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Focusing DSLRs - A Secret Technique

Posted by Michael Covington   09/03/2005 12:00AM

Focusing DSLRs - A Secret Technique
[ARTICLEIMGL="1"]Every digital SLR camera in the world has a built-in feature that will enable you to focus perfectly and get pinpoint star images. But almost nobody knows about it. In what follows I'll reveal the secret.

As you know, the normal way to focus any kind of SLR is through the eyepiece. This only works if the eyepiece diopter adjustment is correct; otherwise, you see a blurry image all the time. So if the eyepiece of your DSLR doesn't
seem to be doing the job, find the diopter adjustment and set it to suit your eyes.

For more accuracy, you can add magnification. The Canon Angle Finder C fits almost any camera that has a rectangular eyepiece - not just Canons - and gives
you a right-angle view with a choice of magnifications. The classic Olympus Varimagni Finder did the same thing.

With a film SLR, those are your only options. But digital cameras have a super-power - the LCD screen on the back.

My secret DSLR focusing technique is simply this: Take a test exposure of maybe 10 seconds, then view the star images, on the LCD screen, magnified as much as possible. Fiddle with the focus, and do it again. After half
a dozen tries, you'll find the best setting, and you'll know what sharp star images look like on your camera.

The Canon EOS 20Da, designed for astronomy, goes even further. It offers "live focusing," the ability to view the magnified image continuously while you focus. With other DSLRs, you'll have to expose and view, expose and
view, refining the focus iteratively.

If you'd like some computational help, DSLR Focus software (www.dslrfocus.com) will automate the process, downloading images from your camera and analyze them. But the camera by itself is usually enough for me.

Incidentally, with a DSLR camera, star images will never be single pixels. Every DSLR performs antialiasing - that is, it spreads each pixel out into several - so that the Bayer color matrix will do its job properly and so that
distant striped objects, such as zebras or skyscrapers, don't form moire patterns with the pixel grid. A small, sharp star image is a fuzzy-edged disk about 3 pixels in diameter. That's smaller than the star images you normally get on film.