Do You Know Your Camera's Image Resolution? Think Again.

by Sean Setters

Here's a seemingly simple question – "What is your camera's image resolution?"

Before you answer, let me warn you. If you're simply going to recite the "Effective Pixels" value listed in your camera's specifications, you'll be incorrect. Your camera's advertised resolution is different than the resolution of the recorded images. In fact, your camera's image resolution is a little lower than the widely accepted value.

Want proof? Multiply the pixel dimensions of your camera's highest image resolution setting and compare it to the advertised camera resolution. For convenience, I'll list a few examples below.

Canon EOS 5D Mark IV
Image Dimensions: 6720 x 4480
6720 x 4480 = 30,105,600 pixels (30.11 MP)
Stated Resolution: 30.4 MP

Canon EOS 80D
Image Dimensions: 6000 x 4000
6000 x 4000 = 24,000,000 pixels (24.00 MP)
Stated Resolution: 24.2 MP

Nikon D810
Image Dimensions: 7360 x 4912
7360 x 4912 = 36,152,320 pixels (36.15 MP)
Stated Resolution: 36.3 MP

Sony a7R II
Image Dimensions: 7952 x 5304
7952 x 5304 = 42,177,408 pixels (42.18 MP)
Stated Resolution: 42.4 MP

Are you seeing a pattern here?

Before I explain why the maximum image resolution is slightly smaller than the advertised effective pixels resolution, it's important to first understand the difference between the total and effective pixel specs. The term "total pixels" refers to the total number of pixels found on the sensor. The term "effective pixels" refers to a subset of the total pixels which fall within the image area.

The pixels that fall outside of the image area are used in determining effective pixel color values at the image's borders. Pixels don't record color information themselves (only brightness); instead, pixels rely on a color filter array placed on top of the sensor to help determine what color value should be assigned to each specific pixel. As the final color of a pixel is determined by algorithms which take into account the colors (red, green & blue) and light intensity of nearby pixels, the overlapping area is necessary for correct color calculations at the recorded image's borders.

So the area outside the portion of the sensor used for recording the image accounts for the difference between the total pixels and the effective pixels. But, the question remains: What is the reason for a discrepancy between the advertised effective pixels and the actual number of pixels in the image?

The answer is that manufacturers use specially allocated pixels within the image area to aid in noise reduction, dark noise cancellation and other various image processing tasks. These pixels are not recorded in the final image, but can enhance the quality of the final image. Thus, the difference between the effective pixels value and number of pixels found in the final image equals the total number of specially allocated pixels used for the various processing tasks.

The next time someone sees one of your images and asks, "How many megapixels is it?", don't recite your camera's advertised resolution but instead pull out your calculator to give them the precise answer.

That said, does the difference really matter?

What do you think? Were you surprised? Let us know in the comments.