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This weekly course covers the most common questions videographers encounter when shooting and editing with DSLR cameras, from choosing a frame size and frame rate to understanding moiré. Authors Rich Harrington and Robbie Carman will also help you understand the impacts of compression and the difference between cropped (or micro 4/3rds) and full-sized sensors in cameras, and much more. This continual FAQ guide is a handy way to find the answers to the questions that plague you the most.
Richard: So, one of the questions that comes up a lot, and I think it's almost entered urban myth sort of status is oh! You know those CMOS sensors, we're going to get that wobble, that Jell-O, that skew. Robbie: The Rolling Shutter effect. Richard: Yeah, and people like how do I apply the Rolling Shutter, like no, it's not something you want. Robbie: You want, right? Richard: But people-- but producers are terrified of this. Like I've had something that's like no, we can't shoot on the camera, it's going to have a Rolling Shutter effect. Robbie: Oh, you know--now you know, sort of--you are right, it's sort of entered urban legend.
Now when camera manufactures talk about any camera that they're coming out with, they specifically say you know, reduce Rolling Shutter artifacts or no Rolling Shutter, or this and that. You know, Rolling Shutter is something to be aware of, but it's not something necessarily to be scared of, right? Richard: Well, and I think even before that-- Let's just clarify, this is not unique to DSLR. Robbie: Well, it's-- Richard: Even a Red camera has Rolling Shutter. Robbie: Right. And we're going to commonly, most commonly find this sort of artifact in CMOS image sensors, right, that are using DSLRs, using other high-end cameras and just for fun sake, what does CMOS stand for? Do you know? Richard: Complementary metal-oxide-semiconductor. Robbie: Oh, well, you won jeopardy tonight.
Richard: Yes. Robbie: Good job, good job, good job. Richard: So you have it for pure trivia. But from a practical sense, older times, video cameras used to be CCDs where you'd have multiple chips, and now we're using a single chip to capture the image. Robbie: Well, even more so than that, a CCD used what we refer to as a global shutter, right? So the entire image sensor was-- Richard: And that was a Charge-Coupled Device. Robbie: There you go. Richard: Double jeopardy. Robbie: A CCD, it was a global shutter, so the entire sensor was exposed at sort of the same time. The way that CMOS chips or CMOS, depending on you know tomato or tomato, where you're from. Richard: If you're from Canada. Robbie: Right, exactly.
The way that these chips work is that they scan, the shutter scans the actual image sensor, right? So if you start at the top of the sensor, you're scanning down to the bottom of the sensor. So in other words, the scanning, at the top and the bottom are actually different points in time. It might be, you know, a really short period of time. Richard: We are talking fractions of a second because if you're shooting 30 frames a second that's refreshing every 30th of a second, but-- Robbie: Every 60th of a second, if you do fields yeah, exactly. Richard: Yeah, so I think that the people get hung up here because they're like oh! You know that's plenty of time.
You know, the split difference sometimes--and winning a race could be 30th of a second. I mean while it's very, very fast when shooting video, you know a 30th of a second is not that fast. Robbie: Well, let's just take a look at this clip that we have here. Richard: Yeah. Robbie: So in this clip obviously, it's just a set of window blinds, shooting out the window, but as we're panning back and forth, what you'll notice is that the strong vertical lines seem to wobble back and forth. And again that's because of the Rolling Shutter effect. The top of the sensor and the bottom of the sensor as the shutter scans that sensor are different moments in time.
And we are most commonly going to see this on fast movement and fast pans, and you're not going to just get this Jell-O effect, refer to this as wobble most of the time. Richard: Yeah. Robbie: You might get things like smearing or skewing. The one that I see all the time because I do a lot of natural history stuff is--or sort of outdoor stuff, say, for example, a lightning strike in often the distance, right? What you might see is the top of the frame is nice and bright, and the bottom of the frame is darker. So to get this partial exposure going on, all of these are symptoms of Rolling Shutter.
Richard: Yeah, and I think what's important to realize here is you're going to start to see this as you've mentioned when movement or subject are faster than your frame rate. So if you're dealing with something speeding through the frame, like you have got a large delivery truck, big boxy truck, and it's driving 80 miles an hour down the street, which it shouldn't be, but if it is, and it goes past your camera shooting 24p, it's very possible that that truck will take on a diagonal shape because it's moving so fast. Robbie: Well, it's not just the frame rate, it's also-- it's really the shutter speed, right? Richard: Yeah. Robbie: So you know you're shooting at 24, you might be shooting at the shutter speed of 150th, right? Richard: Yeah.
Robbie: And that's what's going to cause that, and I think you're right, I mean the thing to be aware of Rolling Shutter is that some people convince themselves that they can see it all the time, right? Richard: Yeah. Robbie: I take argument with that. But to me, the only place that I'm really going to notice is that when we have strong vertical lines or things moving really fast, like helicopter blades are the famous example, they're rotating around and all seem you through like they look like they are bent or something like that. Richard: But we used to see this on wagon wheels that would look like they were rolling in reverse. Robbie: Right. Richard: The human eye and cameras behave differently. Robbie: Right, and so my suggestion is be aware of that it's sort of phenomenon that happens with these sensor types-- Richard: It's an optical illusion. Robbie: But don't let it paralyze you, right? Richard: Right.
Robbie: And later on, we'll take a look at how to minimize Rolling Shutter artifacts when we're actually out filming. Richard: Okay, so to recap I think what we want to point out here is Rolling Shutter does exist, it is real. But it's only going to come up in certain shooting situations, like strong, fast movement, really fast panning, really fast subjects. It's going to manifest itself as image wobble, as skewing where vertical lines start to look diagonal, or smearing where we just start to see blurring or in some very rare cases partial exposures when you're dealing with things like flash photography or lightning where you have really quick changes in the scene where the sensor doesn't refresh as quickly.
Now the big thing I want to say is don't just keep going frame-by-frame through your video, stepping one frame at a time. If you do that, any video looks terrible, because remember video is all about persistence of vision, where multiple frames add up and create smooth movement within the brain and video looks good. If you look at any individual freeze-frame, it's always going to look a little soft, a little smeary, potentially a little bit skewed. It's the cumulative effect of watching that video playback in real time. And if you don't see it there, don't obsess, your clients or your customers may have heard about this effect.
Just tell them it happens in a few situations and in our next movie, we're going to talk about strategies for avoiding it.
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