Gadgets Manual

Using the Aperture and Shutter Speed Together for Great Pictures While the camera’s aperture is concerned with how much light gets to the image sensor, the shutter speed is concerned with how long the light shines on the sensor. In figure 4, we see a test image with three exposures of the same subject. The first exposure is underexposed (too dark), the second exposure is just right, and the third exposure is overexposed (too light).
You must understand how to control both depth of field (aperture) and subject movement (shutter speed) in your pictures. Which is most important? That varies with the type of subject you are shooting.
Let’s say you are taking a picture of a bird flying through the air (see figure 3C). Of course, you want the bird to have a good focus covering its eye, body, and both wings, if possible. You need enough depth of field to be able to focus on the bird’s eye, and still get the rest of him in the zone of sharp focus. You could set a small aperture, which you know will give you a nice deep depth of field so that all the bird is in focus. However, when you use a small aperture for deep depth of field, the opening the aperture blades form is rather small, and not much light gets into the camera. So, to compensate you’ll need to let the light come in for a longer time period, which means using a slower shutter speed. Unfortunately, a slow shutter speed means the bird will blur as it flies through the air. You’ve got yourself a real quandary!
How can you balance your need for stopping action (a fast shutter speed), with your need to keep all parts of your subject in focus (a small aperture)? Great question! Let’s find an answer.
Now, let’s say you are taking a picture of a beautiful scenic view of the mountains. You need deep depth of field to cover the distance from up close, to out past the mountain ridges, which requires using a small aperture. You also want to keep the wind from blowing the tree limbs and blurring them, so you want a faster shutter speed.
This quandary is what makes photography a learned skill. We are always balancing our need for depth of field, with our need to stop motion. When there’s a lot of light, like on a bright sunny day, there may be enough light to have a small aperture and a fast shutter speed, which gives us deep depth of field and stop motion capability. However, if there is less light on our subject, we can’t have both.
We can artificially make our cameras more sensitive to light by raising the ISO sensitivity of the sensor. In the old days we called it “film speed.” That can be helpful, but when we take our camera much above the native ISO sensitivity (200), we start introducing digital artifacts called noise into our picture. We had the same results with very high speed films, and we called it “grain.” High ISO sensitivity can create so much noise that our image is no longer very nice. So, higher ISO sensitivity is only used in an emergency when the shot must be acquired at all costs.

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