What is Aperture and Aperture Value

(Note: This is lesson four of my “How It’s Done Series“)

Aperture is the Most important controllable setting on your DSLR.  Understanding the effects of your camera’s aperture is the key to unlocking the potential of your camera.  Also, once you understand aperture you will better know your camera‘s limitations, and hopefully how to overcome some of these limitations.  This entire lesson of my “How It’s done” series is dedicated to the details camera aperture.  This lesson should give you a good understanding aperture and aperture value.

Let’s start with the most basic question:  What is aperture?  Your camera’s aperture is the adjustable opening in your lens.  The aperture is constructed of metal blades that widen or narrow the opening in your lens.  By changing the size of the aperture the photographer can increase or decrease the amount of light traveling through the lens.  The aperture size is either commanded directly by you the photographer or calculated by your camera’s metering system.  The size of the aperture (opening) is displayed as aperture value.  Most DSLR’s display aperture both inside the viewfinder and on the LCD next to the shutter button. Check your camera’s manual to see where the aperture value is displayed on your camera.

Aperture values are displayed as f-stops (we discuss f-stops later, I hear you groan).  The actual aperture numbers (f/2.8, f/5.6, f/11)  might not make any sense to you now, but  don’t worry.  I’ll explain it all.

Above is an illustration of an aperture.  The number of blades that define the circumference of your aperture is usually directly related to how expensive your lens is — the more expensive the lens the more blades.   With more blades an aperture has smoother edges and more closely resembles an actual circular opening.  It’s believed that a smoother aperture creates a smoother more appealing bokeh.  The science of Bokeh is much contested and involves ridiculously complex math so I’m just not gonna go there.  All you need to know is that your aperture can expand and contract, and the photographer can control how wide or narrow his/her aperture is.

Let’s talk about those crazy aperture value numbers and get it out of the way once and for all.  I’ll add a caveat, if you don’t understand these next two paragraphs, don’t worry about it, because you really don’t need to know this stuff.  I’ll give you the straight dope a little further down. Ok, here is the scary explanation of aperture:   Aperture value is the ratio of the lens’ focal length to the diameter of the aperture.  What did I just say?  I’ll break it down further.  If you take the focal length of your lens and divide it by the aperture value you get the diameter of the aperture’s opening.  That still sounds too much like math class so let me break it down even further.  If you are shooting at a 100mm focal length and your aperture setting is f/4 then the diameter of the aperture is 25 mm.  If you are shooting at 100 mm focal length and your aperture is f/8 then the diameter of the aperture’s diameter is  12.5 mm.  Does that make sense to you now?  You should have noticed that the higher the aperture value the smaller the opening is that lets light into your camera’s senor.  Now, let’s look at it from a different direction.  What if you wanted to build a 400 mm lens with an maximum aperture of f 2.8, how big would you have to make the diameter of the aperture?  The answer is 142.8 mm.   That’s almost 6 inches across!  That is a lot of expensive glass!  Now you can see why those long focal length lenses along the sidelines of pro football games are so big.  Normally, a lens capable of a wider aperture is more expensive than a similar lens with a smaller aperture.

The next question is why did they pick those values for apertures?  Why not a simple 1, 2, 3 approach?  The answer lies in the slightly complex concept of f-stop.  Each full step in the f-stop scale cuts the amount of light in half.  The f-stop scale is:  f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16…  Going from f2/2.8 to f/4 cuts the amount of light entering the camera in half.  To complicate matters modern camera’s usually use 1/3 of a stop scale (f/2.8, f3.2, f/3.5, f/4.0 …).  Each step in a modern camera reduces the amount of light entering your camera by 1/3 of one stop, or 1/3 of ½.  Now, that is complicated.  Don’t worry, you don’t need to know this stuff.  I’m just throwing it out there so your camera doesn’t scare you with all these numbers.

So here is the important take away from my long, rambling aperture value explanation:  The smaller the aperture value the wider the opening that allows light into your camera’s sensor.  When you set higher aperture values you are restricting (decreasing) the size of the opening that allows light into your camera’s sensor.  A setting of f/2.8 is a wider aperture than f/5.6.  It’s kind of counter intuitive.  Smaller aperture value means bigger aperture.

Now that we got all the boring stuff out of the way the next few lessons will be more fun.  I’ll explain how aperture controls depth of field and shutter speed in the next two installments of “How It’s Done”

Continue to the Depth of Field lesson