How to Choose Your Lens Aperture Explained

Looking through a 'wide open' Canon 70-200mm f2.8 lens. This is with the lens opened up to f2.8, its widest aperture. (Copyright 2012 / Andrew Boyd)

Looking through a 'wide open' Canon 70-200mm f2.8 lens. This is with the lens opened up to f2.8, its widest aperture. (Copyright 2012 / Andrew Boyd)

Learning how to choose your lens aperture (the ‘f-number’) can be very confusing for the beginning photographer. The discussion can quickly get into a daunting explanation of the shutter speed/aperture relationship, all of which means absolutely nothing to the novice. Here I’m going to give you all you need to know to make good choices about aperture. Your understanding of the lens aperture/shutter speed relationship will start to become clear as you apply this simple principle.

There are only two things you really need to understand to correctly choose a lens aperture:

If, when you look through the viewfinder, you want everything you see to be sharp in your resulting photograph, choose a small aperture by stopping the lens down. (I’ll explain how to do this in just a moment.)

If, when looking through the viewfinder, you want to isolate your subject from the background, throwing the background out of focus with only your subject sharp, choose a large aperture by opening the lens up. (Again, I’ll explain how to do this in just a moment as well.)

Two more tidbits, neither too complicated, must be understood before we proceed:

First: It sounds backwards, but ‘large apertures’ refers to a large hole for light to pass through to your CCD (charge-coupled device—the thing that captures your image) when you take your photo. Large holes are actually the smaller-sounding f-stop numbers: f1.8, f2.8, f3.5 are all relatively ‘wide open’ apertures.

Second: ‘Small apertures’ refer to small holes for light to pass through. Examples of small apertures (with the lens ‘stopped down,’ as we say in Photospeak) would be f11, f16, f22.

That’s it! Just these two principles will guide you to good aperture choices. Now here’s how to accomplish both of these things:

LARGE APERTURE EXAMPLE

A wide-open aperture allows the player catching the ball to be sharp with the background thrown out of focus, here f4 @ 1/1250th sec, ISO 1600. (Copyright 2012 / The Times-Picayune / Andrew Boyd)

A wide-open aperture allows the player catching the ball to be sharp with the background thrown out of focus, here f4 @ 1/1250th sec, ISO 1600. (Copyright 2012 / The Times-Picayune / Andrew Boyd)

If you’re shooting a football game with a telephoto lens, either indoors or outside on a sunny day, you’re going to want to isolate action shots of players from the distraction of the background by throwing that background out of focus. (Take a look at the current issue of Sports Illustrated and you’ll see dozens of examples of this practice.) To do this you’ll choose a relatively ‘wide-open’ aperture: probably f2.8 or f3.5 or f4. This will guarantee you shots without all of the rest of the commotion taking place in the frame in focus. [Important shutter speed nugget to grasp now: to have that wide-open aperture, with all of that light passing through to the CCD, you’ll need to use a very, very fast shutter speed to keep from overexposing your shot. If you’re using your camera’s ‘Aperture Priority’ exposure setup, it’ll make this adjustment for you. If not, you’ll need to increase the shutter speed until you see your camera’s version of ‘correct exposure’ line up.]

SMALL APERTURE EXAMPLE

Frost-covered mustard greens. The leaves are not all on the same flat plane--some are closer to my lens, others are further away-- so I stopped the lens down to f7.1 to increase the amount of subject matter that I could render sharply. (Copyright 2012 / Andrew Boyd)

Frost-covered mustard greens. The leaves are not all on the same flat plane--some are closer to my lens, others are further away-- so I stopped the lens down to f7.1 to increase the amount of subject matter that I could render sharply. (Copyright 2012 / Andrew Boyd)

Now we’re in a totally different shooting situation. We’re outside shooting plant life details. In this case, we want as much of the information in the frame to be as sharp as possible. So take your lens and choose a small aperture: f11 or  f16 or f22 would be good places to start. [Here’s the related shutter speed factoid to absorb: to use such a small, small aperture, which doesn’t allow much light to reach the CCD, you’ll need to slow your shutter speed way, way, down, effectively giving that small aperture the longer amount of time it need to create a correct exposure. Again, either with manual exposure and your camera’s light meter, or with the aperture priority exposure setup on your camera, you can accomplish this little feat.]

Now of course there are twenty other things we could talk about now to address all of the related issues that you may encounter when you go out and try this. But that would just make things more complicated than I want them to be for you at this point. So instead, go out and give these two ideas a try!  Shoot something with a ‘wide open’ aperture and see what happens. Then pick something that needs things ‘stopped down’ most or all of the way: check your results. Pretty cool, huh?

Once you’ve got this down, here’s another related article that adds some more pieces of the exposure puzzle.

That’s it!  You now know most of what you need to understand about aperture and how to control it….so go shoot some pictures!

Hi, I’m Andrew Boyd, a.k.a. The Discerning Photographer, and I hope this post has been interesting and informative. Please leave me a comment about it, let me know what you’d like to see more of on the site! You can also sign up for email delivery of all future articles or my RSS feed. Or subscribe to our Facebook page or our Twitter feed. Thanks!–DiscerningPhotog

Posted in: How To

About the Author:

Photographer, videographer and photo editor. Host and creator of The Discerning Photographer web site. Currently a Canon shooter.

6 Comments on "How to Choose Your Lens Aperture Explained"

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  1. Christine O'Reilly says:

    I enjoyed the article. I’m a new DSLR user and I appreciate the easy to understand lesson about aperture use. I would like to learn more. I’m having a ball with my new camera but am experiencing lots of “challenges”.
    Thanks again.
    Christine O’Reilly

  2. Glad to hear you’ve ‘jumped in the water,’ Christine. Another article you might find helpful is this one:
    http://thediscerningphotographer.com/2011/09/19/the-three-building-blocks-of-photography/
    Andrew

  3. You did a perfect job explaining this important subject to a beginning photographer. This is the umpteenth time I have had to implant these very important concepts in my brain and I am starting to get it.

  4. That’s great, Patrick! Once you really ‘get it,’ it’ll stick.

  5. Patrick Corrigan says:

    Yes you did a very good job explaining that. I am trying to figure out why my brain resists understanding the reciprocating nature of aperture and shutter speed. I am sure down the road I will look back and wonder why this was ever a mystery.

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