Understanding exposure: shutter speed, aperture, and ISO

When your camera is set on automatic, making a photograph is as simple as pressing the shutter release button. Somehow, the camera magically records just the right amount of light to render an image of the scene before it. But what is really going on? How does the camera know how to do that?

Read on to find out how a little knowledge about what goes into making an exposure can open up new worlds of creative possibilities.

There are essentially three factors that go into making an exposure whether on film or a digital sensor. These three things are the shutter speed, the lens aperture, and the film or sensor sensitivity called ISO. Each of these things individually and taken together affect how much light from a scene is recorded. And photography is nothing if not capturing light.

Shutter speed is probably the simplest to understand. Inside your camera is a movable screen in front of the film or sensor call the “shutter.” When you press the shutter release button, the camera opens the shutter and then closes it again. This controls the amount of time that light is collected. It is measured in fractions of a second from 1/8000 of a second to 30 seconds.

The aperture is a variable-sized hole inside your lens through which all light must pass before it reaches the shutter. The hole can be small (“stopped” down) or it can be large (“opened” up). The wider the opening, the more light that can be collected. Aperture size is expressed in “f-stop” (or “f-numbers”) like this: f/2.8. The smaller the f-stop, the larger the aperture is. When someone says they are shooting “wide open” they are referring to the aperture in their lens being open to the largest size. And when a photographer refers to “stopping down,” he means closing the aperture (setting the camera to a bigger f-stop number).

Everything in the frame is in focus.
1/320s @ f/9

Let’s talk about these a minute before moving on. To record a given scene, your camera has to capture light from the scene and record it on the sensor. This is called the “exposure.” You can do this by capturing as much light as possible for a short time (a wide aperture and a short shutter speed) or by capturing a little light over a longer time (a small aperture and a long shutter speed). But why on earth does this matter? Because these two simple factors give you a huge amount of creative control over the resulting image.

Shutter speed and aperture control how much light gets to the sensor. But they also control something else. Shutter speed also controls time. A very fast shutter speed will capture the briefest of moments on film, freezing action, suspending water droplets in mid-air. A very slow shutter speed will blur a mountain stream, capture star trails moving across the sky, and blur moving objects to give a powerful sense of motion.

The aperture also controls the “depth of field.” This is the portion of a photograph from front to back that is in acceptably sharp focus. A very small aperture opening will keep everything in the frame from the closest flower to the furthest mountain peak in focus. But a very large aperture opening (remember, a smaller f-stop!) will keep only a slice of the photo in focus: just the eyes of a portrait remain focused while the background is thrown into beautiful blur.

Narrow depth of field
Narrow depth of field.
1/60s @ f/2.8

These settings affect each other because you still need a certain amount of light to record a good exposure. So, if you widen the aperture letting in more light, you also need to shorten the shutter speed to compensate. And if you lengthen the shutter speed to let light fall onto the sensor for a longer time, you need to close down the aperture to let less light in.

Fortunately, most cameras come with Shutter Priority and Aperture Priority modes to make these computations simple for you. If you put your camera in Shutter Priority mode (often labeled “Tv”), you set the shutter speed for the effect you want and your camera will compute the correct aperture. You’ll do this when you want to control how much blur is in the photo or to make sure you can freeze the action. In Aperture Priority mode (“Av”), you set the aperture and the camera will compute the correct shutter speed. Do this when you want to control how much of the photo is in focus from foreground to background, either a sliver of a portrait (large aperture, small f-number) or an entire landscape (small aperture, big f-number).

Lenses usually indicate their biggest aperture setting. Lenses that are capable of very large openings like f/2.8, f/1.8 or even larger are called “fast” lenses because they let in a lot of light enabling fast shutter speeds in low-light conditions.

The last factor is ISO sensitivity. The more sensitive your film or sensor, the less light needs to be captured to record the image. ISO sensitivity also affects noise or grain. The higher the ISO, the grainier/noisier the image will be. But higher ISO also allows you to shoot with faster shutter speeds in low light, an option that is preferred by many photographers over using a flash.

Letting go (motion blur example)
Slow shutter speed, small aperture.
1/40s @ f/20

The last thing to remember is that shutter speed, aperture, and ISO are usually expressed in numbers that indicate a halving or doubling of exposure. For example, 1/30s lets in twice the amount of light as 1/60s but half as much as 1/15s. f/4 lets in twice as much light as f/5.6 but half as much as f/2.8. And ISO 800 is twice as sensitive as ISO 400 but half as sensitive as ISO 1600. This is useful to remember if you are going fully manual and want to control all three settings yourself. For example, if you slow down the shutter speed by half, you can either decrease the aperture size to keep the same exposure (1/60s @ f/5.6 is the same as 1/30s @ f/8) or decrease the ISO while keeping the aperture constant (1/60s @ ISO 400 is the same as 1/30s @ ISO 200 with constant aperture).

Shooting in aperture priority or shutter priority modes is an easy way to take advantage of this knowledge without having to remember formulas or do any math. If you want motion blur or frozen action, go with shutter priority mode and use a long or short shutter speed. If you want to control the depth of field, use aperture priority mode and set the aperture to the smaller f-stops for narrower focus (portraits) and larger f-stops for wider focus (landscape photography).

By John Watson

John Watson is the original founder of Photodoto. If you're interested in what John has been up to, you can browse his personal blog.


  1. Aperture is the one technical bit my brain wants to ignore!Before reading about it I imagined the f/ representing the frequency of blades in blocking the light, hence a higher aperture -> more blades blocking light -> a smaller hole for the light. Then I tried thinking in terms of fractions of light entering the lens: f/2.8 allows roughly 1/3rd of avaliable light in, while f/5.6 lets in around 1/6th of the light, double that of f/2.8.

    Your article has expressed it much more clearly than elsewhere for me, and I hope I haven’t confused anyone by posting ‘my way’ to working with apertures!

  2. Nice post, I always keep forgetting to mess with the DOF on my shots. Need to play with that more when taking portraits.

  3. I own an all manual Minolta Maxxuum and would like to learn to use it. I have a limited understanding of FStop, depth of field, shutter speed, and ISO, but I’m having trouble putting it all together to take good camera shots.

  4. That was as crystal clear as a very small aperture opening (great depth of feild and a high f/stop, of course) combined with a very fast shutter speed!

    Much appreciated! 🙂

    P.S. Can you tell us anything about reflective metering?

  5. I had no idea all this went into taking a good picture with a professional Camera, until I started yearbook class this year.
    and Understanding all this and loving what I do, is amazing.
    I have realized I have a HUGE passion for this,
    and its all amazing.

  6. daveyr The way i think about aperture is just see the number as the range of the image that will be in focus. a small number means a smaller range will be in focus, a large number means a large number will be in focus. Unfortunately, when it comes to light, that forumla is backwards (small number = lots of light) i can live with that though and still remember it!

  7. I feel I understand aperture / shutter speed / iso better now. Thank you.
    However, I still don’t understand how to read the aperture and shutter speed settings
    on my monitor on my Nikon D50. I get confused as to how to translate the numbers,
    i.e. – what 3000 means or 30 with a degree mark means. Can you explain?

  8. What a brilliant to the point article. I have read many a book and this tops all of them. Thanks so much. I wish others could follow your example of clear and to the point explanations. The rest we can go figure oursleves.

  9. Without a doubt, these are the “Big Three” concepts to understand, and I agree with Steven that you’ve given us a very clear explanation. Thanks.

  10. This is a great explanation and has really helped me understand everything the photographer has told me today at Uni. Many thanks

  11. thankyou sooo much for this information !
    i found it so easy to understand
    and grasped everything rather easily on here
    compared to in a classroom .

    thankyou again !

  12. Interesting and informative post. Also interesting to note that I just posted the same topic on my blog. I believe yours was more informative so I guess I should have read your first. I think I may be gack and add to mine, LOL!!

  13. John, I want to kiss you!

    Thank you so much! I was going crazy – now i understand – completely!

    🙂 Aisha

  14. I bought a new Digital Camera today thinking..”Great!! I’m going to be the next Ansel Adams”…lol. Or so I thought. It takes much more than just using this camera in full auto mode and pointing and shooting. The camera made me feel dumb. In all honesty, I was ignorant. I had no clue how to do anything. I, then, read this article and, now, have a much more complete understanding to the different modes my camera offers. This article was very educating and I have been experimenting with the different ISO, aperture, and f/s settings and am very surprised how much I am learning and how quickly, as well. Some pictures are great, and some…well, not so great, but that’s the point of experimenting with the settings. I think I have a good grasp on low light settings, now. Tomorrow I will experiment with full light and moderate light settings, and probably drain my camera batteries. But, that is a very small price to pay for having the knowledge to capture that great picture in whatever setting I’m in. Thanks a bunch!! When I become famous, I will remember this lesson! Haha! Best regards.

  15. I am very new at this and I get a headache reading all of this tech stuff. Just wish I had someone to show me, then I’d go ahhhh!

    1. DRuthie, you can always try it on your own, step-by-step. Just take a shot in auto mode, check the settings that your camera set, then turn the mode to Manual, so that you can adjust Shtter speed and Aperture (the f-number). Start with the settings you had in auto mode, and then just try adjusting either aperture or shutter speed little by little. And just see what difference it makes. This is how we learned to speak in childhood – we were not sitting with a textbook, but simply listened, observed, and tried 🙂 But THEN go on reading articles just to make sense of what you already started to understand by intuition 🙂

  16. Just a passing thought. The control of depth works like the human eye. My eyesight is beginning to fail a bit,, (old age) and I notice if I squeeze my eyelids down to a point where I can just see my target, I see it much clearer. For example, reading something on the television.

  17. I’m just starting to get into my photography more seriously … And this is very clear explanation of the basic principles … Thank you very much!

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