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Aperture, Speed & Sensitivity (Part 2)

2013-04-10 17:20:00 GMT

By ACDSee Guest Blogger & Professional Photographer Alexandra Pottier

After aperture, one of the three facts that make a good exposition in a picture is the shutter speed.

It is the time while the sensor is exposed to the light while the curtain is open. If we think of it as a window, it is the time while the window is open.

Usually, we express the shutter’s speed in seconds or fractions of a second.

A long exposure time, 1 sec for instance, exposes the sensor for a longer period of time. That is useful when there isn’t much light in the scene.

On the contrary, a short exposure time, 1/1000 sec, exposes the sensor very shortly to the light when there is a lot of it.

image

Usual shutter speeds are : 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, ¼, ½, 1 second, up to 30 seconds. Then you have the Bulb mode, where you can decide for yourself the length of the time of aperture.

As you can see the numbers are equally proportional. When you double the exposure time (from 1/250 to 1/125 ) you let twice as much light in.

Of course the speed has consequences on the final result of the picture. The use of a fast shutter speed (1/1000s sec) will freeze the action even if the subject is moving.

image

The use of a slow speed (1 sec) will show an amount of panning.

image

The shutter speed must be chosen according to four criteria which are :

The effect you want for your picture. Frozen action or fuzzy yarn.
The movement’s speed. The photographer’s stability is important, below 1/60s, it is better to use a tripod, because the human, even very still, suffers from micro-movements, and the result is a fuzzy picture.
The subject’s speed. The more the subject is moving, the more fuzz there will be. And vice versa.
The focal length. It is common to say that it is better to use a focal length ratio 1/focal length. For example, if you are using a 200mm lens, it is better not to go under 1/200 sec.

If you want to keep the same exposition while changing the shutter speed, you’ll have to change the aperture increasely.

To change the shutter speed, there are two options : use the manual mode or the S mode. This way, you get to choose the speed you want for your camera.
To practice, you can start with the focal length rule (1/focal length) then you can try on different moving subjects, a walking person, a cyclist, a jogger, etc…

Have fun!

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