Why is depth of field affected by focal length?

Question

As the focal length decreases, the depth of field increases as well. Why is this? I'm not so much interested in a physics lesson as I am interested in a simple, down-to-earth explanation.

Answer

Pretty sure I answered this one before but I cannot find it.

• As focal-length gets longer, the angle of view gets smaller.
• With a smaller angle of view, rays forming the image are closer to being parallel.
• With less variation of angle between rays, light travels more before being sufficiently out of focus.

This is a little oversimplified but I hope it is easy to visualize at least.

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