How to make sense of ISO, Shutter speed and Aperture combinations

A simple guide using ISO, Shutter speed and Aperture

There are three key reasons why cameras have ISO, Aperture and Shutter speed controls

  • Control sensitivity of camera sensor to light through the use of ISO.
  • Select optimal exposure for current lighting conditions by combing the best Aperture and Shutter speed for the scene being photographed.
  • Apply creative control over use of Aperture (depth of field) and Shutter speed (freeze or blur movement).


ISO is used to control how sensitive the camera sensor is to the available lighting conditions.

Bright light means that we can use a lower ISO setting and have less chance of getting grainy images (noise). When the light is not so bright, we often need to increase our ISO so that we can arrive at a suitable Aperture / Shutter speed combination for our subject, increasing ISO this can introduce noise into our images.

Rule of thumb for changing ISO

In general, we want to keep our ISO as low as possible. When we can no longer arrive at an optimal exposure for our scene or set the desired Aperture Shutter speed combination, then it is time to increase ISO.

Overview of ISO, Shutter speed and Aperture

Bright lighting conditions allows us to use lower ISO settings, faster Shutter speeds and smaller Apertures, shooting in low light might require that we increase our ISO settings to arrive at the best Aperture and Shutter speed combination for our subject or scene. Therefore, the optimal exposure for our subject or scene is a combination of ISO, Aperture and Shutter speed, generally referred to as the ‘Exposure Triangle‘.

Aperture / Shutter Speed

The Aperture is used to control the amount of light coming in through the lens and also controls the depth of field in a photo.

The Shutter speed controls the length of time a shutter stays open. Fast Shutter speeds freeze movement and slow Shutter speeds create blur when shooting moving subjects. When hand holding a camera, slower shutter speeds can lead to blurred images.

Based on our ISO setting and the brightness of the scene, we combine our Aperture and Shutter speed to arrive at an optimal exposure for our scene and our creative intent.

Bright scenes allow us to use

  • Low ISO settings
  • Small Apertures
  • Fast Shutter speeds

Dull scenes often require us to use

  • Higher ISO settings
  • Wider Apertures
  • Slower shutter speeds.

Optimal Exposure and Camera Shooting Modes

Optimal exposure is arrived at through a combination of ISO, Aperture and Shutter speed settings.

Camera shooting modes allow us to control some or all of these settings.

In full Auto Mode (P mode) the camera sets the Aperture and Shutter speed based on the ISO setting and the brightness of the scene.

Semi auto modes are Aperture priority and Shutter priority

  • Aperture Priority – You set the Aperture, based on the ISO setting and the brightness of the scene the camera selects the shutter speed.
  • Shutter Priority – You set the Shutter speed, based on the ISO setting and the brightness of the scene the camera selects the Aperture.
  • Manual Mode – Based on your chosen ISO, you set both the Aperture and Shutter speed to arrive the optimum exposure, either by using your camera light meter or a handheld exposure meter.
Camera Shooting Modes: A= Automatic (set by camera) M = Manual (set by user)

Choosing which mode to use can be confusing, as a guide I would use

  • Programme mode – For general shooting, snapping pics or when you are not too worried about being creative.
  • Aperture Priority – Shooting still subjects and controlling depth of field
  • Shutter Priority – Sports, action or when you need fine control over shutter speed.
  • Manual mode – When you need precise control over every element of your photography and exposure.

A little more about Aperture Priority and Shutter Priority shooting modes

There is a correlation between the Aperture and Shutter speed being used to arrive at an optimum exposure, which is derived from the brightness of the scene and your ISO setting. So, Aperture Priority Mode and Shutter Priority mode work in a similar way.

In Aperture Priority, you select the Aperture and the camera sets the shutter speed, the wider the aperture, the faster the shutter speed, the smaller the Aperture the slower the shutter speed. By varying your aperture, you can have sone control over which shutter speed the camera selects. This works in reverse for Shutter Priority, you can vary your shutter speed to arrive at a given Aperture.

This means that, it does not matter which of these modes you choose to use unless you are looking to be creative with your aperture or shutter speed.

Revisiting ISO – Trusting your camera

Earlier we talked about camera shootings modes, which vary from ‘Fully Auto”, “Semi Auto’ and ‘Manual Mode’, ISO settings can be set manually or automatically too. Automation makes things easier, if you can automate certain camera settings then you are less likely to choose the wrong settings or miss an important moment when shooting.

Setting your camera’s ‘ISO’ to Auto mode and choosing to shoot in either Aperture priority or Shutter priority means that you only need to control one variable in the exposure triangle. This helps you concentrate on your picture taking and not worry about getting your camera settings right.

You only have to worry about noise when you are shooting in very low light conditions and using very high ISO settings, so exploit your camera’s ability to automate things for you.

Learn to love the Exposure Compensation Dial

When people shoot in manual mode, they are looking to control all three elements of the exposure triangle to arrive at an optimum exposure for their creative intent.

As a rule of thumb, it is good to have control of at least one of these elements (Aperture, Shutter speed or ISO) and exploit your cameras ability to select settings automatically. Choose to shoot in Aperture or Shutter priority mode, rather than full Auto Mode. If you need to adjust your exposure, then learn to use the exposure compensation dial. Doing so means that with one button or dial you can lighten or darken your exposure without having to fiddle with camera settings.

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