Understanding your digital camera’s manual settings for image exposure: Shutter speed, aperture and ISO sensitivity
1 November 2017 § 1 Comment
The three primary settings in a digital camera are shutter speed, aperture and ISO (light sensitivity). Before diving into the advanced picture controls and techniques, it is a good idea to learn these photography basics. This article explains in detail how changing their parameters affects the final image in terms of light, motion, focus and noise grain.
A basic guideline rule for good image exposure
On a bright sunny day, when taking a picture of a landscape at ISO 100, aperture f/8 and shutter speed 1/125, the result is a well-exposed image with most of the objects in focus. If we want to change one of these parameters up or down to use it for effect, we would need to change another one with the same number of steps to keep the same image exposure. This is the guideline referred to in most photography books and tutorials from the times of film photography. Back then, light metering before the shoot and setting the right exposure was the only way to get well exposed pictures.
But what do all these settings mean?
Controlling the shutter speed
The shutter is the part of the camera which controls how long the film or sensor would be exposed to the light coming from the lens. The slower the speed, the more time the sensor is exposed to the light; the faster the speed – the less time the shutter is open. Shutter speed is measured in seconds, or parts of a second – such as 1/125, 1/60, 1/30, 1/1000. Most consumer cameras today support speeds from 30 seconds to 1/2000th of a second. So in the initial example, when set at 1/125, the shutter allows light to expose the sensor for 1/125th of a second.
Let’s set the three parameters to ISO 100, aperture f/8 and shutter speed 1/125 and see how changing them up or down creates a different effect for the final image.
Shutter speed affects two main aspects in photography: the amount of light captured, and movement.
Shutter speed and light
With ISO 100 and aperture f/8, when taking photos on a sunny day and the object is well lit, the shutter speed usually needs to be set somewhere between 1/125th and 1/640th of a second. If set to a slower speed – say, 1 second exposure time, the sensor would be overexposed, the photo would become too bright and most of the detail would be lost. The photo would contain too many white areas, and even the darker objects would look too bright. If set to a higher speed than needed – say, 1/2000th of a second or higher, the sensor won’t be able to collect enough light, so the objects would look too dark, and most of the photo would be black. Again, the detail would be lost.
Using different shutter speed settings for capturing light (while keeping ISO 100 and f/8)
One example of using slow shutter speed is when photographing cityscapes at night. Since the light is not bright enough, a slower shutter speed – usually over 1 or even 2 seconds – helps expose the sensor with the correct amount of light.
On the contrary, when shooting on a bright sunny day in the snow or at the beach, when the shutter speed is set between 1/500 and 1/1000, the image would be exposed properly and most of the details from the scene would be preserved.
Shutter speed and motion
Often forgotten, camera shake is a common problem to keep in mind when planning a shot. If the shutter speed is set to less than 1/60, even the slightest movement while releasing the shutter could result in a blurred image. To avoid that, photographers often use tripods or monopods to keep the camera steady. Another solution is to increase the shutter speed to something faster than 1/60.
A lot of the time the object itself is moving – sometimes it is a moving car or a group of people, other times it is a waterfall or a windy field.
Motion can be used to achieve various effects – for example, when the photographer needs to ‘freeze’ the moving object in the image so it doesn’t look blurry. In this case, a faster shutter speed should be used. Another option is to capture the motion blur of the object in order to emphasise on the movement – a slower shutter speed is the answer. There is another, third option that needs more practice and maybe a dolly, but is definitely worth trying. If the object is moving, the photographer or camera can move in parallel (this is known as motion tracking) – so the background looks blurry, but the object remains crisp.
Controlling the aperture and getting to know the f/numbers
The aperture (or iris) affects the amount of light that reaches the sensor and the area of the image that is focused. The iris of the camera follows the same basic principles as the human iris.
Aperture and light
When the iris is open wide, it allows more light to reach the eye (or in our case: the sensor). The f/numbers are used to control and signify exactly how wide it is open. The opening is measured in a fraction, since a fully open lens is typically not possible. So if the lens is open halfway, this is represented by f/2. The number represents a ratio of the full lens opening size. This is why, f/1.2 is a very open aperture, allowing a lot of light in, while f/16 means the aperture is almost closed, allowing very little light in. The smaller the number for the denominator, the wider the iris is open and the more light it allows.
Aperture and depth of field
Depth of field (often referred to as DOF) is the area of the image that is focused. It depends on the focal length of the lens and the f/number. The higher the f/number, the wider this area is – so for example, if taking a picture of a flower, setting the f/number to f/16 would result in both the flower and the background being focused. But if the number is set to f/1.2, only the flower (or even part of the flower) would be focused, and everything in front and behind the flower would be blurred.
Light sensitivity, ISO and noise grain
The sensor is the part of the camera that captures the image. Its sensitivity to light can be controlled through the ISO settings. The lowest sensitivity setting in modern cameras is usually ISO 100 or ISO 200. When set to a higher setting, the sensitivity is electronically amplified, and the captured image becomes brighter. But since this is an amplification process, it leads to digital noise. The higher the ISO number, the more amplification is applied to the light sensitive sensor – resulting in more noise grain.
Choosing the correct shutter speed, f/numbers and ISO settings in different light conditions
Remember the basic guideline example? Bright sunny day, ISO 100, aperture f/8, shutter speed 1/125.
The settings for most light conditions can be easily calculated as a variation of this guideline. Whenever a parameter is changed with one step, another one needs to be changed in the opposite direction to compensate for the change in light coming in and to keep the exposure correct.
If the camera doesn’t allow ISO settings as low as 100, then one of the other parameters needs to be changed too. Using ISO 200 would mean higher light sensitivity – so either the shutter speed should be increased from 1/125 to 1/250 to control the amount of light, or the f/number should be changed from f/8 to f/11.
If the weather is very cloudy, it would mean less light going through the lens – so in order to keep the image well exposed, the shutter speed needs to be lowered or the f/number changed for a wider aperture. In this case, lowering the shutter speed would mean higher chance of camera shake affecting the image. The other option – opening the aperture – would make the depth of field more shallow. Sometimes this is the best solution – especially in portrait or macro photography. Opening the aperture would result in the background becoming more blurry and the main object standing out as the only thing that is focused. Changing the f/number would also keep the shutter speed in a safe setting – so it lowers the risk of shaky camera.
Working with this guideline helps photographers spend less time on the technical side and more in the creative process. Because however well-exposed a photo is, it is much more important what story it tells. When you use this guideline, you will free your mind from figuring out exposure parameters and will be able to focus on your ideas, experiment with different composition techniques, testing the limits of your camera and your creativity.