Canon EOS 100D Basic Instruction Manual page 278

A Menu Function Settings
Aspect ratioM
The image's aspect ratio can be set to [3:2], [4:3], [16:9], or [1:1].
The area surrounding the Live View image is masked in black when
the following aspect ratios are set: [4:3] [16:9] [1:1].
JPEG images will be saved with the set aspect ratio. RAW images
will always be saved with the [3:2] aspect ratio. The selected aspect
ratio information is added to the RAW image file. When you process
the RAW image with the provided software, this allows you to
generate an image with the same aspect ratio that was set for
shooting. In the case of the [4:3], [16:9], and [1:1] aspect ratios, the
aspect-ratio lines will appear during image playback, but the lines
are not actually drawn on the image.
Image
Quality
3
5184x3456
(17.9 megapixels)
1
3456x2304
4
(8.0 megapixels)
2592x1728
a
(4.5 megapixels)
1920x1280
b
(2.5 megapixels)
c
(350,000 pixels)
The asterisked image-recording quality settings do not match the
respective aspect ratio exactly.
The image area displayed for the asterisked aspect ratio is slightly larger
than the recorded area. Check the captured images on the LCD monitor
when shooting.
If you use a different camera to directly print images shot with this
camera in the 1:1 aspect ratio, the images may not be correctly printed.
162
Aspect Ratio and Pixel Count (approx.)
3 2
4608x3456
(16 0 megapixels)
3072x2304
(7.0 megapixels)
2304x1728
(4.0 megapixels)
1696x1280*
(2.2 megapixels)
720x480
(310,000 pixels)
4 3
5184x2912*
(15.1 megapixels)
3456x1944
(6.7 megapixels)
2592x1456*
(3.8 megapixels)
1920x1080
(2.1 megapixels)
640x480 720x400*
(290,000 pixels)
16 9
3456x3456
(11.9 megapixels)
2304x2304
(5 3 megapixels)
1728x1728
(3 0 megapixels)
1280x1280
(1 6 megapixels)
480x480
(230,000 pixels)
1 1