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The Lowdown On Zooms

Digital And Optical Zooms • How Many Clicks Do You Get? • The Color Of sRGB And Adobe RGB (1998) • Film Vs. Digital Vs. Sensor Size...Again

This Article Features Photo Zoom

The young alligator was photographed at the Tennessee Aquarium in Chattanooga using a Canon G10 compact point-and-shoot camera. I used the zoom at its maximum optical setting of 30.5mm (140mm 35mm equivalent). The camera is capable of a digital zoom of 560mm (35mm equivalent), but the image quality would have suffered greatly.

Digital And Optical Zooms
Q My son has a digital point-and-shoot camera with an 18x zoom, but when you zoom to the extreme end, the viewfinder displays a message that it’s a digital zoom, and the resulting quality isn’t very good. Is it also true that zoom lenses on D-SLRs don’t work well at their maximum ends? My 200mm zoom telephoto has a 1.6x factor. How does this relate to the digital vs. optical zoom on my son’s point-and-shoot?
D. LaKing
Via the Internet

A The issues you raise point out common confusions about the differences between true optical zoom and the “crop factor” of some digital sensors.

Digital sensors, the devices within digital cameras that receive image information and record it, are packed with pixels; each pixel collects a dot of information. A 10-megapixel sensor holds 10 million pixels. The size of a sensor is expressed in terms relative to 35mm film, a 36x24mm frame (864 square millimeters). A full-frame sensor, available in some professional D-SLRs, is the same size as 35mm film; these have either more pixels for resolution or larger pixels for light-gathering capability. The APS-sized sensors available in some D-SLRs are smaller (they range from 329 to 548 square millimeters). A point-and-shoot camera has a very small sensor (as small as 43 square millimeters), so the pixels on it are very small and crammed together.

Your “1.6x factor” has to do with your camera’s sensor, not your lens. It means that your APS-sized sensor has an angle of view that’s narrower than a full-frame sensor or 35mm film. The effect is a crop of your image—making the subject seem larger in the frame—that uses all of the megapixels on your sensor and gathers maximum information.

Optical zoom is a lens-oriented term. It refers to the range of optical magnification of which the lens is capable when using the entire imaging sensor in the camera. D-SLRs have only optical zooms—that is, the zoom happens in the lens, not the camera. In most lenses, you can expect a small amount of drop-off in quality when the lens is extended to the lower and upper extremes of its range and used at its widest aperture. But this is a different phenomenon and not nearly as image-degrading as the “digital zoom” option found on some compact point-and-shoots.

At the end of a point-and-shoot’s particular optical zoom range, many cameras offer “digital zoom.” With digital zoom, the image is cropped on the sensor in the same way you might crop it on your computer. Remember that the sensor is where your pixels reside. More pixels means more detail in your image. Digital zoom crops the image at the sensor to effectively enlarge your subject. The result is that your larger image is created with a smaller portion of the sensor, using fewer pixels, offering less resolution and yielding a poor-quality image.

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