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Old 10-28-2007, 04:07 PM
rushfan2112 rushfan2112 is offline
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Default Depth of Focus

Depth of focus is, as far as I can determine, the distance between the nearest foreground point where the image is as sharp as it can be to the furthest point in the distance where the image is similarly sharp - with "infinity" (denoted by the sideways figure of 8 symbol) being the optimum point in the distance.

The DoF "effect" is governed by the focal length of the lens, the aperture and the ability to keep the camera still (whether that's hand-held, tripod, mirror lock-up......whatever).

On old, manual lenses there used to be lines that gave a good guide as to where the lens would start to be critically sharp (hyperfocal distance?) but, for the most part, these have all but disappeared. If in doubt, I used to use the next narrowest aperture to be 100% safe. But why?

As much as I understand the workings of the above, I wouldn't know how to calculate it. I presume that there must be a mathematical formula to calculate it and would be glad if someone could provide an easy guide to the calculation for a few reasons:

(1) If, given the focal length of the lens, critical focus can be achieved from your toes to infinity at f8 (entirely possible with a wide lens) there's no point whatsoever using f22 or f32 just for the hell of it. Indeed there may disadvantages as you might be forcing yourself to use a tripod for no good reason and/or blur a shot that might have been crisp at a wider aperture and shutter speed combination (Exposure Value / EV).

(2) Although a depth of field / DOF preview button on a camera is useful, it's only a guide and can't be relied on in low light.

(3) The performance of some modern lenses actually deteriorates at very narrow apertures and, if you can get away with f8 or f11, that might give a better result than f16 or f22 for that particular lens.

Does anyone have a more enlightened or technically accurate view on this as I'm going to start experimenting with DOF over the autumn / winter and see what effects I can get.

Thanks.
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Old 10-29-2007, 03:10 PM
nicoz nicoz is offline
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Default Re: Depth of Focus

Hi!
I suppose that I could derive some equations for you but that would be a bit boring... ;-)

I'd say that keeping the camera still won't change your DOF much. However the distance between you and your subject will!

You can check this link which calculates everything for you: http://www.dofmaster.com/dofjs.html

Or a hanfull of other websites found by google: http://www.google.co.uk/search?hl=en&q=dof+calculator&meta=

Hope that helps!
Nicolas
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