Aperture, Shutter Speed & ISO–understanding how these relate and how to exploit them

1/8000th sec @ f5.6, ISO 100: quite a speedy setup!

1/8000th sec @ f5.6, ISO 100: quite a speedy setup!


When I started photographing with my first “real” camera–one with shutter speeds and an aperture ring that I could manually adjust–I struggled with the relationship between these two vital functions. How did they relate? Should I change the shutter speed or the aperture first? What difference did it make? And what about the ISO setting? What was that about?

I read how-to books but still felt befuddled. Where to start?

Finally after lots of trial-and-error shooting and some head scratching, it came clear to me, like a light bulb turning on:

Just as moving the shutter speed knob one turn doubles or halves the amount of light reaching the sensor (or film, back then),  moving the aperture ring one click doubles or halves the amount of light reaching the sensor..

So the aperture ring and the shutter speed knob/dial both serve the same function, always doubling or halving the light used in the exposure!!  THIS IS THE KEY CONCEPT!

The exposure will thus be the same: 1/60 sec @ f8 = 1/30 sec @ f11.

Try this yourself and you’ll see both exposures are identical.

ISO speed works the same way: Doubling or halving the ISO speed doubles or halves the effective amount of light.

For example, going from ISO 200 to ISO 400 will gives you twice as much light, or one extra shutter speed or aperture f stop:

1/60 sec @ f8, ISO 200 = 1/125 sec @ f8, ISO 400.

1/60 sec @ f8, ISO 200 = 1/60 sec @ f11, ISO 400.

Now, modern digital cameras have more aperture “clicks” than my old manual equipment had, so here’s how to judge this:    The “full clicks” are:  f1.8/f2.8/f4/f5.6/f8/f11/f16/f22/f32. Anything in between two of these, say f6.3, will not be a full doubling or halving of the light.  SO LEARN THESE ‘FULL CLICKS’ AND THIS STILL WORKS PERFECTLY.

For me, shutter speed is usually paramount. If there is any action in the photograph, shutter speed becomes your dominant consideration. You usually want action to be stopped, clear and sharp:

Having at least 1/500th/sec insures that you'll be able to stop most action crisply. (Copyright The Times-Picayune/Andrew Boyd)

Having at least 1/500th/sec insures that you'll be able to stop most action crisply. (Copyright 2009 The Times-Picayune/Andrew Boyd)

A general rule of thumb, when hand-holding a camera, is to make sure your shutter speed is at least as fast as your lens focal length: so if you’re using a 300mm lens, you’d want your shutter speed to be a minimum of 1/320th sec (my cameras don’t have 1/300th). In practice, as a working pro, I can hand-hold a camera at much slower shutter speeds, but it’s a good place to start when thinking about this issue.

Now let’s look at a real-life example of how this plays out.

You’re shooting your son’s mid-afternoon baseball game and you’re using your 300mm/f4  lens. You want to stop the action so the pictures are razor sharp. For sports, your minimum action-stopping shutter speed is 1/500th/sec, but you still have good light. How do you set you camera?

Well, since there’s still enough light, begin by using a low ISO setting, say ISO 100 or 200, if your camera allows. (The lower the ISO, the finer the detail, and the less digital noise in the photographs.) Now point your camera at the baseball field and take an exposure reading: 1/1000th @ f5.6. This is good, but for action, 1/2000th @ f4 would be crisper and the same exposure, so make this change. You’re set to go.

Now as the light begins to fade, you’ll have to adjust. When you find that you can no longer shoot 1/2000th @ f4, switch down to 1/1000th, then 1/500th. If the sun is setting and  the game is still going on, then it’s now time to double the ISO, which will allow you to continue shooting at the 1/500th shutter speed setting. You probably want to use the Shutter Priority mode on your camera to do this (Tv mode on Canon, S Mode on Nikon), letting it change the aperture (fstop) as you control the shutter speed. Eventually as the light fades you would up the ISO setting the same way, always trying to maintain that magic 1/500th sec setting.

I would avoid the “Program” mode on the camera when shooting action, if it’s like mine. While it’ll take a proper exposure, you’ll never know what it will do about the shutter speed/aperture relationship, and you may find yourself looking at blurry photographs.

Continue to play this exposure game as it gets darker and you’ll continue to have good pictures of the action.

Another example: you’re planning to photograph a sunset from the porch of your vacation cabin in the mountains.  The view is spectacular and there’s detail you want to capture, both up close and far away. You have no tripod available, though. Where do you start?

Well, in this example, “action” is not part of the equation. You want maximum detail, so you’ll want to use a smaller aperture (higher aperture number) for more depth of field. You’ll want a lower ISO setting, for maximum detail. So you set your ISO to 100 and point your camera at the scene for an exposure reading: 1/125th sec @ f11.  You start shooting at that setting. As it gets darker, the colors intensify but the light fades rapidly. You react by first lowering the shutter speed to 1/60th sec @ f11, then maybe 1/30th at f11, using the railing of the porch for a “tripod” now. Finally you start to lower the aperture to f8, then f5.6 to capture the fantastic colors in the scene. Always your technical concerns are exposure, clarity and sharpness. Again, I’d suggest you stay away from the “Program” mode of the camera, using your own brain to “program” the shot:

Correct exposure is the key to beautiful, dramatic photos. (Copyright 2009/Andrew Boyd)

Correct exposure is the key to beautiful, dramatic photos. (Copyright 2009/Andrew Boyd)

Was this article helpful? I value your feedback, so please leave me a comment and let me know! I’ll try to answer all relevant questions.

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.

9 Comments on "Aperture, Shutter Speed & ISO–understanding how these relate and how to exploit them"

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  1. Ryan Williams says:

    Hi,
    this article was very good, thank you. I’ve always been involved in sports and now at the age of 20 I’ve decided to get behind the lens and try get into the sports photography side of things. I’m aware that to be a great sports photographer you have to be one step ahead of the players on the field. I’ve been shooting for a few months now and value every bit of knowledge I receive from professional photographers.

    Thank you.

  2. eyal says:

    Very helpful! thank you!

  3. El says:

    Very very helpful.. Very nice advice… happy new year in advance…

  4. James Johnson says:

    I know it was posted 6/11/09, but just in case you see this, thank you so much for you article. It helped me so much.

  5. Thanks James! Glad you were able to use it.

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