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09-13-2009, 11:12 AM   #1
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DOF and F-stop question.

Hey guys just hoping to get some answers on a few questions ive been having. It mainly has to do with one thing. If you have a lens that has a maximum app of 2.8 and you compare the DOF to a camera with a max of 4 at a constant f-stop (so both at 5.6 or something), are they the same? as in the DOF? Because all these long zoom sports and wildlife lenses that get down to 2.8 all seem to be the bees knees. Is this because you can shoot a higher shutter speed at 2.8? but with a horrid loss of dof is it really that great a benefit? I guess getting it and not is always worth it to some. Thanks,

09-13-2009, 11:25 AM   #2
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For lens with the same focal length, shooting at the same aperture always yields the same results in terms of DOF and shutter speed. So whether the maximum aperture of the lens is f/2.8, f/4. or f/5.6, shooting at f/5.6 will yield the same results in those respects. Quality will most likely be best from the f/2.8 lens and worst with the f/5.6 lens, because most lenses are at their best stopped down a bit, and because if they're going to go to the trouble and expense of making an f/2.8 zoom lens, they usually try to use the best optics they can.

So the advantages of an f/2.8 are that the image quality is usually better, plus you have f/2.8 available if you need it. And if you do need it, you put up with the reduced DOF. "Horrid" is in the eyes of the beholder. Sometimes you *want* the reduced DOF, as it helps isolate the subject from the background.
09-13-2009, 11:25 AM   #3
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There are a lot of variables that go into DOF. Background distance from the subject is one of the biggest ones, the farther the background is from your subject, the more out of focus it will be ( depending on how close you are to the subject when you take the shot ) F stop makes a big difference too. Wider angles lenses have more DOF than longer lenses generally.
09-13-2009, 11:47 AM   #4
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Great thanks guys, that was what i was expecting to hear. Cheers!

09-13-2009, 11:51 AM   #5
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For a particular camera, DOF depends on f-stop; Increasing f-stop increases DOF.

For particular camera and f-stop, DOF depends on how wide the scene is, increasing scene width increases DOF.

A valid equation for scene depth as a percent of scene width is:

%DOF = Camera_constant*F-Stop*Scene_Width

This simple equation includes the effect of focal length because one way to increase scene width is to decrease focal length.

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