JakTrax
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Jagged edges in high-res files

Thu Jun 22, 2017 3:27 pm

All,

First noticed with my 6D a couple of years ago but now with several other Canon cameras (with the exception of the 50D) and also a Sony. Particularly (but not limited to) areas of low contrast (think the cheatlines of the Air Malta retro scheme - red and dark blue), non-horizontal/vertical lines appear jagged or 'stepped'; I'm aware that pixels by their nature make this inevitable, but I shouldn't be noticing it at a resolution of 20+ megapixels. At first I put it down to poor anti-aliasing filters in newer, higher-megapixel bodies when shooting jpeg, however it's also present in RAW files (although to a lesser degree).

Doing a little research online, it now transpires that the cause is likely the resolution of my monitor, and how it's displaying the files. Since my expertise and knowledge of computer monitors is practically non-existent, I wondered if anyone can shed any scientific light on the subject?

Knowing what I do about digital images, I can't realistically see it being a camera issue, especially since my 2008 15mp 50D doesn't suffer from the problem. Unless of course more modern AA filters are indeed the cause, due to them perhaps being less effective on today's higher resolution sensors?

Cheers,
Karl
 
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dvincent
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Re: Jagged edges in high-res files

Thu Jun 22, 2017 7:11 pm

Is this when viewing the files at 100%? There are likely multiple factors at play.

The first is that normal screen resolutions are indeed low. You can try taking some of your RAW files to an Apple store and check them out on a retina display for a higher DPI look. But this will help in the way a print helps, higher resolutions mean less visible aliasing simply because you have more pixels to describe diagonal lines. A Dell 24 inch 4K monitor would do the job on Windows, but not as well since it's not as dense as a 22 inch iMac or the Macbook Pros.

Another factor is that red and blue have less resolution info available due to bayer pattern arrays on the sensor. Your 20MP camera really only has 10MP available for green, 5 MP for red, and 5 MP for blue. There's interpolation factors going on that can affect the appearance of these red and blue colors, especially in detail areas. False color moire is an issue that this can bring up on cameras with weak AA filters, of course, but that's a separate yet related issue. Ever have a yellow logo on top of a red plane and notice how the edges are fuzzy? That's why. The only solution to this is a Foveon sensor or the hyper-res modes on things like Pentax or Olympus. Or an array that's different than Bayer (e.g. old 3CCD methods).
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JakTrax
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Re: Jagged edges in high-res files

Thu Jun 22, 2017 10:36 pm

Thanks for that. So would you say Sony sensors have weak AA filters? Or was that just a blanket statement? Obviously jagged lines and moire are somewhat related, and both I dare say are to do with detail being too precise for the sensor to accurately render.

I'm not familiar with Oly or Pentax, so therefore have no clue about hyper-res functions - a very good summary but I'm still not sure whether it's a monitor issue, a sensor/AA issue, or a bit of both.

Like I say, I'm loathe to accept that it's solely a sensor issue as a near-10-year-old 50D is fine, and Sony sensors are generally considered class-leading.

Finally, I know you say yellow on red, but red on yellow is just as bad - the amount of times I've photographed the DHL livery and the red bits (registration, titles, cheatlines) never seem to look sharp!

Cheers,
Karl
 
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dvincent
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Re: Jagged edges in high-res files

Fri Jun 23, 2017 1:50 am

Karl,

Yellow on red and red on yellow's basically the same thing to the camera. DHL is a particularly bad color scheme for bayer arrays. It shows exactly the weakness of Bayer sensors sacrificing color resolution.

I don't recall the stats on the 20MP crop sensor (it's one I haven't used), but the 24 MP crop sensors tend to do better on color separation but they forego the AA filter entirely. The 20 MP sensor still has an AA filter, AFAIK, but I do not know its strength. It's a sensor only used in low-end cameras (a58, a3000, a5000). It was cycled out pretty quickly with the a3K and a5k replacements going up to the 24MP sensor used in the a6000. Older cameras had stronger filters, as the lower the resolution the stronger the filter needs to be. Even then, the nature of what is being photographed can result in aliasing. I've had my 12MP a700 produce aliasing and false color effects under certain conditions. United's old blue colorscheme was one that it was particularly prone to, since the U gradient was made of tons of little color strips.

There's another factor at play, and that's color filter array separation, which every sensor differs on. Newer sensors have been taking the tack of weaker filters with less separation to improve high ISO noise performance. This can have an affect on the look of aliasing as well by preventing bleed-over on colors between the filters. RAW developers also affect this, especially with the default Adobe profile and how hard you push the input sharpening.

Do you happen to have any samples of specific troubles? It's hard to judge whether it's debayering, AA filter, or processing that's affecting it without seeing a result. Keep in mind that looking at 100% the higher you go gets to be tougher on images because you're seeing, proportionaly, a higher magnification the higher the res go. A higher res monitor or prints reduce the effect of this because you're not looking at 100%.
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Miguel1982
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Re: Jagged edges in high-res files

Fri Jun 23, 2017 6:25 am

Not trying to hijack Dvincent's wonderful response, but I found the 6D's default sharpness setting (3) to be WAY too high. That easily resulted in jagged edges and light halos around high-contrast borders. I have it now set to 1, and usually reduce it afterwards on Canon's own DPP software to 0,5 - 0,7. Obviously this post-processing can only be done when shooting RAW.

Cheers,
Miguel
 
JakTrax
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Re: Jagged edges in high-res files

Fri Jun 23, 2017 4:11 pm

Miguel,

Yes, shooting RAW mostly eliminates jaggies in 6D images.

Dan, here's a crop at 100%. Hopefully it illustrates the issue; it's only slight but you can definitely see the 'stepped' effect where the orange and white meet. The image does look very sharp, however similarly sharp images from other cameras don't display the same jaggies.

https://www.flickr.com/photos/130333117 ... /lightbox/

Cheers,
Karl
 
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dvincent
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Re: Jagged edges in high-res files

Fri Jun 23, 2017 6:44 pm

Karl,

Thanks for the sample. Looking on my iMac's 22 inch 4K screen at work, the native resolution (e.g. 100% view in Photoshop) is dense and the aliasing does not look like a problem. It looks like a sharp print. Pulling it over to my non-retina Dell 24 inch to the side, I can see on the bottom side of the orange stripe has some aliasing going on but it would probably go away when viewed as a print, or when downsampled. Always have to remember that looking at 100% on a non-retina screen is something like 96-100 DPI, which is only one third of your average print (and most prints can be done at 200-250 DPI on an inkjet without much complaint). I can see some sharpening effects going on around there, was this an export from Lightroom or a screengrab? I'm just wondering if turning down the Lightroom sharpening sliders could help. LR's sharpening in the develop module is input sharpening, and I've always used 50/1/25/25 as my defaults. When I downsample in Photoshop, I always use Bicubic (Best for Gradients) which doesn't try to do any tricks.

What lens is this, btw? Some lenses might have slight chromatic aberration that, when corrected, could result in some edge exaggerations. Is this a Canon lens you were using on an adapter, or an E-mount lens? Are you using CA correction?

To be honest, looking at it on a non-retina display shows something, but it just seems to be a case where turning down the input sharpening could make it less objectionable, by reducing those contrasty edges. On the retina display, it's not objectionable. It may be a foible of that sensor and how it outputs. But I'm far more likely to pin it on processing than an actual flaw in the sensor.

My TL:DR is that I'm more inclined to pin this on sharpening than an issue with a sensor, but without playing with a RAW file it's hard to say. Honestly, I wouldn't worry about it other than to turn down sharpening for a5000 pics.
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JakTrax
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Re: Jagged edges in high-res files

Sat Jun 24, 2017 3:44 pm

Thanks Dan. I opened the original jpeg in Photoshop (Elements 5.0), then cropped to highlight more of the affected area. I then defaulted the view to 100% (actual image size), saved (in highest quality) and uploaded to Flickr.

The lens is the Sigma 30mm f/2.8 Art (E-mount fit), which does have a reputation for being incredibly sharp. As far as I know I'm not using an CA correction (not in processing anyway - the camera might be applying some by default but the manual's so poor I wouldn't know!). I have turned the jpeg sharpness down to -2 in camera (it was set to 0) so will see if that helps.

I'm inclined to agree that it's likely not the sensor - I suspect the jpeg engine is 'over-processing' - however this issue does seem to be most prominent with 20mp sensors (in my experience). The worst offenders I've seen are the 6D, 70D and now the A5000.... all of which just happen to be 20mp. Coincidence?

Karl
 
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dvincent
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Re: Jagged edges in high-res files

Sun Jun 25, 2017 1:58 am

The lens is the Sigma 30mm f/2.8 Art (E-mount fit), which does have a reputation for being incredibly sharp. As far as I know I'm not using an CA correction (not in processing anyway - the camera might be applying some by default but the manual's so poor I wouldn't know!). I have turned the jpeg sharpness down to -2 in camera (it was set to 0) so will see if that helps.


The e-mount lens spec includes distortion, vignette, and CA correction information. Lens manufs are supposed to encode that into the ROMs on the lenses so the camera knows how to adjust for optics. If you're shooting JPEGs, the camera will by default correct vignetting, distortion, and CA. You can turn these off in the Lens Comp. section in the main menu.

This info also gets baked into RAW files which Lightroom will honor if you tell it to do corrections.

I'm inclined to agree that it's likely not the sensor - I suspect the jpeg engine is 'over-processing' - however this issue does seem to be most prominent with 20mp sensors (in my experience). The worst offenders I've seen are the 6D, 70D and now the A5000.... all of which just happen to be 20mp. Coincidence?


I have no first-hand experience with these sensors, I've only used the 24 MP crop sensors (or once, the 18 MP Canon crop sensor). But if you're interested in looking at image quality tests, you could always check out the ones at Imaging Resource and do some comparisons. They have shots of resolution and color charts.
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