The Exposure Triangle

Aperture, Shutter Speed, & ISO


The most important thing that an aspiring photographer can learn about is how aperture, shutter speed and ISO function both independently and in relation to one another. Together these three components form what is known as the exposure triangle: any independent change to one of the components of the exposure triangle will change the entire exposure (brightnessof the image. Exposure is just one of the properties of the image that aperture, shutter speed and ISO control; each component has control over their own respective properties which contribute to how an image is composed. 

Most cameras give users various options over how much control they want over their settings, ranging from fully automatic to fully manual. In program auto, the camera decides all the settings for a scene depending on the lighting conditions. In aperture priority and shutter priority modes, the user is able to choose respective aperture and shutter speed settings, leaving the remaining settings to be chosen automatically by the camera depending on the lighting conditions. Manual mode allows for each exposure triangle component—aperture, shutter speed, and ISO—to be selected manually.

While using the manual setting gives complete control over one’s camera settings, it can be daunting to photographers who do not have a solid understanding of how aperture, shutter speed, and ISO operate and affect one another. Below is what I hope will be a helpful guide for those of you who are confused about how the three components work.


APERTURE

Also known as f-stop, aperture is to the camera what the pupil is to the eye. Aperture is the size of the hole in the lens that allows light into the camera. Aperture is represented as f/N, where N is the f-number (e.g. f/1.4, f/11, f/22).

The terminology surrounding aperture can be a bit confusing. Large apertures are represented by small f-stop numbers (e.g. f/1.4), and are characterized by a larger opening in the lens, which lets more light into the camera. Small apertures are represented by large f-stop numbers (e.g. f/22) and are characterized by a small opening in the lens, which lets less light into the camera.

Aperture controls the depth of field in a given image. Depth of field refers to how much of the spatial distance within the image is in focus. The larger the aperture, the shallower the depth of field will be (e.g. sharp subject and blurry background). As the aperture becomes smaller the depth of field becomes deeper (e.g. a vast mountainous landscape that is all in-focus), but after a certain aperture the image will appear less sharp due to diffraction. Diffraction will happen at different f-stops depending on the lens being used, but it is most commonly noticeable beginning around f/16 or f/18.

Camera lenses are all sold with an aperture listing that represents the largest aperture that the lens can achieve. This number can either be fixed (e.g. 24-70mm f.2/8) or it can be a range (e.g. 100-400mm f/4-5.6). A fixed aperture on a zoom lens (a lens with a variable focal length, e.g. 24-70mm) means that the lens can sustain the aperture across all focal lengths of the lens. A ranged aperture on a zoom lens means that as the focal length increases (“zooms in”), the lens will slightly narrow its widest possible aperture.  The larger the maximum aperture of a lens, the more expensive it tends to be.

The use of aperture will vary for the type of photography that is being shot. Portrait photographers will usually shoot close to “wide open” (largest aperture available) in order to achieve optimal separation between subject and background. Landscape photographers will typically “stop down” (reduce their aperture) to around f/8 – f/11 in order to achieve a sharp image with a large depth of field with minimal diffraction. There are also scenarios where landscape photographers may shoot wide open, such as for astrophotography where the camera needs to gather the largest amount of light possible during a certain time frame,  or fully stopped down to f/22 when the sun is present in order to create a sunstar effect through lens diffraction.


SHUTTER SPEED

Shutter speed controls the appearance of motion in an image. Fast shutter speeds are represented by small fractions (e.g. 1/400, 1/8000), and are characterized by seemingly frozen motion in the shot, such as of a water balloon bursting with the tiny water particles frozen in time. Slow shutter speeds are represented by large fractions or whole numbers (e.g. 1/20, 2”, 30”), and are characterized by blurring of motion, such as a silky waterfall or light trails from car headlights on a highway.

Like with aperture, shutter speed also controls the amount of light that is received by the camera. The longer the shutter stays open, the more amount of light will be received.

The ideal shutter speed varies depending upon what is being photographed. For instance, sports and wildlife photographers often use fast shutter speeds in order to freeze the quick motions of their subjects. Astrophotographers and landscape photographers who enjoy shooting long exposures will use slower shutter speeds coupled with tripods in order to collect more light or to achieve more blurring of motion from the elements in the scene.

Unintended motion blur can be a big source of frustration for many photographers. Unintended motion blur is primarily caused by one of three reasons: 1) the shutter speed was not fast enough to freeze the motion in the shot, 2) the camera was not held steady enough during the shot due to shaky hands or the absence of a tripod, or 3) the subjects (e.g. people, animals) were not standing still enough during the shot.


ISO

ISO is a numerical value that reflects how bright an image will be. ISO is controlled in camera, and can range from as low as 50 to over 4,000,000. Every time ISO doubles in value, the photo becomes twice as bright (e.g. 50 to 100, 3200 to 6400). Unlike aperture and shutter speed, ISO does not affect how much light the camera physically captures.

While shooting higher ISOs will naturally brighten the scene, it will also add more noise (graininess) to the image and will reduce the camera’s dynamic range (ability to capture highlights and shadows).

In general, ISO should be kept as low as possible in order to achieve minimal noise and the greatest dynamic range. Even in the dark, it is best to use a tripod and a low ISO so long as there isn’t any motion that will be unintentionally blurred in the frame.

There are many circumstances however where there is no alternative to using a higher ISO. One of these circumstances is in wildlife photography, provided a scenario such as where a photographer is trying to capture an owl flying through the woods at night. The photographer may be using an f/2.8 aperture and needs a shutter speed of 1/640 or faster to freeze the motion of the owl. Shooting at a low ISO like 100 would not be possible here because the image would appear too dark, and using a slower shutter speed to gather more light would result in a blurry image of the owl's movement. In this scenario, a higher ISO of 3200 or greater may need to be used so that the image is bright enough while the owl's motion is still frozen. ISO should only be raised in order to achieve optimal brightness when circumstances prevent aperture or shutter speed from being able to achieve said brightness. 

Higher ISOs are used in every branch of photography, but they are most common during nighttime shooting  or when lighting conditions are poor.


SUMMARY

Aperture = measured by f/stop values, controls depth of field.

Large aperture = low f/stop number (f/1.4), shallow depth of field, large amounts of light let in.

Small aperture = high f/stop number (f/16), larger depth of field, small amounts of light let in.

Images will usually be less sharp after f/16 or f/18.

Astrophotography = use largest aperture available.

Sunstars = use f/22.


Shutter speed = measured by seconds or fractions of seconds, controls appearance of motion.

Fast shutter speed = low fraction (e.g. 1/800, 1/4000), motion is frozen, less light comes in.

Slow shutter speed = small fraction or whole number (e.g. 1/4, 8”), motion is blurred, more light comes in.


ISO = controls brightness of image.

Lower ISO = darker image, less noise, higher dynamic range.

Higher ISO = brighter image, more noise, lower dynamic range.

Only use higher ISO when aperture and shutter speed cannot be used to correctly expose an image.

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