Exposure 101: Aperture

The effects of changing shutter speed are pretty straightforward to understand. But now let’s look at the second of those exposure controls we mentioned in our introductionaperture.

Aperture is just the name given to the size of  the hole in the middle of the lens that restricts the amount of light it passes to the sensor. Pretty straightforward in terms of exposure.  But opening up or closing down that hole has some other effects that are not obvious: it changes the amount of our subject that is in focus! This effect is called depth of field (or sometimes, though incorrectly, depth of focus) and we can use it to help draw the viewers eye to the parts of the photograph that are important and render the rest a blur. or alternately, to ensure that the whole of the image is in focus.

So remember those photos you took of your children in a garden where the flowers distracted from them so much that no one knew if it was a photo of the kids with flowers in the way, or flowers with the kids in the way? When faced with competing backgrounds like this, it’s aperture to the rescue!

First a slight diversion.  Whilst shutter speed is measure in obvious units like fractions of second, aperture isn’t. Instead it’s measured in fractions of an arbitrary-sized opening. Here’s a guide to some common aperture settings you might find on your camera:

Name On camera Description
f/1.4 1.4 A big hole
f/2 2 1/2 of the size of f/1.4
f/2.8 2.8 1/4 of the size of f/1.4
f/4 4 1/8 of the size of f/1.4
f/5.6 5.6 1/16 of the size of f/1.4
f/8 8 1/32 of the size of f/1.4
f/11 11 1/64 of the size of f/1.4
f/16 16 1/128 of the size of f/1.4
f/22 22 A small hole

If you’ve not come across this before it  could easily be a bit confusing.  The thing to remember is that small numbers represent a large aperture, ie a lot of light passing, and big numbers a small aperture, ie little light passing.

Also note whilst you’re here that doubling the number, as from f/4 to f/8, reduces the amount of light not by one half, but by one quarter! This will be important in the next chapter.

The way aperture works for us is that increasing the size of the hole really reduces the depth of field. If you have a good lens it could easily reduce it to just a few millimeters when wide open, which could be much less than we would like. So how can we deal with a photograph of a subject in front of a distracting background?  Open that aperture up!  Like this:

30d 20758 2010aug28 1418 jpg 1kp scr cr frb1 Exposure 101: Aperture

 Now the background is rendered as just a blur. And, if you look closely, even her left eye is slightly out of focus, directing all the viwers attention to her right eye.  By increasing the aperture even further we can enhance that effect:

30d 17715 2010jan23 1258 jpg 1kp scr cr frb1 Exposure 101: Aperture


If on the other hand you subject is a landscape you might want a really deep depth of field so you have everything in focus.  In which case set a small aperture, as was used in the following image:

7d 12849 2012may11 1846 jpg 1kp scr cr frb1 Exposure 101: Aperture


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