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Canon EOS Crop Sensor for HD
APS-C sensor cameras including the 80D, 70D, 7D Mk. II, 7D, EOS M and Rebel models for HD video recording.


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Old March 15th, 2011, 11:55 PM   #1
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Far away contrasty lines causes rainbow-like video distortion?

While I've been test shooting my 7D I've come to realize something that I hope is fixable. Say I'm 300 meters away from a brick wall and I shoot it from far away. If I focus on the bricks, when I record there will be this odd, subtle video effect where waves of rainbow colors will kind of strobe in and out of the wall. This happens to most things where there are lots of lines in a small area from far away - the fine contrast screws something up. It's VERY subtle but definitely noticeable.

I attached some pictures of what I'm talking about but I don't know if it will be that clear without watching the video of it.

What is this called / is it fixable?
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Old March 16th, 2011, 02:56 AM   #2
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Re: Far away contrasty lines causes rainbow-like video distortion?

That's aliasing.

DSLRs throw away lines and bin pixels to get their ~20MP imagers to deliver 1080p video. For this generation, we're stuck with it.

There are a few ways to deal with it. The image you show has fairly uniform color (were it not aliasing). If you can get your NLE to blur the color but not the luma, the rainbows will be minimized. You'll still get some ratty edges though.

The best way to avoid this is to put a person or object of interest in the scene, focus on them, and open the aperture a bit so the background is slightly out of focus. Problem solved. Just make sure the person isn't wearing finely patterned clothing. Anything in crisp focus can alias with DSLR video.

Another solution, when on a tripod, is to snap a photo of the background. You can then track the image in After Effects and replace the video with the photo in areas of no foreground motion.
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Old March 16th, 2011, 05:16 AM   #3
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Re: Far away contrasty lines causes rainbow-like video distortion?

One way to avoid this "color aliasing" is to modify your; angle, fov, focus.

This is less apparent on the 5D mkii, it is similar among 7D,550D, 60D.
It is very minimal or not present in the GH1 and 2.

Having said that, I still like the overall color and contrast aesthetic of the canons compared to the lumix gh cameras.
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Old March 16th, 2011, 09:58 AM   #4
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Re: Far away contrasty lines causes rainbow-like video distortion?

Also, when you are recording, make sure your picture style has the sharpness turned all the way down. Using sharp glass like ziess can also exaggerate the problem. Don't do any over the top sharpening in post.
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Old March 16th, 2011, 10:00 AM   #5
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Re: Far away contrasty lines causes rainbow-like video distortion?

Oh I see,

Well I think the best way for me to avoid this is going to be either with the chroma blur or just focusing on something else in the frame that's near it. I had no idea it was such a universal thing.
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Old March 16th, 2011, 11:29 AM   #6
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Re: Far away contrasty lines causes rainbow-like video distortion?

Quote:
Originally Posted by Greg Fiske View Post
Also, when you are recording, make sure your picture style has the sharpness turned all the way down. Using sharp glass like ziess can also exaggerate the problem. Don't do any over the top sharpening in post.
Good point about turning Sharpness to minimum. You can then sharpen in post, where you can mask off problem areas.

I've got non-L Canon glass at home and Zeiss ZE lenses at work. I much prefer using the ZEs for video. When I nail the focus on the eyes with the Zeiss glass, they really sparkle. As long as the DOF isn't too deep, aliasing on backgrounds isn't a problem. On the other hand, when my EF lenses are in focus on detailed backgrounds, I still get aliasing. I can nudge either an EF or ZE lens slightly out of focus to knock down aliasing. It's hard to know how much to do that in the field though...

BTW, a diffusion filter, like GlimmerGlass or Soft fx, can help slightly. Both with defocus and diffusion, by the time you've killed the aliasing, the image is simply too soft.
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Old March 16th, 2011, 05:33 PM   #7
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Re: Far away contrasty lines causes rainbow-like video distortion?

I see it with the Canon 70-200mm f2.8 II.

I could be wrong but my thought was that it's more related to the ISO. I don't think I've noticed it as much below 1250 iso.
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Old March 17th, 2011, 12:41 AM   #8
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Re: Far away contrasty lines causes rainbow-like video distortion?

You'll get blocking artifacts and color noise with high ISOs for sure. But aliasing is ISO independent. Typical scenes that show the effect are wide city/town shots with tile roofs, car roof tops, and the little waves that appear on water on a windy day. And you can get it with any lens that is in focus on the alias prone areas.
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Old March 17th, 2011, 03:45 PM   #9
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Re: Far away contrasty lines causes rainbow-like video distortion?

Aliasing and moire are common problems with the Canon DSLR's. As previous posters mentioned, try to stay away from deep focus shots in locations with lots of tight patterns (i.e. brick buildings). I've also seen moire on sand and pavement.

These issues become even more prominent at 60fps / 720p.

A very helpful and inexpensive tool to have is an ND filter. A neutral density filter governs the amount of light that reaches the lens and will allow you to shoot with larger apertures in daylight and maintain a shallower DOF, which will help you avoid this problem.
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Old March 18th, 2011, 12:35 AM   #10
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Re: Far away contrasty lines causes rainbow-like video distortion?

Also, you may want to try this: Now available: my DSLR anti-moire filter for FCP! Free download! Jorgen Escher
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