ACD Systems Blog

Aperture, Speed & Sensitivity Part 3

2013-04-30 16:08:00 GMT

By ACDSee pro photographer & guest blogger Alexandra Pottier

We already talked about Aperture and Speed, it is now time to talk about Sensitivity, and close the chapter on the basics of a good exposure.


Sensitivity is the sensibility of the sensor to the light. It is the measurement of the sensitivity of the surfaces, a sensor in digital photography, a film in silver based photography.

Sensitivity is expressed in ISO, most of the times vary between 100 and 3200. Usual numbers are: 50, 100, 200, 400, 800, 1600, 3200.

A big number (ex: 3200), represents a high sensitivity where a smaller amount of light will be necessary to expose the picture correctly.

We can compare it to human skin. A high ISO (small number) can be compared to a light skin type (as of a blonde or redhead) which is very sensitive to the light and will burn if it is exposed to too much light. On the contrary, a low ISO (high number) corresponds to a dull or dark skin, which will take longer to burn or correctly expose in photography.

We can also compare the aperture to the clouds (the more clouds there are, the less light comes through) and the speed to the amount of time the skin is exposed to the sun.

As for the rest in photography, when you double the ISO number you will need half the amount of light to expose correctly. Therefore, you can double the speed or you can use a smaller aperture (everything is connected together!).

In traditional photography. a roll of film is set for one sensitivity only. On a digital camera you can change the ISO for each picture.

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The sensitivity has a big impact on the quality of the picture. When the ISO is set up high, the grain of the picture increases.

The grain in traditional photography is the bigger silver salts that appear on the prints. It is called noise in digital photography, where the pixels appear.

When you have very little amount of light, the first thing to do is to open up the aperture as much as possible or use the lowest speed before you increase the sensitivity. If you need speed, you’ll have to use a bigger aperture to compensate the loss of light.

Sometimes noise has nice effects. You can also add noise with your favorite software!

You’ll have to juggle between the three parameters speed, aperture, and sensitivity to expose your images correctly, compromise is what makes photography interesting.

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