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Suggestion - Canon 5DS R anti-AA filter explanation

Discussion in 'AP Magazine Feedback & Suggestions' started by Malcolm_Stewart, Feb 18, 2015.

  1. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    Canon's latest 5DS comes in two versions. 5DS & 5DS R.

    I'd appreciate an explanation, from Prof Newman perhaps, of how the anti-AA filter is supposed to work in the 5DS R version, and also what physical profile it must have. I have no problem understanding a Low Pass AA filter, but reversing that effect is beyond me. Fine grating in two orthogonal directions? How aligned?

    Many thanks
     
  2. AlexMonro

    AlexMonro Old Grand Part Deux

    I seem to remember reading a detailed description somewhere of the system Nikon used when they brought out two versions of a camera with and without AA filter (D800 / D800E?). Presumably Canon use a similar system. There's a reason that you can't just "remove" the AA filter - it's part of the optical path, and if it isn't there, the lens won't focus where it should.

    I sort of understood the rest of the explanation at the time - A normal AA filter is in two stages, the horizontal and vertical parts, and if you make the 2nd stage the same orientation, it neutralises the first stage, or something like that, but I'm a bit fuzzy on the details now, and I'm sure Prof Newman would make a far better job of explaining it.
     
  3. Alex1994

    Alex1994 Well-Known Member

    Do you have an AA filter in your glasses? :D
     
  4. daft_biker

    daft_biker Action Man!

  5. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    Thanks for your link - I have several prisms and a spectrometer (sitting on my "dining" table). Pity that the only comment so far has come from someone who clearly doesn't understand the difference between Refraction and Diffraction. I hope he's not a teacher.
     
  6. Andy Westlake

    Andy Westlake AP Staff

    The 5DS and 5DS R bear exactly the same relationship to each other as the Nikon D800 and D800E did, in terms of how the effect of the low pass filter is cancelled. You can see a diagram and explanation here:
    http://www.whatdigitalcamera.com/ro...ikon-d810-vs-nikon-d800e-vs-sony-alpha-7r-190

    Basically OLPFs exploit an optical phemonenon known as birefringence. If a ray of light is passed through certain crystals such as lithium niobate, it splits into two based on polarisation. Cameras normally use two filters oriented at right angles to each other to blur the image in a highly controlled fashion, to minimise sampling artefacts such as aliasing and moiré. The 5Ds does this as normal.

    On the 5Ds R though, the first plate is oriented differently, so it splits the incoming light into two beams, but the second plate then recombines them exactly, based again on polarisation. Hence the low-pass effect is cancelled.

    Why would Canon do this, rather than eliminate the OLPF altogether? Basically because it allows the two models to be made with minimal difference in components between them. Removing the OLPF altogether would modify the light path through the filter stack and require the sensor to be repositioned slightly, complicating the manufacturing process. By doing it this way Canon can test the water as to whether its customers might be prepared to accept the elimination of the OLPF altogether, without risking alienating them by forcing them down one route.

    Now you might argue that Nikon, Sony and all medium format makers have unequivocally already established that OLPFs aren't necessary at high pixel counts, as far as photographers are concerned. But Canon is nothing if not conservative.

    Andy
     
  7. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    Andy,

    Many thanks for your reply, it's much clearer now - I hadn't realised that use was made of birefringence.
     
  8. AlexMonro

    AlexMonro Old Grand Part Deux

    Nah, just a layer of crud - what I need is a built in cleaning system, like the sensor shaking anti dust system that Pentax use! :)

    Andy has made an excellent job of filling in the details I couldn't remember.

    I'll just add that the Pentax K3 uses the sensor shift stabilisation system to simulate an AA filter, by moving the sensor in a small circle during the exposure. This seems to work very well, even with flash, and means you can effectively turn the AA filter on or off as desired.
     

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