Confused by DOF and subject distance...

Started 4 months ago | Discussions thread
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MBob
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Confused by DOF and subject distance...
4 months ago

I have recently started playing with zone focus and the use of hyperfocal distances.  I thought I understood the basic principles, but after taking a few autofocus photos yesterday, I learned that I clearly do not.

Using an APS-C camera with an 18-70 zoom to photograph a mountain, I set focal length to 24mm and the aperture to f/9.  Using a phototools app on my iPhone to calculate the DOF at this settings, I noted that the hyperfocal distance was 10.6 feet.  I was surprised when the camera autofocused and set the subject distance to 6.5 feet.  Using a subject distance of 6.5 feet in the DOF calcs in the app resulted in a DOF of only 12.70 feet, extending from 4 feet in front of me to 16.7 feet in front of me, yet the mountain was in focus.  Why?  How?

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