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A colleague and I were having a discussion about which of these is correct:

"This resolution is 15 megapixels"

or

"This resolution has 15 megapixels"

What it comes down to is our difference in definition of resolution. 5000x4000 and 10000x2000 both come out to 20,000,000. One of us says that these are different resolutions with the same pixel count, and the other says that they are two different resolutions with the same pixel count. What is correct?

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    \$\begingroup\$ This would be better suited for english.stackexchange.com . \$\endgroup\$
    – ths
    Jun 27, 2017 at 18:19
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    \$\begingroup\$ I don't understand your last sentence. Is one of those supposed to say "the same resolution"? \$\endgroup\$
    – mattdm
    Jun 28, 2017 at 7:16
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    \$\begingroup\$ Judging by the answers so far, I'd say the term "resolution" has multiple conflicting definitions. \$\endgroup\$
    – Mark Ransom
    Jun 28, 2017 at 15:58
  • \$\begingroup\$ "The term resolution is often used for a pixel count in digital imaging, even though British, American, Japanese, and international standards specify that it should not be so used, at least in the digital camera field." See Wikipedia \$\endgroup\$
    – TFuto
    Jun 29, 2017 at 14:04
  • \$\begingroup\$ The image HAS 15 megapixels. See more at Wikipedia pixel resolution \$\endgroup\$
    – TFuto
    Jun 29, 2017 at 14:07

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Linguistically, "megapixels" is a property of a device or a recording from a device. Properties follow a "has a" relationship, or could be said that "the property ... of ... is ...". Thus, in your situation, it could be equally said,

  • The resolution of this image is 15 megapixels; or
  • This image has a resolution of 15 megapixels

What it comes down to is our difference in definition of resolution. 5000x4000 and 10000x2000 both come out to 20,000,000. One of us says that these are different resolutions with the same pixel count, and the other says that they are two different resolutions with the same pixel count. What is correct?

Both resolutions are 20 megapixels. They differ in their aspect ratio property, or the ratio of (usually) long dimension to short dimension. Aspect ratio is independent of orientation. Whether your hypothetical images are oriented vertically (portrait) or horizontally (landscape), they have 5:4 and 5:2 aspect ratios, respectively.

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The resolution is defined as points/pixels per length unit, usually inch.

In image quality, it is measured in ppi - pixels-per-inch or dpi - dots-per-inch. That image has resolution of 150 dpi.

In printing, some printers have different resolutions in both directions. In that case it is used X x Y dpi notation sayng it has resolution of X dpi in one direction and Y dpi in the other.

The value of 15 MPx says that the chip has 15 milons of pixels, that means that for usual 3:2 ratio it is capable of taking pictures with 5000 x 3000 pixels. When making 5" wide print, the resolution will be 1000 dpi.

Your good point is that one X MPx resolution can be obtained using many different frame sizes. It is same nonsense as measuring the screen size only by its diagonal length! But, luckily for photography, the usual ratio is 3:2 so the values are directly comparabe (if you ignore cameras developped from mid-format with frame size of 6x6 cm). The screens are not comparable at all.

On the linguistic side of the question, megapixel is a unit. The same unit as mile is.

  • The image has resolution of 15 MPx. / The road has length of 15 miles. (it is 15 miles long.)
  • This is 15MPx image. / This road is an 15mile road.
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  • \$\begingroup\$ The same unit as mile is — Since a pixel defines an area, megapixels are more like "megahectares", "mega-acres", or "million square miles". \$\endgroup\$
    – scottbb ♦
    Jun 27, 2017 at 20:52
  • \$\begingroup\$ @scottbb it doesn't mater whether it is area unit, charge unit, whatever. It is an unit. \$\endgroup\$
    – Crowley
    Jun 29, 2017 at 11:20
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    \$\begingroup\$ Actually, megapixel is marketing unit. 15MPx camera means that it has chip with 15 milion pixels. How are they distributed? Nobody knows. Technically it is not unit at all, "Mega" is a prefix for 1000x multiplication and "pixel" is, well, pixel. A thing. \$\endgroup\$
    – Crowley
    Jun 29, 2017 at 12:57
  • \$\begingroup\$ Fair enough, it's a count of a thing. But without being too pedantic, pixels have an implied area, arranged in a grid. According to Google, the area of Manhattan Island is 22.82 mi<sup>2</sup>. But that doesn't say anything about the shape or distribution of the area. I think it's obvious that Manhattan isn't a perfect square that's 4.78 miles on a side. But its area is still a measured area. \$\endgroup\$
    – scottbb ♦
    Jun 29, 2017 at 14:31
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Resolution is ultimately about how many lines per image height an imaging system can reproduce. Pixels and megapixels are just one of several factors that may be the limiting factor that determines a system's optical resolution.

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Megapixels is Size, how many pixels. 6000x4000 pixels is 24 megapixels, which is a pretty large image, typically a lot of pixels. But if the 24 megapixels covered a huge area like the size of a large road-side billboard, that is not necessarily high resolution.

Resolution involves a spatial dimension, like inches or mm. Resolution measurement could specify pixels per inch. It represents the size of one pixel, or the smallest dot possible, the smallest detail shown.

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