Understanding Digital Photo Quality Factors

Often overlooked amidst the more glitzy, whiz-bang options touted in camera magazine and television ads are the ones that affect the quality of the pictures the digital camera can produce. After all, you probably chose a better camera to take better pictures.

Resolution: How many megapixels?

Digital images are made of colored tiles know as pixels. Camera resolution, stated in megapixels (1 million pixels), indicates the maximum number of pixels that it can use to create a photo.

You need to know these key points:

• Image resolution determines the size at which you can produce high-quality prints. A general guideline is to aim for 300 pixels per linear inch (ppi).
• For onscreen photos, you need very few pixels. Resolution affects the display size of digital photos, but does not affect picture quality.
• High-resolution pictures create larger data files. The more pixels, the faster you fill a camera memory card (the removable storage used by most cameras), a cellphone's onboard storage space, and your computer's hard drive or any online storage closet you may use.

Image sensor: Full frame or smaller?

A photograph is formed when light passes through a lens and strikes a light-sensitive recording medium. In a film camera, the film negative performs the light-recording function. In a digital camera, the image sensor handles the task. The sensor is covered with photosites, which are electronic doodads (that's the technical term) that collect the light data needed to create the image pixels.

Digital-camera image sensors come in two flavors: CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor). In the past, each had a reputation for offering different advantages, but today, both are capable of producing excellent images (although techno-geeks still love to argue about which technology is best).

Sensor size, however, is a different story: A smaller sensor usually produces lower image quality than a large sensor. Why? Because when you cram tons of photosites onto a small sensor, you increase the chances of electronic noise that can degrade the picture.

The three largest sensors commonly used in digital cameras have been assigned these monikers:

• Full frame: The sensor is the same size as a 35mm film negative (36 x 24mm). Why full frame? The term is related to camera lenses, which are still manufactured using the 35mm film negative as a standard. That means that a full-frame sensor is large enough to capture the entire angle of view that a lens produces on a 35mm film camera. Smaller sensors can capture only a portion of that angle of view.
• APS-C (advanced photo system-type C): This is a smaller-than-full frame sensor but with the same 3:2 proportions as a 35mm negative. Within this category, the specific dimensions of the sensor vary from camera to camera. Nikon APS-C sensors measure about 24 x 16mm, for example, whereas Canon's are approximately 22 x 15mm.
• Micro Four Thirds: These sensors are slightly smaller than APS-C sensors, and as the name implies, they have a 4:3 aspect ratio as opposed to the 3:2 ratio of full-frame and APS-C sensors. Note that the term Four Thirds is used for any sensor that has a 4:3 aspect ratio, even for those much smaller than a Micro Four Thirds sensor.

Which is best — 4:3 or 3:2? Well, there's no magic to either aspect ratio. But 3:2 originals translate perfectly to a 4 x 6 print, and a 4:3 image must be cropped to fit. Mind you, you also need to crop 3:2 originals to print them at other frame sizes — 5 x 7, 8 x 10, and so on. And many cameras enable you to choose from several aspect ratios for your pictures or to crop them to a certain proportion using in-camera editing tools.

If you don't see one of these terms, you can find the sensor's dimensions in the camera's spec sheet. But sometimes, size is presented as a single number, such as 1". In this case, that number reflects the diagonal measure of the sensor, which is the same way that TV sizes are presented.

Image file format: JPEG versus Raw

File format refers to the type of file used to record picture data. The standard format is JPEG (“jay-pegg”), but cameras aimed at intermediate and advanced photographers usually offer a second format called Camera Raw, or just Raw, for short.

When it comes to image quality, Raw outperforms JPEG for reasons. The difference has to do with the fact that JPEG files are compressed to shrink file size, resulting in some data loss.

Pro shooters also prefer Raw to JPEG because Raw can record a greater dynamic range (spectrum of brightness values, from shadows to highlights). Additionally, JPEG files are “processed” in the camera, with characteristics such as contrast, sharpness, and color saturation tweaked to provide what the manufacturer thinks its clients like.

Raw files are just that: uncooked data straight from the image sensor. The photographer then does the work of turning that data into a photo using a software tool known as a Raw converter. This gives the photographer control over the final look of a photo.

This is not to say that you should bypass cameras that offer only JPEG, however. Today's digital cameras produce excellent-quality JPEG images, unlike some of the JPEG-only models of past years.

High ISO performance (noise levels)

A digital camera's sensitivity to light is measured in terms of ISO, named for the group that developed the standards for this attribute (International Organization for Standards). Most cameras offer a choice of ISO settings so that you can increase or decrease light sensitivity as needed. In dim lighting, for example, you may need to raise the ISO in order to expose the image.

Being able to increase light sensitivity is great in terms of exposure needs, but there's a tradeoff: As you increase sensitivity, you increase the chances of introducing a defect known as noise, which gives your photo a speckled look. Check out this example, with the noise most evident in the dark background of the picture. Noise is also easier to spot when you enlarge the image, as illustrated by the magnified view.

Today’s cameras are much less noisy than in years past. In fact, if you're using a camera that's more than a couple of years old, better low-light pictures is a perfectly valid reason to justify the purchase of a new model. But because noise levels vary from camera to camera, this is an important characteristic to study when reading camera reviews. Note, however, that a high ISO isn't the only cause of noise; long exposure times also produce noisy images, no matter what ISO setting you use.