Canon EOS-1D X Manual (en)

Take a shot.
The HDR mode cannot be set if you use any of the following 
functions: AEB, white balance bracketing, Multi Shot Noise 
Reduction, multiple exposures, flash exposure, bulb exposure, 
or movie shooting.
HDR shooting of a moving subject is not recommended because 
the movement of the subject will appear as afterimages in the 
merged image. HDR shooting is recommended for still subjects.
In HDR shooting, 3 images are captured with different shutter 
speeds set automatically. Therefore, even in Tv and M shooting 
modes, the shutter speed will be shifted based on the shutter 
speed set.
To prevent camera shake, the ISO speed may be set higher than 
Auto image alignment may not work properly with repetitive 
patterns (lattice, stripes, etc.) or flat, single-tone images.
The color gradation of the sky or white walls may not be 
reproduced correctly. Irregular colors or noise may appear.
With HDR shooting, the three images will be merged after you 
take a picture. Therefore, it will take a longer time to record the 
HDR image than with normal shooting. During the merging of the 
images, “BUSY” will be displayed on the camera and you cannot 
take another picture until the merging is completed.
Using a tripod is recommended. For handheld shooting, using a 
fast shutter speed is recommended.
If [Auto Image Align] is set to [Enable] and the HDR picture is 
shot handheld, the edges of the photos will be cropped, lowering 
the resolution slightly. Also, if the images cannot be aligned 
properly due to camera shake, etc., auto image alignment may 
not take effect. Note that when shooting with excessively bright 
or dark exposure settings, auto image alignment may not work 
If you perform handheld HDR shooting while [Auto Image Align] 
is set to [Disable], the 3 images may not be properly aligned and 
the HDR effect may be minimal.
HDR shooting under fluorescent or LED lighting may result in 
unnatural color reproduction of the illuminated areas.
In HDR mode, the options dimmed in the menu cannot be set. 
Note that when you set HDR mode, for Auto Lighting Optimizer, 
highlight tone priority, and exposure simulation, [Disable] will be 
applied for shooting.