What is a DSLR

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A DSLR is a digital camera that is built like a film SLR (single-lens reflex) and has the same ability to interchange lenses. You can attach a DSLR to anything that will form an image, whether it's a modern camera lens, an old lens you have adapted, or a telescope, microscope, or other instrument.

Unlike other digital cameras, a DSLR does not normally show you a continuous electronic preview of the image. Instead, the viewfinder of a DSLR uses a mirror and a focusing screen to capture the image optically so that you can view

Figure 1.1. The galaxy M31 as the image came from the camera, with no processing except adjustment of brightness and contrast. Canon Digital Rebel (300D); single 6-minute exposure through a 300-mm lens atf/5.6, captured as JPEG. Some noise specks are present which newer cameras would eliminate with automatic dark-frame subtraction.

and focus through an eyepiece. When you take the picture, the mirror flips up, the image sensor is turned on, and the shutter opens.

The reason a DSLR doesn't show the electronic image continuously is that its sensor is much larger than the one in a compact digital camera. Big sensors are good because they produce much less noise (speckle), especially in long exposures, but operating a big sensor all the time would run down the battery. It would also cause the sensor to warm up, raising its noise level. That's why you normally view through the mirror, focusing screen, and eyepiece.

Some DSLRs do offer "live focusing" or "live previewing" for up to 30 seconds at a time. The Canon EOS 20Da, marketed to astronomers in 2005, was the first. Live previewing enables you to focus much more precisely than by looking

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100 Photography Tips

100 Photography Tips

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