 Frank P. Luongo |
Selective Focus I need to shoot a good selective focus shot with blurred foreground and background and central object in focus. I use the widest aperture on my lens (28-80mm f/3.5-5.6 zoom) at 80mm. I tried some floral shots - not yet developed. Do I need a more powerful telephoto lens, such as 200mm to give me even more shallow DOF. At 80mm, how far or close should my subject be from my lens. Thanks.
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Jon Close |
Obviously, longer focal length and wider maximum aperture lenses will give shallower depth of field. Without spending more money, use the lens you have and get closer. At 80mm and f/5.6, if your focused subject is 10 ft away, then depth of field will be from approx. 9.4-10.7 ft. If only 5 ft. away, DoF is reduced to from 4.8-5.2 ft. If focused to 18", DoF is just from 17.85" to 18.15".
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Frank P. Luongo |
Thanks very much John! I appreciate your quick response. Frank
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Rhonda |
John - in the old system they had this information on the lenses. Is there some sort of formulae to this rather than studying some complicated table. I shoot digital and have a whole range of different lenses. From your answer above it seems that the DOF is approximately the distance to the subject (therefore I guess this is why you put the subject 1/3 or so into the image)??? And that is at 5.6 what is it for say 1.4 and 2.8....More info please.
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Jon Close |
Some of the designs of autofocus lenses, especially zooms, makes it impractical to include depth of field scales on the lens. You can download a calculator such as fCalc from http://www.tangentsoft.net/, but it's not very convenient when actually shooting. In it's Help area are the mathematical formulae used in the calculations. Another free download I've found and used safely, DOFMaster is a program for printing customized depth of field scales for your lenses that can be carried in your pocket or attached to the lens cap.
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Steven Chaitoff |
Rhonda, you really can't calculate DOF right off the back while shooting, but you don't need to either. Even the scales written on some lenses tend to be inaccurate. If you have a depth of field preview, that is a tremendous help as the camera stops down to your selected aperture so you can see roughly what's sharp & what's not. It's hardly a mathematical formula but it gets the job done. In Frank's case, he can just shoot as max. aperture to achieve as much DOF as he can in both directions (as long as the subject itself doesn't have too much depth.) This way he can see the effects of the wide aperture straight through the viewfinder since it is already open anyway, Oh just a side note...you will have more depth of field behind the selected focus than in front of it, (because the farther away a point moves, the smaller the rate the angle changes between its reflected light rays.) Anyway, I don't know why Jon's calculations don't seem to show that (except the first) but I've heard that the back/front focus is in a ratio of 2:1. So, if you focus on something say 20 feet away & 1 foot in front is in focus, 2 feet behind are in focus too, so DoF is 3 feet. But I don't know that validity of this whole 1:2 thing...
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Jon Close |
Hi Steve, The range of apparent focus in front and behind the focused subject varies. At close focus it is nearly equal. At farther focus distances it tends towards 1:2 ratio, but it is not constant. Focused to the hyperfocal distance, the apparent focus will be from about 2/3's the focused distance to infinity.
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Jon Close |
D'oh. the last sentence should be "... from about 1/2 the focused distance to infinity."
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Steven Chaitoff |
Oh Ok
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