BetterPhoto Q&A
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f-stops in digital cameras and depth of field

On page 46, Mr peterson discusses how digital fixed lens cameras are "blessed" with infinite depth of field. I understand what he's talking about until I put my Nikon 8700 into macro mode.

I've gotten shots where the DOF is SO shallow that a lot of the shot doesn't make any sense. Quite the opposite of "infinite DOF", where Mr peterson has described my f2.8 as equivalent to f11 on SLR.

OK, I'm easily confused. In this case the physical results aren't bearing out Mr Peterson's book, and if I don't know anything else, I know that he knows a lot more about taking pictures than I ever will.

So my question is this. When I "zoom to focus" I get the described DOF Mr Peterson refers to. However, when I move to manual focus and start dialing in, my DOF disappears. Do these "dual purpose" lens systems we have on our digicams revert to something more like SLR when we shift from zoom (the mountain) to macro (the flower)? I can't find anything in my manual about it.

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June 09, 2005

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June 09, 2005

Thx for your response.

My problem isn't isolated focus; I can get that all day long. My problem is getting enough DOF to make the shot make sense. My DOF ends up too shallow, and some shots are suffering from inadequate DOF, i.e. f-stop is too low -

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June 09, 2005

Bob

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June 09, 2005

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June 09, 2005

My camera has an 8.9-71.2 zoom lens. According to Mr Peterson, an f8 stop equates to f32 on an SLR. The f8.9 equates to f11. That's a fairly deep D
OF to start with, and f32 is a hard one to get to on SLR.

My question has to do with the f-equivalent on the digital lens in macro mode. f2.8 is a hard stop to get to - nearly impossible with a filter because of the need for a tube to carry the filter. Likewise, I can raqrely stop down beyond f7.8 when I zoom, which ought to be somewhere near f28 SLR - lots of DOF - but I'm still getting a shallow DOF, due to the focal length of the digital camera - 280mm at 71.2.

Gregory,

I agree with your comment. Is there a way to quantify the result when shifting to macro?

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June 09, 2005

Depth of field is a phenomenon of human vision, its acuity, and ability of a human to "detect" if something is out of focus. The "apparent" depth of field in a photograph depends on the following:
(a) Visual acuity of the person viewing the photograph.
(b) Distance at which the person is viewing the photograph.
(c) Dimensions of the printed or projected photograph.
(d) Dimensions of the medium on which the photograph was recorded (the film or CMOS/CCD sensor).
(e) Lens focal length (actual focal length, not an "equivalent" one)
(f) Lens aperture setting
(g) Lens focus distance setting (aka critical focus distance)

Change any of these and the apparent depth of field changes. If all else is kept the same, apparent depth of field shrinks as focus distance is decreased. It can be extremely shallow in macros, the magnification of which is expressed as a ratio of the subject size at the lens' critical focus distance as measured on the film or CCD/CMOS sensor and actual subject size. A 1:2 macro is 1/2 life-size on the recording medium. A 1:1 macro is life-size on the recording medium. Macros with 1:4 or greater magnification can easily have an apparent depth of field that's a small fraction of an inch. It's the reason nearly all *true* macro lenses that can focus close enough for these magnifications have much narrower f-stop settings than would be found on a non-macro lens of the same focal length. It allows the photographer to give up some sharpness resulting from aperture diffraction in return for a greater depth of field. I've done high magnification macros with f/16 and f/22 lens apertures in which the apparent DOF is clearly less than a half inch.

-- John Lind

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June 09, 2005

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June 10, 2005