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Old Oct 18, 2006, 6:48 PM   #26
JimC
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Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
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Experiment. ;-)

The larger your aperture (smaller f/stop numbers), the shallower your depth of field.

Ditto for closer focusing (the closer you are, the shallower your depth of field), and focal length (the longer the focal length, the shallower the depth of field).

But, keep in mind that if you use a focal length that's twice as long, you'll need to be twice as far away from your subject to achieve the same framing.

So, that cancels out the DOF difference at most focus distances you'll shoot at (making true depth of field identical if your subject occupies the same percentage of the frame and you're shooting at the same aperture, regardless of focal length at most shooting distances you'd be concerned with). As you get closer to infinity this can change.

However, perspective changes with focus distance. Shooting from closer distances makes foreground and background elements seem more separated. Shooting from further distances makes them appear more compressed and closer together.

So, even though DOF is technically the same at most shooting distances you'll be concerned about it with if you have the same framing and aperture, despite focal length differences (since you'd need to shoot from further away if you increased focal length for your subject to occupy the same percentage of the frame), shooting from further with a longer focal length gives an illusion of a shallower depth of field, just because of perspective, making the out of focus areas more obvious when elements in the scene are more compressed.

Using wider lenses and shooting from closer distances can also cause some perspective distortion. For example, an outstretched arm closer to the camera may seem too large in proportion to the rest of a subject's body. You can use perspective creatively by shooting at different distances to your subject with different focal lengths, outside of depth of field considerations, for more or less background compression, increasing or decreasing the apparent separation of foreground and background elements in the photo.

You're shooting digital. Experiment and see what the difference is.

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