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In this youtube video of "How to Shoot a Basic HDR Photo" from the channel 'Triggertrap How To', at this point what is the person who's doing the lesson, doing?; he rotates the dial on the camera and on the screen, two lines go away from a center line...

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    concur with @scottbb, I had a couple of mins so looked at the video and provided an answer. I'd suggest maybe either finding an image (you can use) of this scale or potentially describing the scale (and where it lives) as well/instead of. – Crazy Dino Dec 29 '16 at 14:26
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The user in the video is using a specialised bracketing function on a Canon body, called Auto Exposure Bracketing (AEB) (complete description from Canon here)

In the "exposure" setting, you can rotate the dial and then 3 lines will appears. Each line represents the exposure compensation that will occur for the 3 photo that you will take immediately after. The first photo will have a "correct" exposure (0 EV correction), the second will be underexposed (line of the right: -X EV) and the last will be overexposed (line on the left: +X EV).

Canon AEB screen

What the user is choosing when rotating the dial is the amount of exposure correction ("AEB Amount") between the first and uncorrected image and the over/under-exposed one (it is symmetric: the "X" in "-X EV" and "+X EV" is the same value).

For example, if the scene has a high dynamic range (very dark areas and very bright ones), you will probably need to capture something like a 15 stop range and your camera can only capture something like 11 stops of dynamic range. You may want to use some HDR technique. By setting an AEB amount of +/-2 EV, the 3 bracketed images will capture something like this for a scene that ranges from EV 3 to EV 17 (EV 3 to EV 17 inclusive is 15 stops) and the base exposure value is set at EV 10.

  1. "0" Exposure centered on EV 10 captures details between [ EV 5 to EV 15 ]
  2. "-2" Exposure centered on EV 8 captures details between [ EV 3 to EV 13 ]
  3. "+2" Exposure centered on EV 12 captures details between [ EV 7 to EV 17 ]

With those 3 images, you will have a dynamic range of around 15 EV (The 11 EV of a single frame plus two more in each direction for the shifted exposures: 2 + 11 + 2 = 15). See also Are RAWs in bracketed exposure (mostly) redundant?

Different Canon models are capable of different numbers of shots in a bracketed series (anywhere from 3 to 7) as well as the maximum step between each shot (usually either +/-2 or +/-3 stops in 1/2 or 1/3 stop increments). Some Canon cameras are capable of doing a series of up to 7 bracketed shots at up to 3 stops per step between shots. For more, please see: Can the 5DS-R take more than 5 bracketed shots spaced by 3 EVs?

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    Deleted my answer, as you're correct. Mine is completely wrong. This is the correct one. – Crazy Dino Dec 29 '16 at 16:25
  • Ah!, so the two lines either side gives the the EV of under and over exposed photos, right? And when we move the middle (correct exposure), then the two lines either side, move with it... .. Hey but, shouldn't it be '12 EV ([-6 EV ; +6 EV])' in the last line? – user152435 Dec 29 '16 at 18:07
  • The user can also select the order of the exposures. I prefer -, 0, + rather than the default 0, -, +. – Michael C Dec 29 '16 at 22:56
  • Different Canon cameras have different maximum offsets, but I'm not aware of any that can do a -6, 0, +6 three shot series. The 5D Mark III and the 5Ds/5Ds R will allow up to 3 stop steps between bracketed shots. Those cameras will also allow up to 7 bracketed exposures in a series (the default is 3, but the user can change the setting to 2, 3, 5, or 7). For more please see: photo.stackexchange.com/questions/75806/… – Michael C Dec 29 '16 at 23:09
  • The edit made to my answer regarding the example and EV values seems wrong. I will study it and add a more graphic example tomorrow. – Olivier Dec 29 '16 at 23:36

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