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  6. Setting Exposure: Shutter Speed and Aperture

Digital SLR Camera Basics

Setting Exposure: Shutter Speed and Aperture

Two of the factors that determine exposure—and thus the brightness of your photographs—are shutter speed and aperture (the other important factor in determining exposure is ISO sensitivity, but in the discussion that follows we will assume that ISO sensitivity is fixed).

Shutter speed is the time the shutter is open. The faster the speed, the shorter the time the shutter is open, and the shorter the time the image sensor is exposed to light. The shorter the time the image sensor is exposed to light, the darker the resulting photograph. On the other hand, the slower the shutter speed, the longer the time the image sensor is exposed to light, and the brighter the resulting photograph.

Aperture (expressed as an f-number) controls the brightness of the image that passes through the lens and falls on the image sensor. The higher the f-number, the darker the image projected on the image sensor, and the darker the resulting photograph. On the other hand, the lower the f-number, the brighter the image projected on the image sensor, and the brighter the resulting photograph.

Exposure is determined by the combination of shutter speed and aperture (f-number).

If you increase the f-number, you can still achieve optimal exposure by choosing a slower shutter speed. To put it another way, if you lower the f-number, you can still produce a photograph of the same brightness by choosing a faster shutter speed.

Shutter speed slowed in proportion to the increase in f-number

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  • The illustration is an artist's conception.

Shutter speed increased in proportion to the reduction in f-number

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  • The illustration is an artist's conception.
High f-number, slow shutter speed
High f-number, slow shutter speed
Low f-number, fast shutter speed
Low f-number, fast shutter speed

Different combinations of shutter speed and aperture used to achieve the same exposure.

Sample Camera Displays

camera information display
camera information display

Shutter speed:

Speeds faster than one second are shown as fractions (e.g.: …1/125, 1/160, 1/200, 1/250…). Some cameras may omit the numerator so that “1/125” becomes “125,” “1/250” becomes “250,” etc. Speeds slower than one second are shown by a double prime symbol following the value (e.g.: 1 ˝).

camera information display
camera information display

f-number:

f-number is shown in steps of 1/3 EV, for example f/4, f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8 etc.


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