Monday, July 2, 2012

Depth of Field Defined

Wide depth of field

I get (mildly) irritated by lots of things...grey skies when I need blue ones for a travel shoot; stale milk when I'm craving a bowl of cornflakes...the way the England soccer team take (or don't take) penalties; a power cut at dinner cooking time in my all-electric house etc, etc.

 There is one thing that seems to be irking me more than usual though, lately. It's when photographers say "great depth of field" when looking at an image that has limited focus in it. Because, that's not "depth of field" at all. It's "shallow depth of field".

shallow depth of field
There is depth of field in every photograph that you take. Sometimes there's a lot of it and sometimes there's a little. Probably the simplest way of thinking about it, is as "the depth of the field (field as in "area") of sharpness". A landscape that is sharply focused from the farmers gate in the foreground of the scene, taking in the cottage in the centre-ground and all the way back to the hills in the background has a wide depth of field. A portait that has only the person's face correctly focused but the trees or wall in the background toally blurred out has shallow depth of field. But they both have depth of field. 

My definition is...

Depth of field is the area in front of and behind the subject that is also sharp when the subject is in focus".  

Depth of field roughly extends 1/3rd in font of and 2/3rd behind the focused point. How far it extends depends on the aperture chosen - wide apertures (low f-stop numbers) give a shallow DoF and small apertures (high f-stop numbers) give a broad DoF - and also on the distance of the camera from the subject (DoF falls off quickly the closer you take a lens to the subject) and the focal length of the lenses (wide angle lenses have broader DoF than telephoto lenses at the same aperture).

When photographers talk about the background being blurred in an image, or objects being more in focus than others, as "depth of field" (as in; "I wanted depth of field in this image so I blurred the background"), this is not technically correct - and it is more accurately described as "differential focus" or "shallow depth of field".

OK, I'm much calmer now that's off my chest. Please; let no one pronounce I.S.O. as "eye-so" (it's an abbreviation - International Standards Organisation- not a word) and I'll have a perfectly relaxed day!

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