Are you an amateur digital photographer who assumes more megapixels in a camera means better pictures? I've been assuming that too, but as it turns out, the equation isn't that...

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Are you an amateur digital photographer who assumes more megapixels in a camera means better pictures? I’ve been assuming that too, but as it turns out, the equation isn’t that simple — pixels, image sensor, lenses and other elements all play roles in any quality comparison.

Recently, for example, I tried a 7-megapixel camera and compared its 11-by-14-inch prints with those of a 5-megapixel camera and found it hard to detect a quality difference.

So what’s the advantage of having a 7-megapixel camera?

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To find out, I started digging into a few photography books and below will attempt to summarize what I’ve learned. If you’re interested, dip into “Real World Digital Photography” by Eismann, Duggan and Grey; and “PC Magazine Guide to Digital Photography” by Grotta and Grotta.


Image sensor and pixels:

Unlike a film camera, which focuses light on a piece of film, a digital camera focuses light on a semiconductor device called an image sensor that converts the light into electrical charges. The sensor is a collection of tiny, light-sensitive, electrically charged pixels. When light hits them, the brighter the light, the greater the electrical charge.

Image resolution is essentially information, and having more pixels provides more information — for an image richer in details, color and other attributes. The more pixels in an image, the more it can be enlarged, and the higher the density of pixels, the more detail. So, yes, more pixels mean better resolution in enlargements.

However, in practice, it takes more than the highest pixel count to produce the best pictures — size of the sensor and the pixels also matters. According to “Real World Digital Photography,” larger pixels can capture more light with less “noise” (visible artifacts in the rendered image), and smaller pixels can resolve finer detail and render a sharper image.

The book’s authors add that it’s difficult to attain the right balance between having the smallest pixels possible for finer detail and other advantages, but not so small as to create tonal and noise problems.

Another influence on image quality is the camera’s sensor. Different manufacturers’ image sensors render colors differently, with different amounts of noise and other variations. Camera manufacturers tend to use the same kind of image sensor in most models, so if you’re happy with images produced by one brand of camera, you’re likely to find similar attributes in other models.

I like to print big, and I often crop images, so I value a large pixel count — pixels that can retain sharp resolution but still capture low light without excessive noise.

However, if you don’t print larger than 4-by-6 or 8-by-10 inches, don’t do a lot of heavy cropping, and don’t mind shooting with a flash indoors, you can get sharp pictures with a moderately priced 4- to 5-megapixel camera.


Image compression:

It’s great to have very-high-resolution images, but saving all those pixels in the camera fills up a memory card fast, plus you can’t e-mail supersized image files or post them on the Web. The standard practice is to compress image files in the JPEG (joint photographic experts group) format, which is adjustable from slightly compressed (high quality) to very compressed (low quality) files.

The downside is that JPEG files use “lossy” compression, which means pixels are removed and the image is degraded every time it’s edited, saved and closed. Degradation (loss of detail and color) generally shows when the images are enlarged, and the larger the image the more it shows.

Lossless compression means no pixels are lost, and the process doesn’t degrade the image and nothing is lost, but the trade-off is the image files are big. TIFF (tag image file format), PNG (portable network graphics), PSD (Photoshop document, Adobe’s proprietary format), and RAW (not an acronym) file formats are lossless.

As for file sizes, in a 6-megapixel camera, for instance, a TIFF file takes about 18MB of storage, a RAW file takes about 6MB, and a JPEG file takes about 1 to 2MB.

Some experts recommend saving images as high-quality JPEG files in the camera, and then save them as TIFF files on the computer for processing without further degradation. Later, if any are to be e-mailed or posted on the Web, they can be converted back to JPEG.

Other experts recommend saving the images in RAW in the camera and converting them to TIFF or another lossless format on the computer. RAW stores the image data directly from the image sensor to the computer without processing it.

Another issue to consider is whether to capture and store images as 8-bit or 16-bit files. According to “Real World Digital Photography,” storing 16-bit files enables capture of up to 65,536 tonal values for each of the three color channels, compared with 256 for each channel in an 8-bit file. “Real World” adds that only the RAW file format currently enables 16-bit capture.

Next we’ll look at mini explanations of aperture and shutter speed, lenses, noise, ISO and more — they all matter for creating the best possible photographs.

Write Linda Knapp at lknapp@seattletimes.com; to read other Getting Started columns, go to: www.seattletimes.com/gettingstarted