Sunday, August 16, 2009

Where is the focus?


My father was telling me about some people who test their cameras to see if the focus is off, which makes some sense. I had always assumed that the camera would focus where I focused it, and not somewhere else. However, things could be off, the sensor might not be aligned, the focal point of the lens might be on the back side of the sensor.

However, it still seems off if the camera focuses on a different plane than I expect. So, I did an experiment.

I am a pilot, and I have a lot of the aeronautical charts around. the distance between the lines on the aeronautical charts is a little over 1/8 of an inch (Los Angeles Sectional Chart if anyone is interested). I put the camera on a very sturdy tripod, put my 28 - 105 mm zoom on it, set it to take a photo at f 4.5 and snapped a photo. Not too much detail, so I put 65 mm of extension tubes on it. That reduced the focus point to around a couple of inches. At 105 mm, I focused on the longest line, and snapped a photo (above).

The results are that the camera did focus where I expected it to. the depth of field on the photo is a little under 1/8 inch, with the two lines crossing in focus.

Glad to know my camera is working as I expected...

Addendum, this topic is actually about whether a camera will auto-focus properly. Since I was manually focusing, this entry is irrelevant to the discussion at hand, but it is a good test to make sure that your camera view-finder is properly aligned with the film sensor. For more information on the auto-focus issue, See here

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