The eyepiece on a DSLR is commonly out of focus. Most DSLRs have an adjustment called the eyepiece diopter (Figure 8.1) which you are expected to adjust to suit your eyes. This is rarely done, because for daytime photography with an autofocus camera, it doesn't matter. To someone who always uses autofocus, the viewfinder is just for sighting, not for focusing, and the image in it need not be sharp.
Here's how to adjust it. Hold the camera up to your eye as if to take a picture and ask yourself whether you can see the focusing screen clearly. (If you need glasses, wear them.) Keep the camera turned off and the lens out of focus so that your attention is directed to the screen.
Adjust the diopter so that the screen is as sharp as possible. Pay attention to its granular texture and the indicator boxes or lines on it. With a Canon, focus on the boxes, not the black or red indicator LEDs just beyond them.
Then do the same thing again in dimmer light. Your eyes focus differently in the dark than in the daytime. If you can touch up the eyepiece diopter under relatively dim conditions, it will serve you better for astronomy.
Now go outdoors in the daytime, set the lens to manual focus, and practice focusing the camera manually. Your goal at all times should be to look at the screen, not through it. That is, keep your attention on whatever features of the screen you can see - boxes, lines, or granularity - and bring the image from the lens into the same focal plane.
Half an hour of doing this will greatly build your skill. Before long, you should be able to focus your daytime pictures better than the camera's autofocus mechanism does. Try it.
When you master the technique I've just described, it will still be very hard to focus on celestial objects. The secret is always to focus on the brightest star you can find, then aim the camera at the object you actually want to photograph. Everything in the sky, from airplanes to galaxies, is so far away that, for all practical purposes, it all focuses in the same plane.
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