, to gray pixel to white pixel in as many steps, but then camera is moving and rotation of camera is changing, thus changing the angle of the grill line. I don't see how one would not experience aliasing in this situation.

As other posted suggested, probably would not have this on tripod, maybe even okay with smooth pan. It is the rotation of camera that constantly changes angle of the very narrow line which causes the "escalator steps" thing. I think in aircraft terms, it is the "roll" of the camera exacerbating the issue.

Re: PXW-Z90 - Aliasing Issues?
 

Thank you all. Yes, I was "rolling" the camera on purpose to make it easier to see.
When I place the camera on a tripod, it is of course much better, but still there.
On most "normal" shots, there is no problem.

Here is the same shot with a Canon XF400
https://drive.google.com/file/d/14l2...ew?usp=sharing

I was just shooting around the house, and saw this issue on window blinds, my roof tiles, etc.

Re: PXW-Z90 - Aliasing Issues?
 

Someone else mentioned to reduce sharpening - if your monitor/TV has sharpening, turn it OFF. First time I shot 4K and played it back, the "shimmer" was so bad I thought I had a bad camera, once sharpening was turned off, the image settled down significantly.

I also started to set shutter speed as low as I could (within reason) so that there was some natural motion blur to cover up overly "sharp" edges that would result from what were effectively very sharp "stills" when the shutter speeds were "too fast".

Because these cameras are capable of VERY crisp "still" frames, it can get pretty out of control whenever there is motion or fine details in the scene (temporal motion looks jumpy or choppy, making any aliasing even worse to the eye), probably something with algorithms trying to make jagged lines "sharp", which is exactly the opposite of what you want when trying to "smooth" out aliasing of fine edge/contrast "detail".

Seems counter-intuitive to buy these fancy 4K cameras that have such great image quality and then have to make 'em go a bit soft sometimes, but that's what it will take to get that shimmer under control.... and the images will look REALLY good with that little bit of softness and blur!

Re: PXW-Z90 - Aliasing Issues?
 

Thanks. I have reduced the sharpening and it is helping.

Re: PXW-Z90 - Aliasing Issues?
 

Andy, reset your detail level to what you had it at previously, and change from TYPE 3 to TYPE 1. The TYPE settings are the black and white edge thickness applied to the detail filter. Type 1 has the thinnest edging thickness, and provides a much smoother look. As the number increases, so does the thickness. In the larger Sony cameras, this setting is called WHITE and BLACK LIMIT. You may find that this would reduce the aliasing. I found it reduced my aliasing when shooting 720p.

Paul

Re: PXW-Z90 - Aliasing Issues?
 

I dont know if this is right or wrong but.....

The Z90 uses Sony's awesome phase detection auto focus. The teqnique asigns an array of pixels on the sensor to create a signal phase analys and comparison. The upside to this its lightning fast AF but the downside is that the pixels use for phase correlation with the "main" image need to removed, or averaged out from the image we all see. Those pixels are "sacraficed" for the cause.

This averaging out of those "holes" can cause some aliasing but it really shouldnt be too noticable under normal viewing circumstances.

CT

Re: PXW-Z90 - Aliasing Issues?
 

Cliff, does this mean that the picture quality improves if you turn AF off?

Paul

Re: PXW-Z90 - Aliasing Issues?
 

In a word No.

Okay let's dot the math. For example an A7111 has 693 AF points on a sensor of 24,000,000 photsites.

1% of that sensor equals 240,000 photsites. 1/10% equals 2,400 photosites. 1/100% equals 24 photsites.

Total Phase detection AF sites on the A7III are 693. Divide this by 24 and you end up with 28.875 100ths of a %. let's call this 29/100ths of a percent. In round figures 693 AF points amount to less that 1/3rd of a percent of the 24 megapixels on the A7III.

Sony's Phase Detection Auto Focus allows the camera to effectively use the differences in light intensity between different points on the sensor to identify if the object that the camera is trying to focus on is in front of or behind the focus point, and adjust focus to the sensor accordingly. This is a huge improvement both in terms of speed and accuracy over the traditional contrast-based autofocus that we’ve seen on many cameras in the past.

On Sony's A7III the 693 photodiodes substituted by AF pixels don’t create dead pixels in the image due to the way a Bayer CMOS sensor works. Bayer is a matrix of red, blue and green photodiodes and a system of interpolation. Each pixel on a Bayer sensor only captures 1/2 the color information in green and 1/3rd in blue and red for any given point in the image. The final pixel color value is actually created by interpolating data from adjacent pixels. For example a green pixel is combined with information from adjacent red and blue photsites to generate the appropriate color for that point in the image. After Debayering the result is less resolution and color accuracy but a faster running sensor. The 693 missing green photodiodes out of the millions on the A7III's sensor just need to interpolate from the adjacent photsites a little more.

In other words you will not see it, even when all these Phase AF pixels get get their color values replaced with the color information from the photosites sitting adjacent to them, Copied to them not interpolated.

For me this is a total non issue, only important to those who are number peeking, and those people are a lot more involved with mathematical theory then those who are pixel peeking. Number peeking is looking at small theoretical differences, differences not to be seen even when pixel peeking. Even if you knew where to look.

All said and done Sony's combining of a Phase Detection system with a traditional Contrast Detection system to give us Sony's new Hybrid Focus system benefits us all with a big leap forward in focusing performance. From my point of view I am more that happy to trade off 1/3% of photsite color information which is being interpolated anyhow in the Debayer process for the massive improvement in focusing. Whether you use the Hybrid AF or whether use manual focusing you won't see the difference in resolution. Test it for yourselves on any chart.

Chris Young