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Old Jun 29, 2006, 10:58 AM   #1
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If I set my camera on manual focus and set the lens to 18 mm (using an 18-55), aperture to say f/22 and adjust shutter speed accordingly, where do I aim the autofocus points to acheive great depth of field?

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Old Jun 29, 2006, 11:30 AM   #2
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You don't want to use f/22 unless you need it for closeups using a tripod.

Using Aperture extremes like f/22 at 18mm with most lenses is going to result in softer photos due to diffraction. Because aperture as expressed as f/stop is a ratio between the focal length of the lens and the area of the aperture iris diameter, you're looking at a very tiny hole at 18mm and f/22.

Using smaller apertures like f/22 will also require slower shutter speeds for any given lighting and ISO speed, which can result in blurry photos from camera shake or subject movement in less than optimum lighting.

Also, most lenses are sharpest about 2 or 3 stops down from wide open (not at their extreme widest or smallest openings).

I'd try to stick with around f/8 if you're on the 18mm end of the lens for most purposes outdoors.

Your hyperfocal distance at 18mm and f/8 is only a little over 7 feet (allowing everything from about 3 1/2 feet to infinity to be acceptably sharp focused there) with a Canon model using an APS-C size sensor.

Plug in your camera model, focal length, aperture and focus distance into this Depth of Field Calculator to get a better idea of how it works:


A good "rule of thumb" shooting with a wider lens (like your kit lens set to 18mm) is to focus about 1/3 into the scene you want to capture for good Depth of Field.

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