When talking about camera resolution, we often refer to the 300 dpi (dots per inch) rule and how we don’t really need more than 8 megapixels. The term dpi is borrowed from print media. It loosely translates into something like this: when you hold an 8 x 10 inch image at an arm’s length from you it needs about 300 dots per inch to make the image look ‘picture perfect’. That means for an 8 x 10 inch print you need only an 8 megapixels sensor (8 x 300 x 10 x 300 / 1,000,000 = 7.2 MP):
When you look at the argument, it’s easy to overlook an important point. Most cameras don’t have a sensor that has the same aspect ratio as an 8 x 10 inch print. Most photo printing services end up cutting an inch from either side of the image to make that size.
To test the DPI theory, Northrup made two identical images of the same scene, except that they were shot by two different cameras and two different lenses. However, both printed at 300 DPI. When the images were shown people immediately identified the top image to be sharper of the two. How is that possible? If the two images have been printed at the same DPI they should have been identical. Is there anything else to this? The answer is lens sharpness; and this is where the concept of perceptual megapixel (PMP) comes into the picture.
“The only system that I can find for actually measuring the amount of detail that your camera generates rather than the sharpness of the lens or the actual megapixels of the camera. When you combine those two you get perceptual megapixel.”
It means that although your camera is capable of shooting at say 6000 x 4000 megapixels (24 MP), the number of megapixels that you can actually squeeze out of that sensor depends on the sharpness of the lens that you use. Some lenses are sharper than others and that means with sharper lenses you can get close to those claimed resolution numbers.
For example, the Nikon 18-55mm kit lens, when mounted on a 24 megapixel Nikon APS-C camera, will only produce about 9 perceptual megapixels. That’s less than half of what the sensor is capable of producing. Meaning, you can only make a 8 x 10 inch print at 300 dpi, although theoretically your camera is capable of producing a print that’s bigger than 11 x 14 inches (at 300 dpi).
Just for comparison, if you put the Zeiss Otus, optically the sharpest lens that DxO has tested, on a 36mm full-frame camera, the 85mm f/1.4 lens and camera combination will produce around 35mm PMP.
Please note it is not the lens alone that is being considered here but the lens/camera combination. That means even with an APS-C 24 MP camera using an optically sharper lens would result in higher PMP.
Roughly, the formula that Northrup uses for calculating how much increase in PMP he can achieve, by upgrading the resolution of his camera, all other things remaining the same, is the following:
This concept, however, is not inclusive of all the factors that affect image sharpness such as diffraction, atmospheric conditions, camera shake, and motion blur.