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On Megapixels

Although some of the uproar about megapixels has died down, I still find too many of my friends trying to compare cameras based on resolution alone.   It is somewhat like the infatuation with automobile engine displacements of the sixties or the preoccupation with stereo output watts in the seventies.   It would be really wonderful if one simple number provided all of the information you need to choose a car, a stereo, or a camera.

A digital camera records light intensity at discrete points called pixels.   Unlike film where the light sensitive grains were scattered at random, pixels are arranged into a neat rectangular array. If the array is four thousand pixels wide by three thousand high, the array would contain 12 million pixels or 12 megapixels.

So megapixels is the number of discrete points that a camera can record. On the surface, more seems like a good thing, as long as the size of the points does not matter.   Size does matter, however.   Cramming more discrete electrical elements into a space reduces the portion of the light each recieves.   Reducing the light that each pixel receives tends to make the image more noisy.   Digital noise degrades the final image causing the darker areas to fill with what looks like multicolor confetti.

More megapixels also increases the file size, filling up your storage and taking longer to save.   So, just how many megapixels do you need?   It really depends on what you want to do with the picture.   Do you want to have a photo in a frame on the wall?   Do you want to have a small photo to carry and show your friends, or do you just want an image on a screen?

How much is enough

I have read that a hand held photograph requires about 300 pixels per inch and that a wall mounted photo only needs about 240 pixels per inch.   That is linear resolution.   We have to square the linear resolution to compare with megapixels.   That means that a hand held photo requires about .09 megapixels per square inch and a wall mount photo requires only about .06 megapixels per square inch.   Pretty small numbers, and I have seen many effective photos that have less resolution than that.

Extrapolating to print size, that means that a 4 x 6 inch print for hand holding needs about a 2 megapixel image.   That is well within the range of a camera phone.   An 8 x 10 inch print is still in the range of hand holding so that would require a 7 megapixel camera.   Wall mounted, it would only require a 5 megapixel camera.   Going whole hog, a 12 x 16 inch photo on the wall would only require a 12 megapixel camera and a really large 16 x 20 print could be made from a 20 megapixel image.

The calculated limits are relative.   In reality I have seen beautiful 8 x 10 inch prints in books made with 5 megapixel cameras and I have seen huge hanging prints made with 12 megapixel cameras.   Most people never expect to print images at all.   My computer screen is far bigger than most and only displays 3 megapixels.   HD resolution is less than 2 megapixels, and think what the screen on your phone might be.

You really don't need that many megapixels.