Megapixel Story

For normal 4x6" (10x15cm) prints, even VGA (640 x 480 or 0.3MP) resolution is just fine. Digital cameras did this back in 1991!

In 1999 when digital cameras were only 1.2 or 2 MP, each megapixel mattered if you were making bigger prints.

Today, even the cheapest cameras have at least 5 or 6 MP, which enough for any size print. How? Simple: when you print three-feet (1m) wide, you stand further back. Print a billboard, and you stand 100 feet back. 6MP is plenty.

Sharpness depends more on your photographic skill than the number of megapixels, because most people's sloopy tehnique or subject motion blurs the image more than the width of a microscopic pixel.

Even when megapixels mattered, there was little visible difference between cameras with seemingly different ratings. For instance, a 3 MP camera pretty much looks the same as a 6 MP camera, even when blown up to 12 x 18" (30x50cm)! I know because I've done this. Have you? NY Times tech writer David Progue did this here and here and saw the same thing — nothing!

Joe Holmes' limited-edition 13 x 19" prints of his American Museum of Natural History series sell at Manhattan's Jen Bekman Gallery for $650 each. They're made on a 6MP D70.

Sharpness has very little to do with image quality, and resolution has little to do with sharpness. Resolution (pixel count) has nothing to do with picture 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. That was a long time ago!

Explanation of Terms


Pictures are made up of little dots called pixels. Pixel stands for PICture ELement. Put enough of them together and you have a picture. They are arranged horizontally and vertically. Get close enough to your computer screen (or use a magnifier) and you'll see them.

Resolution (Linear Resolution)

Image Resolution

Resolution is how many pixels you have counted horizontally or vertically when used to describe a stored image. Digital cameras today have between 2,048 and 4,500 pixels horizontally. 3 MP cameras have 2,048 pixels horizontally and 14 MP cameras have 4,500 pixels. They have fewer pixels vertically since the images aren't as tall as they are wide.

That's not much of a difference, is it? That's the whole point of this article. I'll explain that a little further down.

Print Resolution

Resolution is also how many pixels you have per inch or other linear unit when you print on paper. Most prints are made at 200 - 300 pixels per inch (PPI or DPI, dots per inch). This is the image resolution and has nothing to do with the technology by which the print is made. (For instance, inkjet printers' nozzle sizes are the silly 2880 DPI or other numbers you see. These printer numbers are often used by hucksters to hoodwink and distract you when talking about resolution. These only refer to how the ink is spat out on the paper.)

Screen Resolution

Most computer screens today are about 100 DPI, dots per inch. There isn't much variation from screen to screen so we rarely discuss this. It's easy to figure out: most computer screens are about 1,024 x 768 pixels. If your screen is 10" wide then divides 1,024 by 10 and you have a 102.4 DPI screen. Bigger screens tend to have more pixels, for instance, my 22" CRT has 1,600 x 1,200 pixels and has a viewing area of 16 x 12."

Yes, laptops with bigger screens tend to have lower linear resolution. No big deal.

Pixel Count, expressed as Megapixels

Pixel Count, expressed as Megapixels, is simply multiplying the number of horizontal pixels by the number of vertical pixels. It's exactly like calculating area. A 3 MP camera has 2,048 (horizontal) x 1,536 (vertical) pixels, or 3,145,728 pixels. We call this simply 3 MP.

Small differences in pixel count, between say 5 MP and 8MP, are unimportant because pixel counts are a square function. It's exactly like calculating area or square footage. It only takes a 40% increase in linear dimensions to double the pixel count! Doubling pixel count only increases the real, linear resolution by 40%, which is pretty much invisible.

The Myth

The megapixel myth was started by camera makers and swallowed hook, line and sinker by camera measurebators. 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.

This gimmick is used by salespeople and manufacturers to you feel as if your current camera is inadequate and needs to be replaced even if the new cameras each year are only slightly better.

One needs at least a doubling of linear resolution or film size to make an obvious improvement. This is the same as quadrupling the 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 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.

The Hype

Here's a complete fabrication by a company who is trying to spread the myth to get you to buy too much camera. There's a similar page here. That page is brilliantly done, however it's done with completely fraudulent data to exaggerate the differences. At the low magnifications shown on the screen any and all of those examples should look perfect. Instead the two lower resolution examples have been deliberately degraded to make them look worse. Their page displaying results for a 5 x 7" print actually show how the 4 MP camera would look blown up to 12 by 9 feet, not 5 x 7 inches!

How do we know their 4MP example is what you'd see blown up twelve feet wide, not 5 x 7 inches? Easy: for the 4 MP example at maximum crop I see pixels blown up to little squares measuring 16 pixels per inch on my screen. (Just get out your ruler and measure for yourself.) You divide the number of pixels by the PPI (DPI) to get how many inches you get in print at that resolution. Thus printing a 2,289 x 1,712 pixel (4MP) image at 16 PPI gives (2289/16)" x (1712/16)" or 143" x 107" or, dividing inches by 12 to get feet, 12' x 9.'

I'm sure the designer of that page would feign ignorance of the technology involved if made to own up to it. Page designers don't have Ph.D.s in digital image processing, either. Most likely the designer worked on it till their manager made sure that they showed a clear difference. Their manager, if made to come clean, would probably explain that the page was put up to illustrate the differences as an educational service, not as actual science or a legitimate example. They had to make certain "adjustments" to make the differences clear, namely, to make the 4 MP and 5 MP cameras look much worse than they are.

I taught you above how to calculate the differences among different resolution cameras. The difference between the 6 MP and 4 MP cameras should be (square root (6/4)) or SQR(1.5) or 22.4%. In other words, the size of the pixels or number per inch should be less than 25% different between the 4 MP and 6 MP cameras. They've made the lower resolution cameras look much, much worse by comparison on that page.

nb: thx to KenRockwell

No comments:

Post a Comment