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Understanding the Settings on Cameras: Shutter Speed, F-Numbers and Exposure

Eugene is a keen amateur photographer, primarily interested in nature. He has a portfolio of images on YouPic.

This short guide covers the basics of how an image is formed in a camera and an explanation of the various controls on smartphones and SLR cameras that control proper exposure of an image.

SLR Cameras, Compact Cameras and Smartphones

All the details below apply to both cameras and smartphones. Cameras will have knobs and buttons for changing the settings, whereas camera apps on smartphones will allow you to change these settings in the menus.

How Does a Camera Work?

A camera works by focusing light from the subject (the thing or person you are photographing) onto either photographic film or an electronic sensor known as a charge coupled device (CCD), located just inside the back of the camera. Light firstly passes through the lens at the front of the camera, then through an aperture (hole), and finally through a shutter before finally landing on the sensor or film. When a photo is taken, the shutter opens for an instant to allow light into the camera and create a snapshot in time of the scene. The function of the lens is to gather light and create a focused image at the focal point, on the CCD or film.

A convex lens focuses an image

A convex lens focuses an image

How Does Exposure Work on a Camera?

Irrespective of whether photographic film or a sensor is used in a camera, a certain amount of light must land on the sensitive element. There can't be too much or too little This is because the film or CCD element has a limited dynamic range. This means it can only work over a limited range of illumination levels. Unfortunately, there can be huge variations in the illumination of a subject depending on whether photos are being taken in dim light or bright sunlight.

There are two ways of controlling how much light is allowed into the camera, known as the exposure.

  • Vary the amount of time for which the light enters the camera
  • Vary the size of the hole through which it passes
  • Use different speed films or ISO settings on a digital camera

So how do you know which to vary? Most modern cameras nowadays provide you with auto exposure. However, you still need to understand the consequences and effects of varying shutter speed and aperture size. Depending on the type of camera, there may be no control, little control or a lot of control over exposure settings. Cameras often have several exposure modes (e.g. "sports", "children", "nighttime") which try to automatically adjust exposure and other settings to take the guesswork out of taking photos and make an image appear ok. Which mode you use depends on the application.

What Is an Aperture on a Camera?

This is a variable sized hole behind the lens, through which light passes. Basically, it works just like the pupil in your eye. Changing the "f-stops", "focal ratio" or "f-numbers" setting on your camera results in an alteration of the diameter of the aperture. Large "f" numbers correspond to a smaller hole and less light entering the camera. Small "f" numbers correspond to a larger hole which lets more light in. However, the drawback of a large aperture is a reduction in depth of field or range over which objects are in focus. This may or may not be advantageous as we will see below. Typical f stops on a lens are f/2.8, f/4, f/5.6. On a smart phone, the aperture is often fixed in size, so you can't change it in the camera app.

For a more detailed discussion, see f-number on Wikipedia.

The iris aperture at the back of the lens from an SLR camera.

The iris aperture at the back of the lens from an SLR camera.

Showing the effects of depth of field in an image. a large f-stop was used for the image on the left, resulting in a smaller depth of field.

Showing the effects of depth of field in an image. a large f-stop was used for the image on the left, resulting in a smaller depth of field.

What Is the Shutter Speed Setting for on a Camera?

The second way of varying the exposure is by altering shutter speed. Shutter speeds can be varied from tens of seconds to fractions of a millisecond. So why not have a really slow shutter speed to let in lots of light in dim lighting conditions? The problem with low shutter speeds is that when attempting to capture images of moving subjects, the resulting image suffers from motion blur. This is because the shutter is open for such a relatively long time, that the image formed at the back of the camera actually varies because of the motion of the subject. Using a fast shutter speed, allows you to freeze motion. (In olden times, photographic plates, the precursor of rolls of film, were so insensitive that the plate had to be exposed for several minutes, so if the subject was a person, they had to keep still for this length of time. Hence the stiff poses).

Shutter speeds for lens at normal magnification (non-zoomed)

Type of subjectMovement left to right in front of the camera

Traffic and Pedestrians


Slight or stationary movement


Fast Vehicles




What Does Film Speed and the ISO Setting Mean?

A third way of increasing exposure is to use a faster film speed or the equivalent for digital cameras. A fast film is more sensitive to light, which allows a faster shutter speed or higher f-stop (smaller aperture) to be used than normal. It is advantageous in situations when light levels are low, the aperture cannot be made any bigger, but a fast shutter speed must be used, reducing the amount of light entering the camera (e.g sports photography). Also if you use a long focal length or zoom lens (which is often the case when photographing subjects in sport), the upper f-stop limit will allow less light into the camera than when zoomed out. This may limit the fastest shutter speed to an unacceptable level, so this is a situation when you can increase the film speed setting. This is normally indicated as "ISO" or "ASA" in the setup of your camera. The drawback of a faster film is a "grainier" or sandy looking image.

Exposure Modes on Cameras

SLR and compact cameras usually have 3 basic modes for automatically setting exposure. Not all smartphones have these settings and the aperture may be fixed in size. On an SLR, there is typically a knob for selecting the exposure mode.

Shutter Priority (S)

You set the shutter speed and the camera then varies the aperture size for correct exposure. If you want to freeze motion, a fast shutter speed is necessary. If you choose too high a shutter speed in low-light conditions, the camera may select a large aperture (small f-stop), resulting in a decrease in depth of field, which could be an issue when close to a subject or zoomed in. Also the chosen shutter speed may be too high in dim light (or too low in bright conditions) to give proper exposure. So even though the camera automatically selects the largest aperture possible (smallest f-number), it still can't get enough light in to expose the image properly. A camera will indicate this error with arrows or a bargraph, telling you you need to select a slower shutter speed for proper exposure.

Aperture Priority (A)

You set the size of the aperture to control the depth of field (see discussion of depth of field in Tip 3 below). The camera then changes shutter speed to give correct exposure of the subject. If you want a large depth of field in your image, you can make the aperture small. However, if you choose too small an aperture (large f-stop), it can result in an unacceptably slow shutter speed to freeze motion. This isn't so much an issue of a camera is on a tripod. Also an aperture which is too big or too small may result in overexposure or underexposure in very bright or low light conditions respectively. Again the camera will indicate over/underexposure so you that you can make corrections.

Manual Mode

You can change aperture, shutter speed and ISO. The camera doesn't alter the settings, irrespective of light levels

Program Mode (P)

Both shutter and aperture are adjusted. So in lower light conditions, shutter speed is decreased and aperture made bigger (and vice versa under brighter lighting) to give optimal results. The result is that neither is changed as much as they would have been in "A" or "S" modes. This mode is normally indicated by 'P' on the mode selection dial.

Scene Modes

Newer SLR cameras often have lots of other modes which take the thinking out of taking a photo so that you can just point and shoot. These include an Auto mode which optimizes everything, including setting the aperture, shutter speed, ISO, focusing controls and turning on flash if needed. Other modes optimise the settings for the particular subject. These include a macro mode, portrait mode, child mode, sports mode, landscape mode etc. Smartphone apps often have a few different modes for the type of scene.

Exposure and scene mode setting knob on a digital single lens reflex camera (DSLR). 'A' is aperture priority and 'S' is shutter priority.

Exposure and scene mode setting knob on a digital single lens reflex camera (DSLR). 'A' is aperture priority and 'S' is shutter priority.

Further Reading

10 Tips for Taking Better Photos for Your Website

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Eugene Brennan


Liza from USA on February 05, 2021:

Yes, a camera can be expensive! Today, I went to Best Buy to browse some cameras. I found a few that I am interested in. So far, I got my eyes on Canon - EOS Rebel T7 DSLR video camera with 18-55mm Lens. I might have to spend a little bit more if I want something that has an excellent lens and built-in wi-fi as well.

Eugene Brennan (author) from Ireland on February 05, 2021:

Thanks Liza. I have to remind myself of these details too every so often. I recently purchased a Nikon D5300 which is nice to use (button controls for changing settings on menus, but there's a more expensive version with a touch screen). It's not so good apparently though for shooting video because it's too slow to autofocus.

Liza from USA on February 05, 2021:

I am looking forward to purchasing a new camera. I have an old Nikon DSLR camera but, I am an avid iPhone user as well. Right now, iPhone is my best device whenever I want to take photos. However, I want to take professional food photos, so I need a conventional camera. Your article extended and refresh my familiarity with camera function after neglecting it for a while.