What resolution is right for you?

While most photographers focus on the number of megapixels to gauge a camera’s resolution, the actual resolution is more complex than the number of pixels on an image sensor. For example, the camera may not use all the pixels in the digital image; some pixels may be used for other imaging tasks. Other factors come into play as well, such as sensor size and digital noise, which is the grainy quality of a low-light digital image.

Sensor size must be a consideration in evaluating megapixel count. Many point-and-shoot digital cameras boast an excess of 10 megapixels, but these 10 million pixels are crowded onto a sensor less than half the size of your fingernail! The 10 million pixel sensor on a Digital Rebel XTi is much larger physically than a 10 megapixel sensor on most, if not all, point-and-shoot cameras boasting the same megapixel resolution. Larger photo sensors can gather more light, have less digital noise, and have better dynamic range. In this case, bigger is better.

Each pixel in a camera sensor contains one or more light-sensitive receptors that convert the incoming light into an electrical signal. This signal is then processed into the colors used in the final image, and the brightness of individual pixels is amplified or decreased. A common cause of digital noise is overamplification of the signal coming from the photosite. If the same pixel is exposed several times by the same amount of light, the resulting color values are not always identical but have small variations, resulting in digital noise.

Even with no light present, the electrical activity of the sensor itself will generate some noise (think of it as the photographic equivalent to the background hiss in audio equipment).
Noise in digital images, similar to film grain, is most visible in uniform surfaces (such as blue skies and shadows). Digital noise increases with ISO (sensitivity), and as pixel size decreases. This is why digital compact cameras generate much noisier images than dSLRs. Professional cameras, such as the EOS-1Ds Mark III and EOS-1D Mark III, have higher-quality components and more powerful processors that allow for more advanced noise reduction; subsequently these cameras display significantly less noise even at high ISOs.

If you only plan to have 4 X 6 or 5 X 7 prints made at your local drug store kiosk, then you can be less concerned with differences in sensor size. If you plan to make 8 X 10, 11 X 14, or larger-size prints, then the dSLR’s larger sensor provides a bigger payoff. As you can see in 2-2 and 2-3, bigger pixels can be enlarged or upsized, with much less noise and better results. When a digital image is enlarged beyond its native resolution via software, it results in an effect called pixilation, an unsightly effect where jagged pixels are very obvious to the naked eye. Some editing programs will attempt to create new pixels to the best of their ability if an image is enlarged; one method for doing this is called interpolation. Generally speaking, the results are of less-than-adequate quality.

Notice the differences in these two images viewed at 100 percent, produced from a Canon G-1 point-and-shoot digital camera and a Canon 30D.

In general terms, resolution correlates to the size of the prints that you can make at the camera’s maximum quality setting. Table 2-1 provides an overview of the general sizes at which you can print images from Canon dSLRs at both 240 and 300 dots per inch (dpi).

Table 2-1. Sample Printing Sizes at Native Resolutions
Camera Model Output Size Print Size @ 240 dpi Print Size @ 300 dpi
EOS-1Ds Mark III 5,616 x 3,744 23.4 x 15.6 18.72 x 12.48
EOS-1D Mark III 3,888 x 2,592 16.2 x 10.8 12.96 x 8.64
EOS 5D 4,368 x 2,912 18.2 x 12.1 14.56 x 9.7
EOS 40D 3,888 x 2,592 16.2 x 10.8 12.96 x 8.64
EOS Rebel XTi 3,888 x 2,592 16.2 x 10.8 12.96 x 8.64
EOS Rebel XT 3,456 x 2,304 14.4 x 9.6 11.52 x 7.68

NOTE

Digital images are made up of pixels, which are individual dots of information that combine in a matrix of millions to form a photograph. Zooming in on a digital image in an image-editing program reveals the pixels that fool the human eye into believing it’s seeing a fluid, smooth photo. A megapixel is one million pixels, and the term is used to represent a camera’s maximum resolution (for example, 6.2 megapixels). For example, the EOS-1Ds Mark III has a maximum resolution of 21.10 megapixels; its maximum RAW file produces images of 5,616 x 3,744 pixels (5,616 x 3,744 = 21,026,304 pixels, which rounds to 21.10 megapixels).

Category: Photography System

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