New Year's Resolution

Even my most sincere New Year's resolutions rarely last for more than a few weeks. That's probably because I bite off more than I can chew.

Case in point? Last year I planned to write the definitive article explaining how resolution works in a digital camera, how it contributes to image quality, how you can improve it, and how we test it. But I  realized that this was impossible to achieve in a single attempt. So this year I plan to take smaller steps.

If you've been led to believe that digital cameras with more megapixels have higher resolution and image quality than less-endowed cameras, you're not alone. Unfortunately, a camera's megapixel count is often used as a quality yardstick.

In some cases, cameras that have more megapixels produce better images. In others, the opposite is true. Many variables affect the outcome, including the lens, AF system, file format, and image-processing system. But even when these are equal, it's still possible for state-of-the-art cameras with a wide range of megapixel counts (7.5- to 10.2MP) to capture images with the same, or very similar, detail.

An equalizing factor: the aspect ratio of the camera's sensor. Most compact digital cameras capture photos with a 4:3 aspect ratio, so the horizontal width of the image is just slightly longer than the image height. It's no coincidence that older CRT televisions and computer monitors had 4:3 aspect ratio tubes, as many of the sensors found in compact cameras are direct descendents of the CCDs first used in camcorders. That makes it easy to display these images full-screen on older TVs and monitors, but images with a 4:3 aspect ratio must be cropped in order to make common 4x6- or 8x12-inch prints. (The exceptions among compacts are the Panasonic Lumix DMC-LX2, with its 16:9 aspect-ratio sensor, and the recently announced Sigma DP1, with its 3:2 sensor.)

On the other hand, most DSLRs have sensors with a 3:2 aspect ratio. (Exceptions: the Four Thirds standard DSLRs from Olympus and Panasonic, which maintain the 4:3 format.) The 3:2 ratio was either a compromise to get 35mm film-shooting pros to convert to digital in the 1990s or a convenient format designed to fit the viewfinders of early DSLRs built from modified 35mm SLR bodies. Images with a 3:2 aspect ratio fit the 4x6-and 8x12-inch print model without cropping, but don't fit older TVs and computer monitors as well.

Since the Panasonic Lumix DMC-LX2 has a switch on the lens barrel that allows you to change aspect ratios to either 3:2 or 4:3 from its native 16:9, it shows how you can have images with three different megapixel counts (10.2, 8.5, and 7.5, respectively) that share the same resolution -- albeit with different fields of view (Click photo for a larger version).

Aspect ratios also help to explain why a 7.5MP Olympus Evolt E-330 (with a 4:3 sensor) can achieve similar resolution numbers to an 8.2MP Canon EOS 30D (3:2 sensor), since widening the Evolt sensor to a 3:2 aspect ratio while maintaing pixel density would give you 8.5- megapixel sensor.

So the next time somebody tells you that a 10MP camera takes sharper images than an 8MP camera, fire back "At what aspect ratio?" and see if they know how to settle the score.

Comments

[Jonas Fields]

I don't like your review of the 10mp shootout for one reason...You didn't use Sony's Zeiss standard in comparison with the other cameras. I know it makes it more than "a grande"...but I bet that combination of minolta slr and zeiss optics would blow away those cameras..

[Ales Litomisky]

Yes, education regarding the picture quality in digital teritory is necessary. For example test the 10 Mpixel APS-C sized digital cameras at lens aperture 16 as in your Popular Science shout-out is the mistake, becase the picture quality at this aperture is limited by the difraction limit. Sure, you can use the APS-C sized sensors with the aperture 16 if depth of field is more important then the resoulution, or for small prints - but not for pictute quality tests. The aperture 16 is OK for old film format 35mm (or for full frame sensor), but not for the 10 MPixel APS-C sized sensor. Back to school, gentlemen (or simply try google search on difraction limit).

[Gary Riedel]

The above article is so much gobblygook (sic) that I just have to add a comment. What does it mean when a sensor is rated at 1/25 as opposed to one that is rated as 1/76, for instance? What do those numbers represent? is a lower /number good or bad? All other things considered, is/25 more efficient than /76? Please help.

[Alsharif M. Ali]

To Mr.M.Macnamara Exectutive technology Editor
I Read your January report on aspect ratio and resolution. my question:
In order to buy a 3:2 dslr comparable with the 4:3 Olympus E510 with 10 megapixells would I have to look for a 3:2 dslr equal or higher to 11.2 megapixels?
Thank you
Alsharif M.Ali