Megapixel myth
INTRODUCTION
Forget the silly debate over pixel counts among digital cameras. There is little visible difference between cameras with seemingly different ratings. For instance, a 3MP camera pretty much looks the same as a 5MP camera.
Resolution has little to do with image quality. Color and tone are far more important technically. Even Consumer Reports in their November 2002 issue noted some lower resolution digital cameras made better images than some higher resolution ones.
THE MYTH
The megapixel myth was started by camera makers and swallowed hook, line and sinker by camera measurbators. Camera makers use the number of megapixels a camera has to hoodwink you into thinking it has something to do with camera quality. They use it because even a tiny linear resolution increase results in a huge total pixel increase, since the total pixel count varies as the total area of the image, which varies as the square of the linear resolution. In other words, an almost invisible 40% increase in the number of pixels in any one direction results in a doubling of the total number of pixels in the image. Therefore camera makers can always brag about how much better this week's camera is, with even negligible improvements.
One needs about a doubling of linear resolution or film size to make an obvious improvement. This is the same as a quadrupling of megapixels. A simple doubling of megapixels, even if all else remained the same, is very subtle. The factors that matter, like color and sharpening algorithms, are far more significant.
The megapixel myth is also prevalent because men always want a single number by which something's goodness can be judged.
Unfortunately, it's all a myth because the number of megapixels (MP) a camera has very little to do with how the image looks. Even worse, plenty of lower MP cameras can make better images than poorer cameras with more MP.
PRINT SIZES
Want to know how big a print you can make before you start to lose sharpness compared to film? It's simple and here's the formula for general cases:
Long print dimension in inches = 4 x (square root of megapixels)
For example, for a four megapixel camera the square root of four is two. Two times four is eight. Thus the biggest print you can make without losing sharpness compared to film at normal viewing distances is is 6 x 8." From a sixteen MP camera likewise you could go 12 x 16." Of course you can print bigger, just you won't have the sharpness of film. Also few people are able to get all the sharpness of which film is capable, making this harder to compare.
Of course you can print much bigger, since sharpness isn't as important in color as most people worry. You can get great results from a 6MP camera at 20 x 30" if you want, since normal people view big images from further away. This is all art and in the eye of the beholder; I prefer huge prints made from my 4 x 5" film camera, and for portraits I prefer the smoothing of digital cameras.
Don't worry too much about this, since sharpness is not as important in color as it is in B/W. I make 12 x 18" color prints all the time from 3 to 6 MP cameras and they look great, since I only print images that are good to begin with.
The entire resolution issue is one of scale and viewing distance. Sure, more resolution is better at bigger sizes, but how sharp your image is has little to do with how good it is. Far more important technically is whether or not the colors are correct and whether or not any sharpening was done tastefully. Many digital cameras add nasty looking sharpening that puts very artificial halos around sharp lines, making the image look obviously digital to those of us who recognize these things. Sloppy sharpening is done to impress the innocent by overemphasizing the lines around things if real sharpness and resolution is lacking.
TO REPLACE FILM
Digital does not replace film. Just look here for why a magazine like Arizona Highways simply does not accept images from digital cameras for publication since the quality is not good enough, even from 11 megapixel cameras, to print at 12 x 18."
If you do fret the pixel counts, I find that it takes about 25 megapixels to simulate 35mm film, which is still far more than any practical digital camera. At the 6 megapixel level digital gives about the same sharpness as a duplicate slide, which is plenty for most things. Honestly, I have actually had digital files written back out onto film to see this. See also my film vs. digital page here.
Of course I use much bigger film than 35mm for all the pretty pictures you see at my website, so digital would need about 100 megapixels to simulate medium format film, or 500 megapixels to simulate 4x5" film. This is all invisible at Internet resolutions, but obvious in gallery size prints.
For images seen at arm's length you need to have about 300 real pixels for every inch of your print's dimensions. If you are looking too closely, as with a contact print, then you'll love to have 600 real pixels or more for every inch of your print. Stand further away as you would from a huge print and even 100 pixels per inch (DPI) can look great. By real pixels I mean real optical pixels, not phony interpolated ones. Multiply the inch dimensions by these DPI figures to get the total resolution (horizontal and vertical, typically thousands in each dimension) you need for a decent image, and multiply these together to get a total number of pixels (usually in the millions, or megapixels.)
For instance, for a fairly decent 8x10 you need [8" x 300 DPI] x [10 x 300DPI] or 2,400 x 3,000 pixels, or 7,200,000 pixels, or 7.2 megapixels. This is what the formula at the top calculates the easy way.
