Rolling shutter

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    • #55723

      Under what circumstances does one encounter the dreaded " rolling shutter "? Is it used exclusively with single sensor cameras, or do some mfgrs use it with 3 chip cameras? Is it used with CMOS chips only, or is it also used with CCD sensor cameras? Is there any way to overcome the motion distortion imparted by rolling shutter cameras?


      And while we're at it . . . . how do CMOS sensors compare with CCD sensors other than cost?


      Rick Crampton

    • #206707

      I have no direct experience with CMOS, my knowledge consists of only articles that I've read and video clips that I've seen. CMOS does tend to be cheaper and uses less power than CCD which will increase battery life, which from what I've read are it's only advantages. The number of sensors has no bearing on rolling shutter, it's the type of sensor. CCDs don't have it but CMOS does. A CCD sensor captures each frame (every scan line) all at once like a still camera. Although an interlaced camcorder will scan two different "fields" per frame, the odd numbered lines first and the even ones second. (The reason for this dates back to when TV was being invented but I won't go into that now.) This amounts to sixty half frames per second which essentially combine to form thirty frames per second. When there's fast motion, say a passing car, this will give you something like motion blur. If you freeze frame on it you will see two ghost-like images of the car, one where it was in the first field and the other where it moved to in the next sixtith of a second for the second field. When seen in motion it is not that noticeable but progressive scan (indicated by the P after the frame rate) camcorders don't have this problem which is why serious videograpers seek it out. CMOS is progressive but it doesn't scan all lines at once, instead it scans one line at a time seperately, one after the other so when that car passes by the camera the roof will appear on the right side of the frame, the door will be in the center, and the wheels will be on the left. It's motion blur but on a different caliber, giving you an unnatural, sometimes distorted image. Stobe lights will also give you a problem. The light may flash during the first half of the frame as it's being scanned and be off during the remainder of the scan so only half of the frame is lit, or the lit part may scroll depending on the speed of the strobe. If you can manually set the shutter speed of your camcorder, you can minimize this effect by choosing a faster shutter speed. Your only other choice is to avoid fast motion in your shots. I have read that some manufactureres have overcome this problem but these claims were made while the camera was in the prototype stage. Whether they're on the market yet and which models they are, I couldn't tell you. Research may be advised. Hope this answered your questions. Good luck.

    • #206927

      We use Canon XF300s, which are 3CMOS cameras, 1/3" chips. I can confirm that they do sometimes suffer from rolling shutter when filming a live screen or there is flash photography in the scene. It's all to do with they way the chip reads the light hitting it – it "scans" the light top-to-bottom, so there is a very slight delay between the top of the image being porcessed and the bottom of the image being processed. This procuces "rolling shutter" when there is fast fliker-like movement, such as screens or flashes, that occur faster than the light can be read by the chip.


      Conversely, CCD cameras read the whole frame at once, and so there is no delay in reading different parts of the image. CCDs have their downfalls, though, such as colour depths (some might say) – whereas CMOS tend to produce a deeper image in terms of colour. CCDs are also more expensive, but CMOS are cheaper to produce and so they are used in DSLR cameras and cheaper HDVs.


      Overall, I wouldn't worry too much about it unless you will be filming a lot of fast flickering action – the benefits of CMOS are plentiful and the sensors produce some great-looking images. There are also filters in FCP, PP and AE that takle the resutls of rolling shutter.



    • #206941

      I have to agree 'Josh' regarding rolling shutters. They make high performance available to those, like me, who work on a budget dictated by the circumstances of retirement. I use a Panasonic SD-HDC900, purchased partly for its excellent lens and high-quality image processing. The sharpness I obtain, plus the fidelity to the original colours, never ceases to amaze me. However, being occasionally involved, also with some active wild-life, I do sometimes come up against rolling-shutter effects. The sensible thing to do then, is to use a high quality stabiliser, (Mercalli in my case, with the 'stabicam' setting on 'Rock steady' since I work from a tripod). The numbers of shots needing to be processed this way, would be fewer than 20 percent of a typical day's 'shooting', and not all, of those would necessarily make it onto the timeline, anyway.Typically, I might shoot up to 150 clips a day.


       Since the dawn of photographic history, there have been such problems. For example, I had similar problems with a Leica M3, which I used for years, in that fast-moving subjects such as racing-cars, would have a 'trapezoidal' effect, with, frequently, the auto tending to 'leaning-backward' at the top.Just about every item of gear I have ever used, had its limitations of some kind. Being able to work around them, especially without 'blowing-your-cool', (there's a good 'sixties' term for you), is the secret; and if not, be philosphical.


       Incidentally, I am just checking my emails after a fascinating day on our coastline, wandering through piles of Neocene era (60 to 65m years old) rocks on our coastline at a place sometimes called 'Fossil Point'. Glorious day, even if I had to rock-hop for more than a kilometre, then, to avoid a rising tide, finally climb a cliff, because since my last visit,the farmer has built an electric-fence, right acoss the original track. I've just seen the shots, which have exceeded all expectations.


      'Keep the faith'


      Ian Smith

      'back-to-the-drawing board' video

       Dunedin, New Zealand.

    • #206956

      Hhmmm . . . I thought progressive scanning created more motion blur and/or strobing? I was under the impression that film cameras were limited to the rate one could pan a scene, or the rate an object could pass by the lense because of the way the film is exposed, albeit 24 frames per second, " progressively ".


      Or maybe I misunderstand . . . . but isn't shutter speed more responsible for blur or lack thereof than whether Progressive or Interlaced? Yeh, I understand that it's all interconnected; and perhaps not as simple as perceived by some. Film look? Nostalgic perhaps . . .


      Rick Crampton

    • #207087

      Regardless of what you shoot on, the faster you pan the more motion blur you will get under the same set of shooting circumstances (i.e. matching frame rate, shutter speed, etc.) It's more a function of shutter speed than anything else, so you are corect there. Prog vs interlaced is a very small factor.


      If you pan a film camera or video camera slowly you will get far less blur than if you do it quickly so it became standard not to pan too quickly if you wanted to avoid motion blur. If you increased the shutter speed too much it gave the film a whole different look (think opening battle in Gladiator.)


      As for rolling shutter, the more movement – the more 'jello-cam' you will see whether that's an object in the frame moving quickly or the whole thing panning. CMOS is worse than CCD. In fact, I don't think I ever heard of anyone complaining about it until CMOS came out.


      Some software can fix (to one degree or another) the rolling shutter in post. I know After Affects can do it, not sure what else.


      I would also definitely agree with Ian, all equipment has limitations. Know your gear and what you'll get from it. Work within or around its limitations and enjoy. πŸ™‚

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