Confirmation by magnified playback

No matter how well you can focus optically, you should always confirm the focus electronically. The easiest way to do so is to play back the image on the LCD screen

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Figure 8.3. Confirming focus on the Canon 300D LCD screen. Compare the star images to the dot that indicates magnification (marked by the arrow here).

and view it with maximum magnification (Figure 8.3). On a Canon DSLR, it is easy to compare the star images to the dot that indicates the portion of the picture being displayed. As you press the + button repeatedly, the dot gets smaller and the star images get bigger. If, at maximum magnification, the star images are still smaller than the dot, they're in focus.

Your test image can be a much shorter exposure than what you eventually plan to make. All you need is to capture a few stars. In most situations, a 5-second exposure is enough.

If your DSLR provides magnified live focusing, you don't even have to take a test exposure. Just aim the camera at a star and adjust the focus while watching the image. This feature was introduced on the Canon EOS 20Da (now discontinued) and is gradually spreading through the world of DSLRs.

Live focusing is a delight to use. My very first picture with a Canon EOS 20Da took only 5 seconds to focus perfectly. Canon warns us that if live focusing is used for more than 30 seconds at a time, the sensor will warm up and the noise level will increase. The reason is that when the sensor must constantly take images and output them, all the transistors in it are working hard and emitting heat. Simply taking an image (even a long exposure) and outputting it once is a lot less work.

An ersatz kind of live focusing is provided by the Zigview Digital Angle Finder (www.zigview.co.uk) and other gadgets that aim a tiny video camera into the eyepiece of the DSLR. It's important to note that you are viewing the focusing screen, not the actual image captured by the camera. The Zigview device displays the viewfinder image on a small LCD screen. I suspect that it does not have enough magnification to be useful to astrophotographers. Its main purpose seems to be framing, not focusing.

Figure 8.4. Analysis of a focused star image in MaxDSLR. The camera takes short exposures over and over and downloads them immediately for computer analysis.

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