Good read from David Pogue, NY Times:
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Deconstructing the Megapixel Myth
My column in The Times last week, about the general irrelevance of megapixels in consumer cameras, stirred up an enormous volume of feedback.
Most of the 700 e-mail responses, including many from experts and engineers, agreed that megapixel count has been promoted (by camera stores and camera companies) as a misleading marketing gimmick for years. My argument-and my test-maintains that 5 or 6 megapixels is plenty for enlargements up to, say, 16-by-24 inch posters.
There were still a few naysayers who don't believe the results. This one was typical: "Why do you insist in calling it a pixel myth? Other things being equal, 8 megapixels DOES give a higher quality than 5; you just can't see it."
(Um, dude? If you can't see any difference, then it's not higher quality.)
In this e-column, I'd like to expound upon a couple of points that my print column didn't have room for.
First, one reason WHY a few more megapixels don't produce any visual differences in the print.
Let me tease you first with this question: How much bigger can I print a 10-megapixel photo than a 5-megapixel photo?
Most people answer, "twice as big" or even "four times as big."
But they're wrong. In fact, doubling the megapixels of a photo actually lets you add only 20 percent more area to each edge.
Here's the math. A 5-megapixel photo might measure 1944 x 2592 pixels. When printed at, say, 180 dots per inch, that's about 11 by 14 inches.
A 10-megapixel photo (2736 x 3648 pixels), meanwhile, yields a 180-dpi print that's about 15 by 20 inches-under three inches more on each margin!
Upping the resolution by even smaller amounts (from 5 to 7 megapixels, for example) produces an even tinier difference-too small to bother with.
Meanwhile, cramming more pixels onto a camera's sensor can actually LOWER the quality of the photo. A former Kodak manager wrote to explain it this way: "Too many megapixels can actually impair a camera's performance. For example, the typical sensor in a consumer camera is 0.5-0.7 inches. The more millions of pixels, the smaller each pixel must be-and the smaller the pixel, the less light-gathering efficiency it has, and the worse the camera performs in low-light or stop-action shots."
Now then: At the end of the column, I pointed out that megapixels have become a handy crutch for consumers shopping for a camera. They'd come to rely on it as a letter grade that made comparison shopping easy-and camera makers exploited this fact. "Well, heck, this one has 10 megapixels! That's a lot better than this 6-megapixel model!"
But a million factors are far more important than the megapixels. The question is: Can we come up with a new one-digit crutch? Can we propose a more meaningful comparison factor?
Lots of you said yes, the sensor size is far more important. After all, it's undisputed that a 6-megapixel Nikon D40 digital S.L.R. takes better pictures than a 10-megapixel shirt-pocket camera, because its sensor is relatively gigantic. Its individual pixel sensors can be larger and soak in more light, even if there are fewer of them.
Unfortunately, the camera makers and salespeople aren't going to help you out here. You're not going to see starbursts in the ads saying, "3/4-INCH SENSOR"! But you should.
In fact, the industry seems to go out of its way to prevent you from knowing what the sensor sizes actually are. It reports digital S.L.R. sensor dimensions in millimeters, like 23.6 x 15.8 mm.
Consumer cameras' sensors, meanwhile, are reported as a ridiculous fraction, like 1/1.8"-and that's the *diagonal* measurement. Not only does that mean you have to do a lot of math in your head, but it's also counterintuitive. The measurements with a bigger denominator actually represent *smaller* sensors. A 1/2.5" sensor is actually smaller than a 1/1.8" sensor.
And how does that relate to a 23.6 x 15.8 mm digital S.L.R. sensor? Only Einstein knows.
If you can do the math-you can find sensor sizes reported at camera-review Web sites, like steves-digicams.com, dpreview.com, and dcresource.com-you'll be well rewarded. There are a million factors to consider when you buy a camera, but this one's a fairly good predictor of picture quality.
A better one might actually be on the horizon. I also received this intriguing message:
"Hi David. I am the author of Imatest software, which is used for measuring sharpness and image quality by imaging-resource.com, DigitalCameraInfo.com, and CNET, as well as many print publications.
"You asked: 'So what replaces it [the megapixel statistic]? What other handy comparison grade is there?'
"Imatest measures system sharpness as Spatial Frequency Response (SFR), which is pretty much the same thing as Modulation Transfer Function (MTF). These geeky technical terms have great value to engineers, but they scare off consumers, and they don't quite answer the question, 'How sharp does an image look?'
"I recently added a measurement to Imatest that does, but it's unfamiliar, even to most camera reviewers. It's called SQF (Subjective Quality Factor). It includes print height, viewing distance, and the contrast sensitivity of the human eye. It was used internally by Kodak and Polaroid for years, and it is the basis for Popular Photography's lens tests-but it was tedious to measure until I added it to Imatest. See
www.imatest.com/docs/sqf.html.
"Incidentally, where the megapixel myth really goes berserk is with cameraphones. Limited real estate forces the camera modules to be tiny, which means that the pixels get tiny--well under 2 microns--when the marketing people force the engineers to increase the pixel counts, because megapixels sell. Unfortunately, tiny pixels are noisy, work poorly in low light, and may not be utilized due to a physical phenomenon called lens diffraction. Engineers are well aware of the problems, and they keep butting heads with the marketing people. Guess who has the real power?
"An industry group, I3A,
www.i3a.org (worth checking out), is working to come up with better measurements to rectify this situation. It will be quite a battle."
Visit David Pogue on the Web at DavidPogue.com.
