Tech Tuesday: Aperture

Last week we did an overall view of the exposure triangle. This week we will look more in-depth at one leg of the triangle, aperture.

What is aperture? It this diameter of the lens opening, expressed in f stops, that controls the amount of light allowed in the lens. The smaller the f stop number, ie. f2.8, the larger the opening in the lens. Conversely the larger the number, f22, the smaller the opening.

You may ask why this should be of concern? Can’t I just use the largest opening, or let the camera decide what I want? You can if you don’t want control of your photos or if you just want snap shots.

The aperture sets how fast or slow the shutter speed is and how much of the photo is in focus. The smaller the f number, and larger the opening, the faster the shutter speed but the shallower the depth of field.

Example here I am shooting a yard stick from 3 feet from the near end, focusing at 14 inches. This is f3.5.

Click the photos to see them larger

Notice how fast the in focus part tapers off. Very little in front of and behind the 14. This was ISO 100 and the shutter speed was 1/400

Now we change to f22 and shoot the same scene.

This is also ISO 100 but now the shutter speed is down to 1/13. There is more apparent depth of focus.

If you were shooting a close up of one flower and wanted to focus just on it you would choose f3.5. If you were shooting a bed of flowers you would choose f22. See the correlation?

Another example is when I shoot portraits. Have you ever seen them, shot outdoors, where the background is smooth and out of focus? That uses a small f stop.

Here I focus on a tree about 20 feet away. f3.5 1/400

And f 22 1/10See the difference in the apparent depth of focus. The first one draws more attention to the main tree. The second shows more overall view of the trees.

Here are two taken even farther away

f3.5 1/640f 22 1/15If you are taken a broad vista for a scenic then select a larger f stop for maximum DOF. If you are doing a portrait or want to draw attention to one specific spot then use a smaller f number.

I said before and you can see that the aperture affects the shutter speed.  If you need to stop action, such as football, or in dim light, like a gym, you need a small f stop/large opening to keep the shutter speed up. This is why you will see photographers on the sidelines with huge lenses. These are 300 or 400 mm f2.8 or 500-600 f4 lenses. They shoot maximum opening and high ISO to get a fast shutter speed.

You will see nature and landscape photographers with wide-angle lenses and shooting f16-32 for maximum DOF.

Portrait photographers that shoot outside will use fast lenses of f1.4-4 and shoot close to throw the background out of focus.

Next week we will look at shutter speed.

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