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Opposite Viewpoint:

When would it not be desirable to change the depth of field for emphasis on a specific subject with aperture priority? When taking a group photo of your classmates or coworkers you would want everyone regardless of depth to appear in focus.

Taking Photographs

(4 - The Evolution - Aperture) or Back to Table of Contents

Changing the aperture is something that normally your camera will do for you in "Automatic" mode much like changing the shutter speed. Likewise there will be moments when you will want to know how to do this yourself because the camera won't give you the desired creative effect. If you get lost in the terminology of F-stops or numbers read the description at the bottom of the page.

STEP 4:

The aperture is the circular opening in the lens much like the iris in your eye. It determines how much light reaches the sensor in the camera. While the shutter-speed is the amount of time light is allowed into the sensor and the ISO is the quality of sensitivity of the sensor itself, we also have the third element called the aperture. The aperture is where the quantity of light to the sensor is controlled. These three work together. These three factors will help you get the correct image lighting. If your image is too dark it is said to be under-exposed. Likewise when an image is too bright then it is over-exposed and of course you want the least amount of noise possible.

The aperture helps get a perfect exposure and also creates entirely different effects than the shutter speed. Changing the aperture helps you alter what is called the Depth of Field (DoF) . This is the place in the picture where objects are in best focus. So what kind of effects can I expect by altering the aperture settings? By changing the depth of field you can blur out backgrounds when you have a subject you want to emphasize in the foreground. This is called shallow depth of field. You can blur out foregrounds when the background is more interesting. This is called the deep depth of field. Think of all this like the iris of your own eye and you can intentionally set the focus like you would when you stare at an object.

Changing the depth of field can purposefully emphasize foreground only.

On the brush image above the background was intentionally blurred out using aperture settings. On the snow leopard image below, the bars on the cage are located on either side of his face. By changing the aperture we see the bars were blurred out and it became easier to get the image desired. There are two bars still in this image. One bar is present on either side of the leopard's face. If you look carefully you can see where they are and where his fur changes color slightly. The aperture is so intently focused on the background that the bars seem to disappear.

How does this work? As a lens is wide open this causes the best blur of the background. This is because more stray light is allowed into the camera while it is looking at the item of interest. The reverse is true as well. When the lens is closed down to a soda straw size or smaller then the light coming from that specific object you are photographing is from nowhere else but straight ahead. This makes the depth of field more clear from that object.

Changing the depth of field can intentionally emphasize background only as this 300mm lens shows.

One important and perhaps confusing note is that the size/magnification of your lens changes the aperture characteristics. The longer and longer your lens becomes, the more blurring occurs because of lens properties. What is the reason? It has much to do with where the image clarity crosses as it enters the camera. As a rule, the focal length of the lens has a significant effect on the depth-of-field. The longer the focal length, the smaller the depth of field becomes for any given lens setting. In summary, if you have a telephoto lens and aim it at a flower you will get the blurred background without even changing your aperture settings. You could just use a telephoto for all your background blurring shots but what would the trade-off be? For one the shutter speed would be slower because of the longer focal length. In some circumstances this would confine you to using a tripod. Lastly, telephoto seems to visually compress images and this makes portrait shots not look as rich with depth when it comes to facial features.

The bokeh effect using a 200mm lens not extended and a beam-girder bridge in the background is blurred.

When you blur the background of an image this is called the "bokeh" effect. The word is Japanese and the silent "h" on the back was added for the western hemisphere so we didn't walk around rhyming with the word 'broke'. Many photographers feel that having large blurry shapes in the background is less pleasing than a totally blended blurry background. This is personal preference of course. Either way it is a great way to emphasize your subject.

Okay blah-blah-blah, I get the idea now how do you create this effect...right? On the Canon series you have different settings. First try using the "Aperture" mode. You turn your dial to 'Av'. Push the shutter button only half way down to 'meter' on your subject. Metering is where the camera figures out what light it needs for a decent picture. On the display it might say something like F8.0 aperture with a shutter speed of say 1/250 of a second. Assuming you want to blur your background a bit more you turn your dial on top. You dial the Aperture open to F5.6 or more wide. If it's bright enough in your location you may not have any other adjustments to make. Since you are now letting more light in at F5.6 keeping the shutter the same may wash out your image in some circumstances with too much light. Notice on the image below how the F5.6 with a wider opening blurs the background slightly more than F8.0? It's subtle in this case but you can see it.

In a technical sense be aware of this balance between shutter speed and aperture opening. This is called the reciprocity law. As you now know this is where light exposure in photography equals the Intensity multiplied by the Time (E= I x T) . This rule can seem complicated but it means only that a proper exposure provided by ƒ8 aperture and 1/250 seconds will give the same exact results as ƒ11 and 1/125 seconds. Simply said, if one choice of settings provides proper exposure, then the others will as well if the numbers follow each other. As the iris gets wider the shutter speed gets faster and of course as the iris gets more soda straw small the longer the shutter speed is set for. If you are in aperture or Av mode and open the aperture then calculate or play with the shutter speed on the camera till your image is once again in the right exposure. One can take an image and keep turning the shutter dial till the desired image is achieved.

Technical mumbo Jumbo: (F-Stops)

As you start using different lenses then how wide the aperture opens and closes becomes important for reasons we just discussed regarding background and foreground emphasis. The measurement used when talking about apertures is F-stop or f/stop.

Simply explained the notation looks more complicated than it is. Smaller and smaller lens openings are noted by a numeric fraction to show how divided the light gets going into the lens. The aperture opening is shown as a fraction. If our example lens was open as far as it could go, then the fraction of 1/1 would be its description (which 1/1 as a fraction is just 1 obviously). The opening would be full open and called f/1 in size. Each division is called a "stop" and divides the light in half from the previous F-stop. So an f/2 would give you half as much light as an f/1.2

If this lens was dialed down 22 sizes smaller, the fraction of 1/22 would be the size and gets described as f/22. This nearly pin-hole size opening in the lens would give you many different effects and certainly need more light than an f/1.2 or similarly large setting. Each time move the dial down a "stop" or setting the light is half of what it was. Maybe it helps by remembering that the larger the F'number the smaller the hole and vice a versa?

Technical Mumbo Jumbo: (Focal Length)

If you zoom or can change lenses on your camera, then your camera can see further with a longer lens attached. In this case the camera will most likely take longer to get all of the light it needs for an image. Let's put a couple ideas together that we mentioned earlier.

The longer the lens the more "focal length" it will have. Focal length is the magnification of the lens. The focal length of a lens also affects the camera's angle of view. Here is the trade-off you'll see. The longer a telephoto lens zooms, then the more narrow the cameras view. This also goes the other way as well. That is to say that a very wide view lens like a fisheye will not have far zooming abilities.

To go a bit deeper, we talked about the f-stop as the aperture's opening narrowed. There is a relationship with focal length. That relationship is between the opening of the aperture in the lens and the focal length of the lens. For example, on a 300mm lens we set it for an f/2 aperture. That is saying that the diameter of the aperture is now divided in half. It's like simulating a 150mm lens using your 300mm. To visualize the f/2 = 300mm/2 as you recall. So effectively you are using a 150mm lens at f2. These facts will all be helpful when you are visualizing the kind of photograph you want to have at the end of a picture taking session.

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Written Jan2008 and updated Mar2010 - Tracy Lee Rose