Aperture is defined as “A space through which light passes in an optical or photographic instrument, esp. the variable opening by which light enters a camera.”  Many photographers get pretty hard core on the physics behind these terms but I’d prefer to take a more practical look at this.  I often find that if I can’t describe something at a very basic level then I probably don’t understand it myself.

Aperture is essentially an adjustable “hole” in your camera.  You can make it bigger or smaller as you wish.  Now why would you want to do that?  Like so many other things in photography, it all has to do with light.  If you make the “hole”, or aperture, smaller then not as much light can get through.  If you make the aperture larger then it allows more light to get through.  The amount of light on your image sensor is what controls your overall exposure for the picture.  So aperture allows us to adjust the amount of light reaching the image sensor by adjusting the size of the hole.

There are two other things that impact exposure – shutter speed and ISO.  The former is the amount of time that you leave the “hole” open and the latter is the sensor’s sensitivity to light.  So all three of these factors – aperture, shutter speed, and ISO, are what define the exposure for any given picture.

Now that we’ve got the baseline let’s delve deeper into aperture, starting with how it’s measured.  Aperture is measured in f-stops and this is where things can get a bit counter-intuitive.  The larger the f-stop, the smaller the hole.  So if someone says the aperture of the camera was set to f/16, this means that the aperture was actually very small, i.e. the “hole” was small and sometimes referred to as “closed.”  On the opposite end, an aperture of f/2.8 means the lens is allowing in a great amount of light, sometimes called “open.”  This is illustrated below, and you can see how f/2.8 would allow more light in than f/16.

Continuing on from this, you can see below an image that shows how aperture size corresponds to the f stop number.  The actual f stop number is defined as a ratio of focal length to aperture diameter.  In my effort to keep things simple (both for myself and for you) I’m not too bothered by how that number is derived.  Let’s focus instead on what it actually means.

If you shift the aperture on your camera from f/2.8 to f/4 you go through a few other f stops along the way (f/3.2 and f/3.5).  But the move from f/2.8 to f/4 is what is known as a “stop.”  The move from f/2.8 to f/3.2 is a third of the way, the move to f/3.5 is two thirds of the way and finally the move from f/2.8 to f/4 is the full stop.  The full move from f/2.8 to f/4 will decrease the amount of light coming into the lens by half; this will of course work in the opposite direction as well, so moving from f/16 to f/11 will increase the amount of light in the exposure by a factor of two.

You’ll find when looking to purchase a new lens that the manufacturer will make a big deal about the widest aperture.  For example the Canon 24-70 L series lens has a consistent aperture of f/2.8.  This means that regardless of what zoom setting is used (24 mm, 70 mm or any place in between) the lens is able to maintain a consistent aperture of f/2.8.  Other lenses may say something like f/4.5-f/5.6.  So for example the 100-400 mm lens will have an f/4.5 maximum aperture at 100 mm but as the focal length increases (i.e. you zoom in) the aperture will go up until at 400 mm the maximum aperture will be f/5.6.  Professional photographers pay a lot of money to get lenses that are capable of consistent apertures through the zoom range.

Why is aperture such a big deal?  Because if you can shoot with the aperture “open” you allow more light in, and this has a direct correlation to the amount of time the shutter needs to be open and the ISO setting.  So for example in situations where light is at a premium (for example weddings or indoor sports) it’s critical that the photographer has a “fast” lens, i.e. a lens with a small f stop of f/2.8 or better.  You can set the lens to f/2.8 and this allows you to have a shutter speed that will keep your images sharp.

Aperture is also critically important for another, more creative reason.  Aperture is how a photographer controls depth of field, and this has a direct impact on how much of the frame is in focus.  This is what is used when you see an image where the foreground is in focus, but the background is slightly blurred.  The technical term for this blur is called “bokeh.”  This blur, and how to capture it, is a great first step to improving your photography.  For example looking at these pictures of Adriana I have shown two basic  examples of how aperture impacts the photo.  In the left picture, taken at f/3.5, the Summer Palace in the background is completely blurred; you can tell there’s a building or tower of some sort but no detail.  In the right photo, taken at f/11, you can see more of the building in focus and it gives a better sense of place.

Why does the aperture of the lens impact the depth of field?  This is a pretty significant question of physics, and something I’d rather not get into.  But suffice it to say it has to do with the angle of light striking the sensor

There are countless tutorials and definitions out there on the web for those of you that are interested in learning more about this or really want to get into the science behind how a camera actually works.  If you are looking to improve your photography and learn more about your camera, I heartily recommend shooting in Av or A mode so that you can begin understanding more about how adjusting the aperture can give your photos a new, more creative look.