The Aperture

Jan 14 2010

the aperture concept

the aperture concept

Every still camera has essentially two control mechanisms that allow to limit the amount of light hitting your sensor. One of these controls is the shutter in front of the sensor. The other one is the aperture, usually a set of blades in your lens, similar to the iris in your eye. When there is little light, your eyes (the aperture) are wide open. The opposite is true when it is very bright. You are squinting, trying to close your eyes to reduce the amount of incoming light.

How do you adjust your aperture? If you are lucky your (old-fashioned) lens has an aperture ring, next to the manual focus ring, with a bunch of weird-looking numbers on it, that read like 2.8 – 4 -5.6 -8 -11 – 16. This is no math quiz, although it looks like one. I am not going into the theory behind these numbers (the f-stop), rather have you remember two things:

  1. The larger the number, the smaller the aperture and vice versa.
  2. With every full step, or full stop (e.g. from 4 to 5.6), you decrease the amount of light hitting your sensor two-fold!

If you have a “modern” lens you may not have an aperture ring, but your lens still has an aperture that you can adjust through a dial on your camera. If you have been using the threaded “P” or “Auto-everything” mode, you may never have touched that dial, but the computer in your camera has adjusted the aperture for you. Remember, one reason for the aperture’s existence is to control light. To obtain well-exposed images your camera’s brain measures the incoming light and adjusts one or two (or more) of the available controls.

But wait, there is more! Besides the ability to control the light amount, the aperture has also an effect on the overall “sharpness” of your image, technically speaking it is the depth of field (DoF). You may have noticed that it is next to impossible too take a macro picture of a tiny bug, with the rest of flowers, the meadow and the sky in the background being in focus at the same time. On the other hand, you may have seen landscape photographs, where everything seemed to be in focus at the same time. DoF is determined by several factors, focal length, focal distance, and you guessed it, the aperture.

Again, if you have an old-fashioned lens, you may have some indicators next to the focus ring, showing the depth of field. You’ll notice a rather complex relationship between focal length and aperture, but that’s ok. When you are using the aperture for aesthetic purposes, you don’t care about the laws of physics. The image just has to look good. So what do you need to remember?

  1. Large aperture means shallow depth of field.

A shallow DoF is good to separate an object from the background; a large DoF is sometimes desirable to show an object completely in focus.

If this was not cut and dry enough for you, please read Matt Cole’s excellent explanation of the f/stop.

Aperture and shutter speed go hand-in-hand. Here is how…

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