Myths And Facts About Megapixels

Megapixel count is a quantitative, not a qualitative measure. It simply has to do with the size, not the quality of the picture. Still, every manufacturer persists in promoting devices with more and more megapixels, as though they were the Holy Grail of digital photography. Why?
Because it is much easier and cheaper to design and manufacture small, low quality sensors with high megapixel numbers than larger, high quality sensors with fewer megapixels yet strong elements that really improve the quality of the picture. These elements are low noise level, colour uniformity, contrast and sharpness, particularly at high ISOs.

There is a fundamental limit to how tight the pixels of an image sensor can be packed. The closer the pixels are to this limit the more light they “bleed” to their neighbour pixels, something which is unwanted because it corrupts the image. The level of this light leakage is called “noise level” or “SNR” (Signal Noise Ratio), more formally. The higher this value is the less is the image noise, so the crisper the picture. At lower light levels, where higher ISOs are required, SNR drops dramatically, up to a point when the image can look like this : 


This is a crop of a picture that was shot, from left to right, at 6400 and 12800 ISO. As you can see, the noise level essentially doubles with each ISO doubling. Of course we are talking about an advanced DSLR here, with a quite fat sensor; even with the best cameraphone you could never get any results past 800 ISO, even if it was supported.

Cameraphones have even smaller sensors than compact cameras so the more pixels manufacturers cram on these tiny chips, actually the worse images they produce, at least at night / low light conditions. My 10 MP Sony R1, even at a humble 1 MP settinng, shots much clearer images than a high-end 12 MP cameraphone! The only real uses of lots of mexapixels are when you want to print really huge photographs, or when for some reason you want to crop your high-MP image, in order for example to highlight specific parts or details of the image, as you may see in this example.

In conclusion, megapixels have absolutely nothing to do with picture quality, they are only a measure of how big our shot will appear on screen and how large prints we could potentially have. If you neither print nor crop or want to “zoom in” your pictures there is no point in shooting anything at higher resoltion than your computer screen or your digital photo frame. This is just a waste of space and on top of it you lose quality, since the image has to be downscaled to the resolution of your screen, and this juggles the images.
One other element that impacts the quality of the image is lenses, but they certainly deserve a separate article.

Originally published :

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