Updated: Oct 24, 2020
THE EXPOSURE TRIANGLE
ISO, Shutter Speed and Aperture – the three pillars of photography.
A thorough understanding of these three essential elements of photography is key to taking great pictures. While there is nothing fundamentally wrong with relying on the automatic modes of your camera, these modes aren’t foolproof and don’t allow you to take full creative control of your camera and overcome exposure problems.
There are two fundamental ways to control how much light reaches the sensor of your camera - the aperture in your lens (like the iris of your eye) and the shutter in your camera (like your eye-lid or curtains shutting).
This is how sensitive your camera’s sensor is to the available light.
ISO stands for International Standards Organisation. It is the standard industry scale for measuring sensitivity to light.
A lower number such as ISO 100 represents lower sensitivity, while a higher such as ISO 1600 means higher sensitivity. If you change your ISO from 200 to 400, you’re making your sensor twice as sensitive to light. The important thing to remember here is that the higher the sensitivity the higher the noise and grain of your images - and the lower the quality of your images. Keeping your ISO to the lowest possible number e.g. 100 is therefore desirable in most cases.
As you increase your ISO (sensitivity) you need less time to properly expose your image and can therefore use faster shutter speeds. For example, increasing ISO from 200 to 800 will allow you to shoot at faster shutter speeds, which is especially useful in low-light photography to keep your photos sharp and un-blurred.
So, to reiterate, the ISO setting allows you to control the sensitivity of your camera’s sensor to light.
Lower ISO settings (e.g. 100, 200, 400) = make your sensor LESS sensitive to light. Lower settings can be used for bright conditions or when you’re using a tripod to support your camera.
Higher ISO settings (e.g. 3200, 6400) = make your sensor MORE sensitive to light. Higher ISO settings also make your shutter speed faster – which is useful if you want to freeze movement and avoid blurry images. This is particularly useful if you’re shooting handheld, in low-light, without a tripod.
Higher settings are generally used when you’re hand holding your camera in low light and require a fast enough shutter speed to achieve a sharp picture.
Remember that ISO and your shutter speed are directly linked. Every time you increase or decrease your ISO by 1 stop (doubling or halving) e.g. 100 to 200 your shutter speed will also increase or decrease by 1 stop e.g. 1/60th second to 1/120th second.
The downside of higher ISOs – Noise & Grain
Remember that as you increase your ISO you generally diminish the quality of your images by introducing noise. Noise comes in two forms: Grain and Colour. There comes a point when your images will be too blotchy and grainy to make them useable. Therefore, try to always aim as close as you can to ISO 100 to get the best possible quality – although remember that it’s always better to get a sharp image with a bit of noise than no image at all!
The benefit of Auto ISO
This setting lets your camera automatically choose the lowest possible ISO it can. The great advantage of this is that your camera will ensure fast enough shutter speeds to avoid blurry images and camera shake problems. This can be very useful in low light.
ISO and Low Light Conditions
If you have to shoot handheld on a dark, dull day or in very low light, you may have no choice but to shoot at higher ISO settings and introduce noise to your images. This is something you may just have to acccept – or use a tripod.
The shutter speed is the length of time the shutter is open for to allow light into the camera sensor. You can think of the shutter as curtains which open or close, or as an eyelid which blinks.
The difference between adjacent shutter speeds is known as a ‘Stop’. Each stop represents a halving or doubling of the light reaching the sensor. As the shutter speed is made faster, the shutter is open for less time and vice versa.
A slow shutter speed allows more light into the sensor (typically used in low-light), while a faster one allows less, helping to freeze any action e.g. flying birds, racing cars or athletes. You might use a slow shutter speed at night e.g. 1-30 seconds to allow more light in and capture your subjects and a fast shutter speed during the day for sports or fast moving animals e.g. 1/1000 or 1/2000 of a second to freeze the action.
The aperture is the hole within your lens through which light travels to the sensor in your camera. You can control this hole to make it larger or smaller and therefore control the amount of light reaching the sensor. The larger the hole (aperture), the more light that passes through.
Aperture operates a bit like the iris of your eye. The larger the hole (aperture), the more light that passes through.
Aperture can be adjusted in a set series of sizes known as ‘f-stops’ (expressed in ‘f’ numbers and known as the ‘focal ratio’). A typical lens will have a range of f-stops ranging from f2.8 to f16 and beyond. Each f-stop in the range represents a halving or doubling of the amount of light reaching the camera’s sensor compared to the nearest value. For example, the camera sensor receives half the light at f5.6 as it does at f4.
As well as regulating the amount of light into your camera, aperture also controls your depth of field (the amount of the scene that appears sharp in your picture).
Smaller apertures like f11 or f16 mean less light to the sensor - but a larger depth of field meaning more of the scene is in focus.
Larger apertures like f2 or f4 means more light to the sensor, but a smaller depth of field and blurred backgrounds.
BALANCING THE THREE FOR A GOOD EXPOSURE
Aperture, shutter speed and ISO are all linked. Adjust one and at least one of the other two must also be changed to maintain the same level of exposure. To properly expose an image, you need to balance the shutter speed, aperture and ISO.
To illustrate this, lets take the example of shooting in broad daylight, when lots of light is entering the lens, with the ISO set to a sensitivity of 100:
1. SMALL APERTURE (HOLE) E.G. F11-22 = LOTS OF LIGHT GETS BLOCKED = SENSOR NEEDS MORE TIME TO COLLECT LIGHT = SHUTTER NEEDS TO STAY OPEN LONGER
So, given a constant ISO OF 100, choosing a smaller aperture (hole) means a longer shutter speed for the sensor to have enough time to gather enough light to make a good exposure.
2. LARGE APERTURE (HOLE) E.G. F2 or F4 = MORE LIGHT ENTERS = SENSOR NEEDS LESS TIME TO COLLECT SAME LIGHT = SHUTTER DOESN’T NEED TO STAY OPEN AS LONG
So, a larger aperture means a shorter shutter speed for the sensor to gather enough light to make a good exposure.
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Author: David Wheater
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