Thread: Aliasing

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    Aliasing
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    Last edited by Larry Rutledge; 11-05-2009 at 04:50 PM. Reason: Applied standard DVXuser Article formatting


     

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    Thanks for the great article!

    From what I've seen the aliasing on the Canon 7D appears worse than on the Panasonic GH1. So is it really fair to lump all the HDSLRs together?


     

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    Quote Originally Posted by noirist View Post
    Thanks for the great article!

    From what I've seen the aliasing on the Canon 7D appears worse than on the Panasonic GH1. So is it really fair to lump all the HDSLRs together?
    The ones in the price bracket I've been following, which is the 7D, 5D, GH1, and K-X, all do the same basic thing, to one degree or more. The GH1 doesn't appear to do the severe color contamination that the 7D and 5D do, but the amount of aliasing is still quite substantial.


     

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    Still Alive Mod Jack Daniel Stanley's Avatar
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    Shallow depth of field look pretty.
    Low light be good.
    Price tag am nice.
    That's a moire.

    *Jack puts on dunce hat and sits in corner*
    Does Barry make anyone else feel like they road to DVXuser on the short bus?

    Wow, wow, wow. Thanks for the great article Barry. Obviously it's important to know what's what. And this amazing article will undoubtedly help many, many of us get the best out of our cameras. But as fore the questions you pose, I'd say, IMO, filmmaking is smoke in mirrors anyway. I'm looking for truth in content and truth in performance, but as for the rest I want an illusion with the appearance of authenticity that's either aesthetically pleasing, rough, or life-like heightened as appropriate to the story, that feels real whether it is or not.

    The plastic surgery analogy works to illustrate the potential complications / problems that can arise, but doesn't work as well (for me) in the picking a date vs. picking a camera I think. Most of us want authenticity in a mate, but all of art is artifice - hopefully with the illusion of truth - and that seems to be what the DSLR's offer 99% of the time.
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    Senior Member visugeek's Avatar
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    Great article Barry, thanks!


     

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    When people say that they see aliasing only occationally in their hDSLR videos they are usually confusing aliasing and moire, one of the many artifacts caused by aliasing. Moire is more obviously wrong than normal aliasing. Most shots I have seen of men with beard stubble dramatically show how aliasing introduces false detail. The beard looks much stronger than it really is. This false detail is what Barry, thank you, is talking about.


     

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    Senior Member John Froton's Avatar
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    As always, your explanations are very enlightening Barry. Thank you for the article.

    I wonder what steps the manufacturers of HDSLRs would need to implement in order to reduce the amount of aliasing in these or next gen cameras? Where specifically in the hardware is so much aliasing happening? In the h.264 encoding chip?

    It makes me wonder if there are other h.264 encoding chips out there, in some video camcorders, that are producing images with considerably less aliasing.


     

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    It's not in the encoder, I believe it's in the way the sensor is being read and subsequently downconverted. These sensors are not designed for video, they're designed for still camera use, and they have a limited speed at which they can be read out. The still-frame rate on a 7D is 8fps, on a GH1 it's 3.5fps. You cannot read the entire sensor at 60fps or even 24fps. So, they use pixel binning (or, as some insist, line-skipping) to reduce the amount of data read from the chip, and it is that process that is introducing all the aliasing. Then, there's the matter of converting that down to a 1920x1080 frame; according to Alan Roberts' examination of the 5D, he thinks that they're simply employing very poor downconversion hardware.

    Until we get a camera specifically designed for video purposes, we will probably continue to see these issues. Red custom-designed their own sensor to get away from this, and we're probably going to have to see such an evolution in DSLRs to get away from it too. In the meantime, we have aliased performance, but we also have incredibly low price tags, so one could even look at the aliasing as an enabling technology -- it enables us to get sharper-looking images at unheard-of price tags. It's just that the images aren't accurate and can lead to serious image problems too, but ... then the old "you get what you pay for" adage comes into play.


     

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    Great article
    Last edited by Windjammer; 04-12-2010 at 07:43 AM.


     

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    That's what I would consider the ideal. But not 2mp, you'd want at least 3mp. You have to factor in the resolution loss due to the Bayer pattern and demosaic process, so you need about 3 megapixels to deliver a truly sharp 1080p image. But yes, that would be the best of all possible worlds: incredible dynamic range, incredible sensitivity, tiny noise, razor sharp images, and could have an optimally-tuned anti-alias filter. Only problem would be that it would be lousy for shooting still photos, hence why it's not likely to happen on an HDSLR.


     

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