Considering the Aperture Setting

This post is part 2 of a 4 part series:

1. Exposure Explained
2. Considering the Aperture Setting
3. Selecting the Shutter Speed (coming soon)
4. ISO (coming soon)

Considering the Aperture Setting

In the previous post in this series, I explained the three settings on your camera that you can manipulate to attain a correct exposure. I explained how they relate to each other. Today, I'm going to talk about when and why you would emphasize the aperture setting over the shutter speed or the ISO speed.

The main consideration, when it comes to selecting the aperture setting, is depth of field. Are you wanting to maximize the depth of field to have sharp focus from near to far? Are you wanting a wafer-thin area of sharp focus to set off your subject from the foreground and background? Are you trying to blur a distracting object in the background? Are you trying to show a foreground object in the context of its environment?

All of these questions relate to choosing the desired depth of field. The aperture, the opening that light travels through to get to your sensor or film, is expressed as a number known as an ƒ-stop. It's a ratio of the diameter of the opening to the lens' focal length. But what's important to know is that the smaller the aperture (a bigger ƒ-number, like ƒ-22) the greater the depth of field you will have. Setting a small aperture is known as "stopping down" in photography lingo. The opposite is also true, the larger the aperture (a small ƒ-number, like ƒ-1.4) the shallower your depth of field will be. Unfortunately, opening up the aperture in not know as "stopping up."

So, do I set my camera to aperture priority mode? Yes, if controlling the depth of field is your priority.

Below is an example of a shallow depth of field. Notice how the dishes in the background are out of focus. Also notice how the sharp focus on the ice in this Margarita draws your eye back there repeatedly as you look around the image. An aperture of ƒ/1.4 was used for this image.

Below is an image with great depth of field. Notice how we have sharp focus from the cactus in the foreground to the mountains in the background to the clouds in the sky. An aperture of ƒ/11 was used for this image. Stopping down a little further to ƒ/16 or ƒ22 might havebeeneven better.

Are there times when depth of field doesn't matter (much)? Yes, when the entirety of your subject is in sharp focus, there's no need to stop down any further. This is often true for distant objects. The greater the focusing distance, the the greater the depth of field you will have, the less you will need to stop your lens down to get that subject in focus. Photograph a building six blocks away and you don't need to stop down very much to get the whole building in focus. But if you want the person 5 feet in front of you to be in focus as well as that building stopping down will be in order.

Most SLR cameras have a feature called "depth of field preview." It's usually a button on the front of the camera. When you press it, the aperture closes to the setting that you have set, everything in the viewfinder gets darker, and you are then looking at the scene the way the light will hit the sensor (or film). You are previewing the depth of field. You will see what is and is not in focus. Refer to your SLR camera's manual for how to use this feature and give it a try.

In my next post in this series, I'll talk about why you would emphasize shutter speed over ISO or aperture and why you would set it long or short.

Exposure Explained

This post is part 1 of a 4 part series:

1. Exposure Explained
2. Considering the Aperture Setting
3. Selecting the Shutter Speed (coming soon)
4. ISO (coming soon)

Exposure Explained

There are three variables you can manipulate in a camera when making a photograph--aperture, shutter speed and ISO speed. Striking equilibrium between the three is what gets you the ideal exposure. I refer to it as an equilibrium because you can raise one and lower another and still maintain the same exposure. Changing each of those variables however affects the image in differing ways.

First, let me try to explain a way to think about exposure, then I'lltouchon the most obvious ways that changing each of the three variables affect your captured image. I'm going to use the analogy of filling a container with water from a faucet. Think of the water in the container as light in your photograph. Too much water is overexposure and a half-full container is underexposure. So let's take a look at the three variables:

Aperture:The aperture is the diameter of the opening of your lens. (For the geek types reading this, it is expressed as a ratio of the focal length of the lensto thediameter of the opening.) Think of the aperture as the faucet in my analogy. If you open it only a tiny bit, the water trickles in, but if you open the faucet full blast, the water gushes out.

Shutter Speed:The shutter speed is a measure of time (most often expressed in fractions of a second). It is the length of time that the shutter remains open exposing your film or digital sensor to light. In our analogy, shutter speed equals the amount of time you leave the faucet running.

ISO Speed:ISO speed, is the measure of the film or sensor's sensitivity to light. So for our analogy, think of ISO speed as the size of the container. Higher ISO speeds require less light (water) so think shot glass. A lower ISO speed requires more light (water), think swimming pool. ISO stands for International Organization for Standardization.

So, if I have a bucket that I want to fill and I put it under a faucet set to a trickle, it will take a relatively long time to fill. If I twist that faucet wide open, the bucket will fill in a fraction of the time. Now, if I open the lens on my camera all the way (ƒ/1.4), it will expose my film in much less time than if I let in a trickle of light (ƒ/22). Make sense?

In upcoming posts, I will explain how changing each of these variables affects your images. This will help you determine which variables you'll want to change in which direction to achieve the optimum exposure and the desired visual effect.