The Three Key Elements of Exposure
Shutter Speed, Aperture and ISO Explained
It could easily be argued that exposure, along with focus, is the most important element of every good photograph. And we tend to agree with this point of view; you could capture the most interesting subject at just the right angle and at the most perfect moment and still end up with a photograph you can’t use, because it’s been over- or underexposed. Sure, some fiddling with RAW files in Adobe Lightroom might help, but not if you have clipped highlights or shadows where there is zero information to be extracted.
That is why it’s so crucial to understand the three key elements of exposure: shutter speed, aperture and ISO. Once you learn how each of these plays a role in exposure, and how they interplay with each other, you will be all set to take great photographs which require minimal interventions, at least when it comes to exposure. Even though today’s cameras are pretty smart, simply choosing the Auto Mode and letting your camera make all the decisions is not a guarantee that you will end up with the best possible image. Keep on reading to find out all you need to know about shutter speed, aperture, and ISO, and how they each affect the final image exposure.
What Is Shutter Speed?
Simply put, a shutter is a part of your camera which determines the period of time for which the camera sensor is exposed to the light. The longer the shutter stays open, the more light will hit the sensor.
You will usually find that this time interval is referred to as shutter speed, and is usually measured in fractions of a second, such as 1/125s, although slower shutter speeds that are measured in seconds, minutes, or even hours are not all that uncommon, especially in long-exposure photography.
Pro Tip: Shutter speeds that are slower than 1/60 or 1/90s require a tripod for stabilisation. The slowest shutter speed you can get away with without camera blur or shake also depends on the focal length of the lens. For instance, if the focal length is set at 200mm, you would use a shutter speed no slower than 1/200s
For example, shutter speeds of 1/4000s will allow for very little light to hit the camera sensor, whereas shutter speeds of two or three seconds will let in a lot of light. Of course, how much light you need for correct exposure depends on the shooting conditions. In addition to controlling the length of time the sensor is exposed to the light, you can also use shutter speed in order to infuse your images with a sense of movement. In other words, you can freeze your subjects in time, or you can intentionally create blur to make them appear faster.
For instance, if you want to photograph a speeding motorcycle on a track, you could use a shutter of 1/60s in order to make it blurry. But, if you want to freeze it when it’s crossing the finish line, a shutter speed of 1/1000s would work a lot better. One is not necessarily better than the other. Shutter speed not only depends on shooting conditions, but also the artistic effect you are trying to achieve.
What Is Aperture?
Aperture is an opening inside the lens of the camera which can be made wider or narrower in order to control the amount of light hitting the sensor. Whereas shutter speed is measured in seconds, aperture is measured in f-stops, and you will often see them listed as f/1.6 or f/16, for example.
This is where it gets a bit tricky: the larger the f-stop number, the smaller the opening of the aperture, and vice versa. If you need more light for correct exposure, you would choose a smaller f-stop number, such as f/1.6. If you need more, then opting for a larger f-stop number, like f/16, would be the way to go.
Fun Fact: Aperture works in much the same way as the iris of your eye does. Your iris contracts itself, allowing the pupil to become smaller and let in less light, and vice versa.
Apart from using it to control the amount of light captured by the sensor, aperture also allows you to manipulate the depth of field on an image. In other words,the depth of field determines how much of your image will be in focus. If you are looking for a large depth of field, which means that both your background and foreground will be sharp and in focus, you should choose a smaller aperture. However, you may not always want a large depth of field, especially if you are looking to isolate your subject from the background or foreground. In that case, you would need a larger aperture to create a shallow depth of field.
For example, if you are interested in landscape photography, where it’s preferred to have the entire photo, including both the foreground and background, in focus, you would use a smaller aperture. For portraits, however, you would do the exact opposite and choose a larger aperture in order to make your subject pop, as well as to create a blurry background, which is also known as “bokeh”.
What Is ISO?
Unlike shutter speed and aperture, ISO doesn’t determine the amount of light captured by the sensor. What it does is control the sensor’s sensitivity to light. The higher the ISO number, the more sensitive the sensor, and the lower the ISO, the less sensitive the sensor.
Why would you want to change ISO in the first place? Well, there are plenty of situations in which you might need a large depth of field, for which you need a smaller aperture, and fast shutter speed at the same time. This means the sensor will capture very little light, so in order to achieve the desired exposure, you would simply increase the ISO, which would result in a brighter image.
A useful way to remember this is by imagining a microphone plugged in to a PA system. The louder the singer, the less amplification the voice needs, and vice versa.
However, there is a trade-off with ISO: noise. Noise is something you generally want to avoid in your photographs. If you really want to go for that grainy, film-like quality, it’s usually recommended that you add it in post-production. It helps to think of ISO as a distortion on an electric guitar amp. When you turn the gain knob all the way up, your guitar will be louder, but the sound will also be more distorted. Without gain, you get a clean sound. The exact same principle can be applied to your image and ISO, only instead of distortion, you get noise.
Ideally, you should keep your ISO as low as possible, which is usually around 100 on most cameras, and use shutter speed and aperture to control the exposure. However, in low-light conditions, for example, sometimes the only option will be to raise the ISO. How much? If you want to double the brightness, you double the ISO, it’s as simple as that. You will have to experiment with your camera in order to determine the maximum ISO value which still produces quality images. Whereas some cameras can capture great images at ISO 5,000 or higher, others will produce a lot of noise even at ISO 2,000.
The Exposure Triangle*
As you can see, shutter speed, aperture, and ISO are the three most important elements when it comes to image exposure, with none of the three being a one-size-fits-all type of solution, since each has its advantages and its flaws.
Achieving the correct exposure is always the result of a compromise among these three. Sometimes, you will need your aperture to remain fixed, which means you will have to balance the two other elements to properly expose an image.
The same goes for aperture. Achieving the correct exposure should always be done with those two. Only after you have exhausted that option should you raise the ISO.
Even though this might seem like a lot to think about each time you take a photo, don’t stress, because you will only have to consider all three if you are in full Manual mode. In most cases, you will rely on some of the automatic or semi-automatic modes which allow you to control one or more options, while the camera calculates the rest for you. For example, if you want to control the depth of field, you would choose the Aperture Priority mode (A or Av on most cameras) and select the desired aperture, while the camera calculated the shutter speed. In Shutter Priority mode (S or Tv), you get to choose the shutter speed, while the camera adjusts the aperture.
With these semi-automatic, you can usually select ISO manually in order to compensate for lack of light, but it also means you need to start worrying about not making your image too noisy. The only real way to get good at achieving the correct exposure would be to take your camera and simply practice as much as you can, until it becomes second nature to you.
*Purists will tell you that Exposure Triangle isn’t an accurate term, and technically they are correct. ISO doesn’t control any aspect of the original exposure, or light coming in to the camera; it only boosts the signal after the fact. Remember the microphone analogy at the beginning of the article.
Even though you can get good photos by relying on automatic modes of your camera, you will become a better photographer if you take the time to learn about all the elements of exposure. Your camera, while advanced, is not perfect. The more control you have over your images, the better they will end up becoming. Have fun and experiment as much as possible. Good luck!
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