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#11 | |
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Join Date: May 2005
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Tom LaPrise wrote:
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#12 |
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Join Date: Oct 2004
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I hadn't thought of that. I should have said, "For a given distance..." Really close-up/macro shots, even at f/16 on my camera, can have less than 1/8" DOF, and I have a shot of a butterfly taken from about 4 feet away with decent blurring of the background just inches behind. (I don't have a copy with the EXIF data with me right now, so I can't tell what the aperture was.)
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#13 |
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Join Date: Aug 2004
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DOF is controlled byseveral factors:
sensor size, focal length, aperture and distance to subject. They all play a part in the equation. Also, while DOF is controlled by those factors, background blur can also be aided by distance between your subject and the background. So, while DOF may be 2 feet - how blurred the background looks is also dependent upon how close that background is to your subject. But, back to DOF. Here's a handy on-line calculator that allows you to compute DOF: http://www.dofmaster.com/dofjs.html Want to see how they all play a part, simply change some of the parameters. When doing macro work, distance plays a huge part. But if you're doing other types of work - say sports, then focal length, sensor size and aperture becaome greater factors because you're not as close (i.e. going from 6 inches to 24 inches makes a huge difference but going from 30 feet to 35 feet does not). Here, also, is a good write-up on depth of field and the circle of confusion: http://www.luminous-landscape.com/tu...ries/dof.shtml |
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#14 | |
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Join Date: Oct 2006
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Tom LaPrise wrote:
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FOr example if I understood right it sould be the following: I have Canon S3IS; Lens: 6.0-72mm f/2.7-f/3.5 So in shorther focal lenght and f/2.7 I'll have diameter = f/2.7 = 6/2.7 =2.22 But on largest focal lenght 72mm i have f/3.5, so diameter = f/3.5 = 72/3.5 = 20.2 I'd like to if the diamater can be so opend 20, why we have only 2.22 in short focal lenght? If the formula is so simple then with x12 zoom the AV should be 12 times bigger in shortetst FL. What about the lens that keeps AV fro whole FL range? Please explain me what I have missed? |
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#15 |
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Join Date: Dec 2006
Location: Davenport, IA
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I suspect that if we measured the diameter of the front element of the example f2.8-3.5, 6mm-72mm lens it would be 20mm in diameter and that is the ultimate physical limit for the "speed" of this lens. While the diameter of the front element would theoreticaly permit an f0.3 lens at the 6mm setting it's not a physically realizable piece of glass. That is we can't practically produce the curvatures required to produce a sharp image across the entire picture area for all colors at 6mm and still have it work at 72mm. The fastest general purpose 35mm film lens ever produced was the f0.95 50mm that Canon produced for both their rangefinder and SLR 35s. In our example the iris is located at a point in the optical path that produces a near constant aperature (optical diameter) regardless of the focal length.
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