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How to Know What F-Stop to Use

Like a little mumbo jumbo for lunch? Actually, f-stops and focal lengths aren't all that bewildering once you know what the numbers stand for. See more camera stuff pictures.

Photography deals with capturing light in a way that appeals to your artistic sensibilities, whether you prefer perfectly-lit, tack-sharp portraits worthy of presidential candidates or blurrier, artistic renderings of NASCAR competitors roaring by at 180 mph (290 kph). To consistently create the kinds of pictures that will make your creative spirit soar, you need a firm grasp of common camera settings such as shutter speed, ISO setting and f-stop (or focal stop).

F-stops in particular have a tremendous effect on image characteristics, some of which may not be obvious to amateur shooters. For the bulk of this article, we'll discuss and improve your grasp of the mysterious f-stop.

Shutter speed and ISO settings are also important concepts that will help you flesh out your understanding of how cameras work. For more information on these subjects, check out our articles "What is ISO speed?" and "10 Important Photography Terms."

When you take pictures, you're using the interplay between shutter speed, ISO and f-stop settings to control exposure. F-stops especially seem to baffle many novice shooters, perhaps because of the weird alphanumeric symbols used to indicate these settings. Happily, figuring out f-stops is really pretty simple once you disassemble the term and break it up into understandable parts.

For starters, your camera has a mechanical aperture that controls how much light enters the camera. This aperture can change in size, and it works a lot like the pupil in your eye. In general, the brighter the scene, the more the pupil constricts; in low light, the pupil is larger, letting in as much light as possible. The same goes for your camera's aperture in most situations.

The f-stop number is determined by the focal length

of the lens divided by the diameter of the aperture. Focal length refers to a lens' field of view (sometimes called angle of view), which is the width and height of the area that a particular lens can capture. Focal length is often printed right on the camera lens.

A lens with a 100mm focal length set to an f-stop of f/10 has an aperture diameter of 10mm. Keep in mind that doubling the f-stop number halves the size of the aperture opening. So, moving from f/10 to f/20 decreases the size of the aperture from 10mm to 5mm. If you use an SLR (single-lens reflex) camera that lets you affix a range of lenses, the f-stop numbers will be different for each lens.

All technical details aside, you won't need to spend a lot of time sweating the finer points of f-stops. What you really need to know is how to adjust the aperture diameter to your advantage as you shoot. Keep reading to find out how aperture control affects the way your pictures look.

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