Shutter speed, aperture, and ISO

In Photo Basics by Jim Harmer

Exposure is the most basic element of photography

If you want to be a photographer, you absolutely must learn how to control the exposure of a picture.  It will take a minute, but this page has everything you need to know.  Click on the different camera settings on the interactive exposure tool above to practice your exposure!

In the next few paragraphs, I'll explain exposure in the simplest and easiest terms I know how.  But you should also know that I teach a 30-Day Online Photography Class JUST for beginning photographers, where I teach exposure and other basic topics in much better detail. Check out the beginning photography class here.

Exposure is the total amount of light which is captured on a camera sensor during the process of taking a photograph. Digital cameras have three techniques to control the amount of light which is captured by the camera: shutter speed, aperture, and ISO.

Shutter speed

There is a “blinder” in your camera which covers the camera sensor.  The blinder quickly allows a slit of light through the lens to touch the imaging sensor in the camera, which captures the light. The longer the shutter is open, the brighter the image.

Have you ever wondered why your pictures are often blurry in low-light situations or when the subject of the image is moving quickly? Slow shutter speeds allow lots of light to touch the sensor, but if anything in the image has moved while the sensor is open, the photograph will reflect that movement. We can’t hold a camera perfectly still because our hands shake–that’s why we use tripods. The answer to every blurry picture is to make the shutter speed quicker, but for some situations, such as a moving object in the dark, quick shutter speeds can have the negative effect of a darker image, since the camera has less time to capture the light. Shutter speed is the total amount of time the blinder allows for light to pass through to the sensor, and it is measured in fractions of a second. A shutter speed for shooting a party in a dim restaurant might be 1/60th of a second, while an image at the beach on a bright day might be as fast as 1/4000th of a second.


The aperture is a circular (sort of) hole through which the light passes to the sensor. A smaller hole allows less light and consequently produces a darker image. Perhaps even more important than the amount of light is the depth of field. It basically controls the amount of the picture that is in focus and the amount of the picture that has a nice creamy blur. Aperture is generally expressed in f-stops on a camera. If you want a portrait of a baby with a nice creamy background, a low f-stop, such as 3.5 is used, whereas if you want to take a landscape image with everything in focus, then you’ll want an f-stop like 11 or 22. The low f-stop of 3.5 allows lots of light, whereas a high f-stop like 11 or 22 would not allow much light to get to the sensor and produces a darker image. In essence, low aperture allows lots of light and short depth of field; a high aperture allows little light and makes the entire image in focus.


ISO can best be described as magic. It makes the camera sensor more sensitive to light. The low-light sensitivity is one area in which technology is moving forward rapidly as professional photographers have pushed manufacturers toward this objective. A low ISO such as 100 will produce a darker image than an ISO of 800. However, ISO comes at a high price. If a photographer uses a high ISO such as 800 or above, digital noise becomes visible on the image. (Click here to read about the dangers of expandable ISO.) Digital noise is a grainy look on an image and is never desirable. Different cameras deal with digital noise better than others, but most entry-level cameras should not be raised above ISO 400 unless the aperture and shutter speed cannot offer an acceptable alternative. In short, ISO is a last-resort bit of magic for more light.

Shooting Modes

The aperture, shutter, and ISO buttons on a DSLR camera

Select the shooting mode by using this dial on the top of your DSLR

Your camera has many shooting modes. On all DSLRs, you can choose a shooting mode by twisting the dial on top of the camera. You’ll see several icons to describe different situations in which the camera can adjust to choose the settings for you. For example, if you choose an icon for a portrait, the camera will choose a low aperture value so you can get a creamy background. These semi-automatic modes are okay for those who aren’t interested in photography, but if you want professional results, you will need to move into the creative shooting modes 100% of the time.
DSLRs have four creative shooting modes: Program mode (P), Shutter Priority (Tv or S), Aperture Priority (Av or A), and Manual Mode (M).

Program mode (P)  is never used by professional photographers. It is similar to the fully-automatic “idiot mode,” but allows a couple creative choices.

Shutter Priority (S or TV)  has some use if depth of field is not as important to an image as getting a moving object to be sharp without any motion blur. Choosing this mode means that you will choose the shutter speed and the ISO and the camera will choose a matching aperture value to properly expose the image. Some people might choose this for sports photography, where the important thing is the shutter speed and the aperture is not as important.

Aperture Priority (A or AV) is the mode that professional photographers use 90% of the time. Aperture priority mode allows the photographer to choose the aperture and the ISO, but lets the camera find the corresponding shutter speed to achieve a correct exposure. The reason that this is the best mode for all-around photography is that the photographer can pay attention to depth-of-field without being distracted by shutter speed when it isn't as important.

Manual Mode (M)  is very useful in some specialized situations where the other exposure modes cannot accurately capture the photographer’s vision. As a landscape and HDR photographer, I'm in manual mode most of the time, but I would rarely use it to shoot portraits or sports.

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About the Author

Jim Harmer

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Jim Harmer is the founder of Improve Photography, and host of the popular Improve Photography Podcast. More than a million photographers follow him on social media, and he has been listed at #35 in rankings of the most popular photographers in the world. He blogs about how to start an internet business on