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Archive for Sunday, February 7, 2010

Behind the Lens: Three functions control camera exposure

This aperture diagram shows the relationship between the f-stops, designated by numbers 2, 2.8, 4, etc., and the size of the corresponding aperture. In any sequence of f-stops, you are either doubling or halving the amount of light with each selection. If you keep in mind that the numbers represent fractions, it's easier to understand how an aperture setting of f2 or 1/2 allows in more light than f16 or 1/16. For the proper exposure of a scene you need the correct combination of shutter speed, ISO film speed and aperture.

This aperture diagram shows the relationship between the f-stops, designated by numbers 2, 2.8, 4, etc., and the size of the corresponding aperture. In any sequence of f-stops, you are either doubling or halving the amount of light with each selection. If you keep in mind that the numbers represent fractions, it's easier to understand how an aperture setting of f2 or 1/2 allows in more light than f16 or 1/16. For the proper exposure of a scene you need the correct combination of shutter speed, ISO film speed and aperture.

February 7, 2010

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For the next few columns I'm going to cover some camera basics geared for beginners or those not always familiar with the settings on their cameras. Exposure is controlled by three functions.

  1. Shutter speed, which determines the length of time you allow light into a camera.
  2. Aperture, which determines the size of the opening through which light enters the camera.
  3. ISO, which corresponds to the sensitivity of a digital cameras image sensor to light.

The aperture is probably the least understood, so I'll attempt to explain it this week and review the others later. All camera lenses have apertures, which are different-sized holes that allow light into the camera. They work like pupils, regulating the amount of light entering your eyes. Apertures are described by the term, f-stops and identified by numbers. Look at your own camera and lens. Printed around the front is a description of the focal length and the maximum aperture (largest hole) of that lens. The lens on my camera reads 24-105mm 1:4. The first two numbers refer to the focal range of the lens. It is a zoom lens with an angle of view from wide-angle (24mm) to short telephoto (105mm) focal length. The last number, 1:4 is the maximum aperture, or f-stop, of that lens and can be read as 1:4, 1/4 or f4. It's all the same and it means that at maximum aperture, the size of the hole letting in light is 1/4 the diameter of the length of the lens. You don't have to understand the math, but you do need to understand the relationship between aperture choices and your other exposure controls - shutter speeds and ISO. Just like you have choices of shutter speeds on your camera, you have aperture choices. From largest to smallest, the aperture range on my lens is f4, f5.6, f8, f11, f16 and f22. In this sequence, each f-stop represents a halving of the amount of light entering the camera.

I know from experience, that to get a proper exposure at a Free State basketball game, my setting has to be 1/250th of a second shutter speed - the minimum to stop sports action, f4.0 aperture - to let in the most light and an ISO setting of 1600. Anybody that walks into the same gym would have to set their camera to the same setting to get a proper exposure. The problem is that everyone has different cameras. Maybe your lens reads 24-105 1:5.6. If so, you have only half as much light coming into your camera as I do. Unless you change your ISO or shutter speed to compensate you will end up with an underexposed photo. If I'm covering the Firebird game with your lens I would be forced to choose an exposure of 1/250th at f5.6 at 3200 ISO. I have to double the ISO to compensate for losing one f/stop in aperture. I'll continue with more on apertures and exposure next week.

Comments

Donnuts 4 years, 2 months ago

This is a great topic for articles. Looking forward to seeing more. You posted today?

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Kontum1972 4 years, 2 months ago

This is a true art, I am an old school photographer, i do have one digital and it is a pocket size that i carry with me all the time, my other cams are old school, classics,... real film.

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