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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 August 10th, 2011, 11:28 AM   #1
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moire / aliasing

I posted this over on another forum but didn't get much response.

My question:

What methods / tips / tricks do people have for eliminating (or at least reducing) aliasing / moire? I find it a very frustrating limitation of my t2i . . . in almost all of my inner-city clips there is aliasing—somewhere. It might just be in a hat design, or on pavement, or on a building or a screen . . . it always seems to be somewhere and it ruins a lot of my shots. I'll throw away much footage that I think is otherwise great because of it. And my final vids almost always have a lot of it otherwise I'd be throwing out all my clips!

HELP!
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Old August 10th, 2011, 01:37 PM   #2
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Re: moire / aliasing

The easiest way to avoid moire is to shoot with shallow DOF. Nothing out of focus will alias. In narrative, you can choose costumes carefully. Unfortunately, that's not practical for documentaries.

You can use a diffusion filter. It won't stop aliasing, but will reduce it somewhat. I find that a GlimmerGlass #1 is very subtle. A #3 is somewhat strong. A #2 would be ideal, but they are only available in square formats. It might not be the way you'd want to go for a gritty look. The more subtle the diffusion, the less the anti-aliasing.

For shots with deep focus, you can snap a photo of a static background. If necessary, one could replace the texture of a brick building or tile roof, but it takes time in post. Do it with a heavily feathered mask, and you can be fairly sloppy with it and still get good results.

You can also purposefully defocus slightly.

Frankly, if I were shooting inner city documentaries, I'd want a Scarlet. It won't alias. It will have a moderate DOF appropriate for a gritty look. It will allow strong grading, so you can give the environment a strong feel. The rolling shutter effect should be minimal, so it should work well handheld. It also does slow motion, when you want a pensive mood.
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Old August 10th, 2011, 04:50 PM   #3
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Re: moire / aliasing

good findings. Also there are various plugins you know can get, try to avoid shooting against anything pattern like cushions, brick walls stripy shirts.

It has a lot to do with transcode and color space as well as sharpness.
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Old August 10th, 2011, 06:02 PM   #4
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Re: moire / aliasing

There is a fundamental problem with trying to remove aliasing in post. Sometimes the high frequencies get converted to very low frequencies. No softening filter will remove the bogus low frequencies.

Plug-ins can be effective on single, aliased lines.If there is a telephone wire that looks chopped up by aliasing, a low pass filter can help it look smooth again. That said, it has fundamentally lost information, so it might still dance around when there is motion. Sometimes the sensor sees the wire and sometimes it doesn't, making the whole thing non-linear.

Where it won't help is on a high frequency pattern, such as a shirt with a low thread count. On a given frame, you might have one swath of the shirt where the sensor picked up the dark part of the pattern and another swath where the pixels hit the bright parts. Now you have a big dark splotch and a big light splotch. There are the low frequencies that I mentioned above. Add a strong blur filter and all you get are blurred dark and light "islands" on the shirt.

Maybe you could take a photo of the fabric and do a 3D animation to lay it in, but that would be hugely time consuming. If you pay for post, you might as well have spent the money on a RED ONE and avoided the aliasing in the first place.

Of course, the other solution is to turn a blind eye and say, "well, I guess that's good enough for the web..." :)
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Old August 10th, 2011, 06:24 PM   #5
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Re: moire / aliasing

Solutions then are:

1) shooting with a shallow DOF when possible
2) avoiding patterns (not really possible in a city environment)
3) use a diffusion filter
4) purposeful defocusing (but I hate OOF shots)
5) get a Scarlet (hey, I'm just a hobbyist—a poor teacher at that!)

I've heard some other ideas: turn sharpening down all the way in camera, reduce resolution in camera (can you do that for video?), reduce contrast. Will these things help?
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Old August 11th, 2011, 12:42 AM   #6
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Re: moire / aliasing

Turning down sharpening and contrast doesn't really help, assuming that you will apply sharpening and contrast in post. The sharpening in the camera happens after the sensor has already aliased the image. Using lower resolution definitely doesn't help; it makes it worse.

In addition to Scarlet, most any dedicated video camera will do better than DSLRs regarding aliasing for deep focus shots. Using a camcorder for establishing shots and a DSLR for personal shots is one way to go.
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Old August 11th, 2011, 10:11 PM   #7
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Re: moire / aliasing

I had a post entitled "elimination of moiring" awhile back. It might be of some help.
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Old August 13th, 2011, 10:00 AM   #8
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Re: moire / aliasing

It's not the perfect solution but if you lock down your shot with a tripod you will still have moire but it won't be dancing all around.

I also have a Canon 550D. I found a way to eliminate all the moire. I bought a Sony FS100 :-)
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Old August 13th, 2011, 10:27 AM   #9
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Re: moire / aliasing

1) defocusing
2) use tripod
3) shoot with shallow DOF
4) use video camcorder for wide shoots without shallow DOF
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