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Old Dec 7, 2004, 4:40 PM   #11
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378

cable wrote:
nhl and perdendosi: thanks for the information!

ya, that's what i was curious about. how the camera can select a larger f/number than the lense allows.
A lens is rated by the largest available aperture at wide angle, and the largest available aperture at full zoom. For some higher quality lenses, you'll only see one number, since the lens is capable of maintaining a constant aperture throughout it's focal range.

The larger the f/stop number, the smaller the aperture.

Aperture is a ratio, and is determined by dividing the focal length of the lens by the size of the iris opening.

So, if you had a 50mm lens with an 18mm iris opening, then it's aperture would be f/2.8 (50/18~= 2.8 ). Yet, if the physical size of the iris opening (18mm in this example) did not change, and the lens was zoomedin to 100mm, then your aperture would become approximately f/5.6 (100/18=5.6).

As a general rule, metered aperture (amount of light reaching the film or sensor) and physical aperture ratio are roughly the same. That is, less light reaches the sensor through the lens when longer focal lengths are used (for a given physical size of the iris opening).

The aperture scale (in one stop increments) goes f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, etc. With each one stop move to a smaller aperture (represented by larger f/stop numbers), you will need shutter speeds twice as long for proper exposure.

Here is achart you can use to get anidea of the shutter speeds required for any EV and Aperture (but make sure to use your camera's metering, as lighting can vary -- this is only to give you an idea of how it works). It's based on ISO 100. So, each time double the ISO speed, you can use shutter speeds twice as fast:


Since a brighterlens (i.e., f/2.8 or larger apertures available) allows faster shutter speeds, it'scalled a fast lens. A lens that stops down tof/5.6 is a slower lens (because slower shutter speeds are needed at the smaller available aperture). So, a zoom lens that can maintain f/2.8 throughout it's focal range is popular for low light use requiring faster shutter speeds.

Again, just because you buy a brighter lens with a larger available aperture (i.e. f/2.8, f/2.0, f/1.8, f/1.4, etc.), doesn't mean you can't set smaller apertures. Most lenses allow you to stop down to f/22 or smaller apertures (f/32 is common and you'lleven see some macro lenses with f/45 or smaller apertures availablenow).

So, a brighter lens gives you more flexibilty to go with larger or smaller apertures for depth of field purposes, and gives you the ability to use larger aperture settings for faster shutter speeds to help prevent motion blur in low light.

For example,a 50mm f/1.8 is popular forindoor exiting light use, since even a bright (i.e., f/2.8 throught it's focal range) zoom lenswould require shutter speedsmore than twice as long as f/1.8 for proper exposure for any given lighting condition and ISO speed. Remember, for each one stop move to a smaller aperture (for example f/1.4 to f/2.0 which is a one stop move to a smaller aperture, or f/2.0 to f/2.8 which is a one stop move,you need shutter speeds twice as long for proper exposure for the same lighting and ISO speed.

But, you may want to use to smaller apertures for more depth of field in other conditions.BTW, as a general rule, you'll usually get sharper shotsif you're not shooting with the aperture wide open orstoppeddown too much either unless you need to, since a lens is usally a little softer at both ends of the aperture scale.

Here is an online depth of field calculator. Select your camera,then change the focal length (for example, 28mm), aperture and focus distance to different values, and you'll see how these parameters work together for depth of field purposes.


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