The ultimate depth-of-field primer (free/shareware book download)

jfriend

If you really want to understand depth of field and lens resolving power, this online book (free/shareware PDF download, 87 pages) http://www.trenholm.org/hmmerk/download.html is an amazingly educational read. I thought I understood it pretty well before and, after reading this, I realize I hardly knew anything.

Warning, the book is quite technical and includes all the formulas and calculations that lead to his conclusions. Non-technical folks can still get something out of the conclusions the author draws, but you may have to skip over the technical parts.

To encourage you to check out at least the rules of thumb in the book on page 71, here are a few of them paraphrased:

1. Depth of field is ONLY a function of magnification and aperture. It is NOT a function of focal length. So, if I take a picture of the same subject at the same size on the sensor with an 18mm lens and a 200mm lens at the same aperture for both, the two images will have the same depth of field. I will obviously be a ton closer to the subject with the 18mm lens and the two images will have a different perspective (the background will be larger on the 200mm shot), but both will have the same depth of field.
2. If you want something at infinity to be sharp, focus at infinity, not at the hyperfocal distance. It makes a significant difference.
3. The zone of acceptable sharpness falls equally in front of and behind the point of exact focus (not 1/3 in front and 2/3 behind as a popular myth says).
4. Stopping a lens down 1-stop gives you 40% more depth of field. Opening a lens 1-stop gives you 30% less depth of field.
5. Depth of field scales linearly with distance (for the same lens). So a lens focused at 20 feet has twice the depth of field as the same lens focused at 10 feet.
6. Diffraction places the ultimate upper limit on the resolution that a lens can produce. Practical experience suggests that apertures in the range of f/8 to f/11 produce the most depth of field without losing significant sharpness due to the effects of diffraction.

And, there's lots more in the book. I highly recommend reading it.