Let’s get this out in the open. Yes, it’s true that in high-end cameras like the Nikon D800, the “pixels per inch” number is a reference point that doesn’t refer to the full theoretical resolution of the sensor. So the D800’s sensor has a higher resolution than most current HD cameras, and this is true for both video AND still images.
However, to claim that any camera can produce a “24.3 megapixel” is a gross error — there’s no way to measure the resolution directly in any real way. The D800 sensor can’t be more than 2800 x 1800 inches, so even if there are two or more megapixels, that still doesn’t mean any single pixel is “24.3 megapixels”.
If we can compare camera pixels to other things like a leaf or a tree, the definition is much more straightforward. So let’s use one of my favourite examples to explain how the “pixels per inch” term is used in an apples-to-apples comparison with the D800 sensor:
Let’s say you took a picture of a small tree and wanted to figure out the approximate size of the tree in pixels. So you take one pixel, put it in a box (a camera and lens), shoot the picture and store it. You’ll be asked to estimate the number of pixels in the box, and you get the answer “six”. Then you say to the camera, “I’ll put a pixel here and you just add it to the pixels in the box”. Your answer was “seven”. You then tell the camera, “I’m going to add 7 to that number so it’s 10-11 pixel”. As you see here, it is a common mistake to think that because your answer is more than two pixels bigger than the answer provided, that you’ve answered everything correctly. It happens a few times each time you take a guess, just to make sure you weren’t doing it wrong.
The numbers are also a good tool for measuring how many megapixels a lens can use, and the resolution of a camera’s sensor. But that’s a very crude comparison. For many reasons — including the size of the crop (lens size). It’s also a very good way of measuring a lens’ “sharpness”, which is how much a lens is able to cut out of a picture.
Let’s say you are using a 24.3 megapixel camera to take a
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