ISO, Aperture, and Shutter Speed Explained

Exposure is an important element in photography that affects the quality of what is recorded on a camera’s image sensor.

Three elements can be adjusted to control exposure: ISO, Aperture and Shutter Speed.

The Exposure Triangle

ISO SpeedThis is the camera’s light sensitivities. Each value represents a “stop”, and each increment of ISO (up or down), represents a doubling of or halving of that sensor’s light sensitivity.

ApertureThe lens’ diaphragm controls how much light travels through it to the film plane. Each f number represents a stop of light. The aperture setting is indicated with the f-number.

Shutter SpeedThis indicates the speed at which the curtain opens and closes. This indicates how long the light can reach your sensor after you press the shutter-release key. A shutter speed value is also a stop of light. The shutter speed can be measured in fractions per second.

The Exposure Triangle

These three elements can be combined to give a given exposure (EV) in a given setting.

Any changes in any of these three elements will have a direct and measurable impact on how they react to expose the film frame, or image sensor, and ultimately how the image looks.

You can increase the f stop by, for example, decreasing the size of your lens’ diaphragm. This reduces the light hitting the sensor and also increases the DOF depth of field in the final image.

The shutter speed can affect how motion is captured. This can lead to blurred backgrounds or subjects. But, by reducing shutter speed (keeping your shutter open longer), you can also increase the amount of light hitting your image sensor, making everything brighter.

You can shoot in low light conditions by increasing the ISO. However, this will increase the digital noise in your photo. It is impossible for one element to be changed independently of another, and have the opposite effect on how the other elements affect the image. This will ultimately impact the EV.

ISO Speed

ISO Sensitivity

ISO stands for International Standards Organization. This is the organization that standards sensitivity ratings for cameras.

The specific light sensitivity is indicated by the ISO rating. It can range in value between 25 and 3200 (or more).

The ISO rating is a measure of how sensitive an image sensor is. This makes the image more smooth because there is less digital noise.

Higher ISO ratings (more sensitive) mean that the image sensor must work harder to create an effective image. This results in more digital noise (those multicolored speckles in shadows and midtones).

What is digital noise, you ask?

It’s any light signal that doesn’t originate from the subject and creates random colors in an image. The image sensor was designed by digital camera engineers to work at the lowest ISO, just like film.

This setting is ISO 100 on most digital cameras, but some DSLRs with higher end features have ISO 50 or 25.


Large vs. Small Aperture

The aperture of a lens is the opening in its diaphragm that controls the amount focused light passing through it.

A small f-stop (e.g. f/2), allows a lot of light to pass through. However, at f/22 when the diaphragm may be at its smallest, very little light is allowed in, even at slower shutter speeds.

The interesting thing about aperture and f-numbers, is that the focal length of the lens doesn’t matter as long as the constant f number is maintained. The arithmetical equation which determines the aperture’s f-number shows that the same amount light passes through a lens with a focal length of 35mm as it does on a 100mm lens at a shutter speed 1/125s.

While the diaphragm’s size is different, the amount of light that passes through it is the same.

Shutter Speed

Shutter Speed Comparison

Shutter speed can be measured in fractions per second and is used to indicate how quickly the curtains at the film plane close and open.

The shutter speed is the time that light enters the lens, hits the film plane or image sensor. You can capture the entire world in a split second, but the shutter speed allows you to absorb it at speeds up to three or four seconds. Or the curtain can remain open until the photographer closes it.

You can control how much motion is captured by snapping the shutter in fractions of a second. The shutter speed should be faster than the background or object to ensure sharp images. You’ll see blurred objects if your shutter speed is slower.

Consider the speed at which rain falls in a storm. At 1/30th, the raindrops appear streaks of undistinguishable white. At 1/250th the raindrops hover in mid-air and you can clearly see the full swelling of each drop.

What is “Auto Bracketing?”

Auto Bracketing

Auto Bracketing allows you to ensure the best exposure by taking at minimum three (3) exposures with the exact same composition, one at the metered Ev, one at 1/30 of a halt below the metered Ev and one at 1/33 of a stop above the metered Ev.

Auto Bracketing is when you set the EV value and release the shutter. The camera will automatically adjust the EV to give bracketed exposures. You can then review the three or more exposures and see the subtle differences, which are crucial, and choose the best one for you.

The three images shown in the example above might be your favorite. It is because the setting sun shines brightest.

Bracketing is a technique that was developed from the slide film photography. It was due to the limitations of the darkroom’s ability to fix the image. The technique is still used by many photographers today to get the exposure they desire.

The three images in brackets reduce the post-processing time they may need.

Overexposure & Underexposure

Exposure Comparison, Overexposure to Underexposure

What is the best way to define overexposure or underexposure?

Overexposure simply means that the highlights are rendered unreadable. This type of loss is irreparable and it is impossible to retrieve the missing image information from the digital darkroom.

Subexposure is essentially the same concept, except that there is no image information within shadows. Post processing cannot retrieve this non-existent information.

Digital photography is not possible to retrieve image information once it’s gone.

In the world of film photography, this is not always true. Film processing is more efficient than digital. It is possible to “find” image information in a frame that is too dark, or to print information for images that are severely overexposed.


Auto Exposure Lock allows you to lock the exposure value in your camera settings (when you are shooting in one of the semi-automatic, fully automatic modes such as. Shutter priority

This mode allows you to maintain the same EV regardless of any lighting changes in the scene.


Digital photography has a great practical advantage. You don’t have to spend anything to play around with the controls. So get out there and start shooting.

You should become more proficient in all three elements of exposure triangle to make it easier to adjust on the fly and to know the effect.