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Shutter Speed, Aperture and ISO

Digital cameras are based on a sensor that reacts to the light reflecting off the subject of the photo you are taking. A certain amount of light is required in order to get a good shot, and that optimum amount of light is basically the same in any situation.

The three things we have to play with to adjust the amount of light are shutter speed, aperture and ISO.

Shutter speed and aperture are closely related. If you increase the aperture (lower F-Stop number) more light gets to the sensor, so the shutter speed has to be increased to adjust the amount of light. Conversely, if you choose a small aperture (bit F-Stop number) then the shutter will have to stay open longer to let in more light. Think of filling a bathtub. If you open the tap all the way (aperture), the tub will fill in less time, and you will turn the water off sooner (shutter speed).

In order to get a good exposure, you need the right combination os aperture and shutter speed. Most cameras have settings where you can adjust one and the camera will automatically adjust the other to compensate and give you a great shot every time. If you have a more professional camera, you may be able to override this feature and choose both manually, and alas, some point and shoot models give you almost no freedom. I have one model that has no aperture settings at all.

In terms of exposure, a narrow aperture/slow shutter speed will give you the same result as a wide aperture/fast shutter speed combination, but varying the aperture also affects the depth of field (DoF) or how much of your subjects background will be in focus. If you are trying to achive a blurry background like in our cherry blossom shot above, you will want a narrow depth of field which can be achieved by choosing a wide aperture setting (low F-stop number), or if you can’t control that, and fast shutter speed, which will force the camera to change the settings to increase aperture.

ISO used to indicate the sensitivity of the film you were using. A high ISO film was more sensitive, and needed less light. Now that most of us are using digital cameras, the ISO setting is actually just a simulation. Increasing the ISO number will cause the camera to have a higher shutter speed which may result in a small depth of field, but use it in moderation, as you may see more digital noise with a highter ISO (that awful ‘grainy’ effect)

Once again, if you want to achive that ellusive blurry background:

Use the widest aperture (lowest F-stop) if you can control it. If not try a high shutter speed or high ISO setting

Have the subject as far away from the background as possible.

Try to get the subject as close as you can. If possible use the macro setting on your camera and take the subject just inches from the lens.

Thanks to Elizabeth Laurel for the great shot of last spring’s cherry blossoms!

Tags: aperture, depth of field, f stop setting, ISO, shutter speed

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