Motion Blur
Motion Blur
[White Hat is holding a camera up to his face looking to the left away from Cueball and Ponytail standing to his right. Cueball is holding one hand, with a balled up fist, up towards White Hat] White Hat: Okay, I'm going to pan around. Cueball: No, wait, your shutter speed is too fast, it will look choppy if—
[White Hat turns clockwise towards Cueball and Ponytail, with the camera held up partly behind his head, so it points in the panel. Cueball clenches both his fists, held down now, and hunches his shoulders. Ponytail stands as before next to him.] Cueball: Hnnnnngh
[White Hat has turned around and is now pointing the camera towards Cueball and Ponytail. Cueball now appears blurry while Ponytail looks normal.]
[Caption below panel:] Expert photographers can learn to generate their own motion blur to compensate for other people's bad camera settings.
This comic remained on xkcd's front page even after the midnight between Monday and Tuesday (in Eastern Time), which is a rare instance that Randall has missed a midnight deadline.
This is analogous to something much more common that people do, by practicing moving their bodies relative to the motion of the camera: reducing blur when the shutter speed is too low.
The title text refers to the fact that only one object - in this case, Cueball - appearing blurry while everything else in the frame is sharp is even more exotic. It can also be seen as a celebration and sarcasm regarding the rare experience of valuing having exotic knowledge and skills. It seems likely Randall has practiced reducing blur, but not succeeded at increasing it, and was possibly exposed to somebody saying "high" shutter speed when they meant "low" shutter speed, but this is presently unverified.
This is somewhat similar to a trick 'used' by some fictional characters who have the power to make themselves unclear to observers or cameras alike. In real-life it is the difference between "stop motion" and "go motion" tricks.
When light hits a human's retina, it is perceived for a short while even after the light has ceased. This means that objects moving across a human's field of vision at a sufficient speed will naturally appear blurry – in our perception, the light arriving right now from the trailing part of the object will mix with the light that arrived a moment earlier, from the leading part of the object.
A camera's shutter speed is the amount of time that the shutter is open for each frame, allowing the image sensor to capture light. If the shutter speed is too high (relative to the frame-rate), this blurring will not occur, and the motion will look unnaturally crisp – if something is too small and/or too quick, the illusion of motion may disappear altogether; the object instead will appear as a brief flash of multiple objects standing still, like in the case of a fast-moving mouse cursor on a screen. See for instance this Videography - Slow Shutter Speed vs. Fast Shutter Speed Comparison .
In cinema, the shutter speed is generally set to double the frame-rate, e.g. 1/48 s for footage shot at 24 fps (one of the lowest standard frame-rates, a remnant from the age of mechanical motion picture cameras and film projectors).
An opposing problem is that of a camera not sufficiently matching the relative motion of a moving object, with a shutter speed that is too slow (and may need to be, given the choice of aperture and lighting conditions). Sports photographers must learn how to scan-and-pan their subjects (runners, horses, vehicles, etc) with enough synchronicity to capture them sharply, and possibly seemingly hanging frozen in mid-air against an artistically-blurred background.
It is unclear how Cueball makes the motion blur include both his feet, as the friction with the ground should hinder them from vibrating horizontally in the manner that may cause for motion blur. Additionally, creating the kind of motion blur he does (with evenly distributed horizontal blur) requires extreme acceleration at both ends of the movement.
[White Hat is holding a camera up to his face looking to the left away from Cueball and Ponytail standing to his right. Cueball is holding one hand, with a balled up fist, up towards White Hat] White Hat: Okay, I'm going to pan around. Cueball: No, wait, your shutter speed is too fast, it will look choppy if—
[White Hat turns clockwise towards Cueball and Ponytail, with the camera held up partly behind his head, so it points in the panel. Cueball clenches both his fists, held down now, and hunches his shoulders. Ponytail stands as before next to him.] Cueball: Hnnnnngh
[White Hat has turned around and is now pointing the camera towards Cueball and Ponytail. Cueball now appears blurry while Ponytail looks normal.]
[Caption below panel:] Expert photographers can learn to generate their own motion blur to compensate for other people's bad camera settings.
This comic remained on xkcd's front page even after the midnight between Monday and Tuesday (in Eastern Time), which is a rare instance that Randall has missed a midnight deadline.