I was planning to write about the three things that affect depth of field this week, but by pure coincidence (seriously!), I discovered this great blog post that Brian Auer wrote a few days ago:
I’d highly recommend checking it out. Brian did a nice job of explaining the three things that affect depth of field (aperture, camera to subject distance, and focal length), including some sample images too.
There’s a few things I’d like to add though:
If the f-number is doubled, then the depth of field is also doubled. Note that “doubling” here literally means doubling (e.g. from f/4 to f/8), it does NOT mean one whole stop (e.g. from f/4 to f/5.6).
If the subject distance is doubled, then the depth of field is increased by four times (depth of field is proportional to the subject distance squared).
If the focal length is doubled, then the depth of field is decreased by four times (depth of field is inversely proportional to the focal length squared).
So, if you’re looking for small depth of field (i.e. to help isolate your subject against an out-of-focus background), then the key is to use a small f-number, get close, and use a long focal length lens.
The only place I’ve seen these three things explained so precisely like this, is in Ansel Adams’ wonderful book, The Camera. It’s 195 pages of pure magical goodness, and it’s a book that I constantly reread and go back to. I just wish there was a pocket size edition too 🙂
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About the Author: Steve Berardi is a naturalist, photographer, computer scientist, and founder of PhotoNaturalist. You can usually find him hiking in the beautiful mountains and deserts of Southern California.