Aperture, Shutter Speed, and ISO: The Photographer's Trinity

There’s three interlinked metrics I refer to a lot in these posts: Aperture, Shutter Speed, and ISO. It’s vital to understand the basic use of these and how they work together to truly begin understanding how to take a good photograph. Without that knowledge, it would be like trying to cook a meal without knowing how temperature and cooking time works. Your chicken may look great at first glance but when you find out it’s pink on the inside and your entire family is chucking their guts up the next day…

Okay so that wasn’t the best analogy.


Peekaboo! (kurtmunger.com)

Peekaboo! (kurtmunger.com)

An aperture is a fancy word for a hole or an opening. When your camera’s shutter opens to allow light to hit the sensor, the aperture blades in the camera’s lens will close to a certain point to control the amount of light getting through. The aperture figure is the measurement of how wide that hole is.

Like squinting, only mechanical. (verificauto.com)

Like squinting, only mechanical. (verificauto.com)

As you can see, the number scales the opposite way you’d imagine it to; the smaller the number, the larger the gap. Now if this was all there was to it, life would be a lot simpler. However, changing the aperture size also controls the “depth of field”. When you focus on an object with your camera, the aperture will allow everything in a certain distance towards and away from you to stay in focus too. The narrower the aperture is, the wider this distance will be.

Say hello to Steve.

Say hello to Steve.

Above are two images spliced together. One was taken with a wide aperture, the other with a narrow aperture. The difference is that in the shot with the wider aperture, you can see the background behind the focused subject is blurred. The same has happened in the narrow aperture shot, but to a lesser degree. That’s because the depth of field extends out further with the narrow aperture to mitigate the loss of focus at further distances. This is vital when it comes to something like a landscape shot; the massive distances involved means you want as much as possible to be in focus, rather than one particular area.

To summarise: the wider the aperture you use, the more light enters the camera, but also the lower the depth of field is.

Shutter Speed

If your shutter looks like this, you might want to go looking for the rest of the camera. (mydigitalcamerahq.com)

If your shutter looks like this, you might want to go looking for the rest of the camera. (mydigitalcamerahq.com)

Nestled snugly over the sensor and beneath the mirror sits the shutter. When you press the shutter button on your camera to take the shot, this small covering with move to allow light onto the sensor, and then move back into position to cover it again.

Your shutter speed defines how fast this process happens, and thus how much light is allowed to touch the sensor. The faster this is, the less light is allowed to get by. This has the effect of darkening or lightening the end result.

However, much like in aperture there is also a side effect of this. Next time you’re walking alongside a main road, stop for a moment to look at the traffic passing by and close your eyes. Open and close them as quickly as you can. How much motion did you perceive in the cars passing by? If you’re quick enough, it will seem to you as though the cars are all sat completely still, but when you leave your eyes open they will be clearly moving.

Your camera has the same problem. When the shutter opens, the sensors starts to receive light. However, if something moves while the shutter is open, the definition of that object gets worse and worse and you experience a blurring effect. A faster shutter speed has a “freezing” effect on movement, allowing everything to maintain definition, but then this also results in a darker picture.

In summary: the lower the shutter speed, the brighter the image, but the less definition there is in moving objects.


ISO stands for International Standards Organistion, which is a governing body that controls standards in various subjects. In photography, this standard was Film Speed, but since modern digital cameras no longer use film, the term ISO has become the shorthand for the digital sensor’s take on it. A “standard” ISO setting would be 100, but as this increases, so does the sensitivity of the sensor, thus creating a higher sensitivity to light and a brighter image.

Again, there’s a side effect. The more sensitive the sensor is to light, the more distortion you begin to see. This takes the form of “noise” or “grain”, which are tiny spots or specks in the image. The greater this distortion is, the less definition your images will have. How much noise you get depends entirely on the camera you’re using; more modern and expensive camera tend to have higher quality sensors equipped with methods of reducing noise and coping with higher sensitivity. My camera for instance can get to about ISO 800 before you start seeing any noise, whereas some of my more established colleagues can push their cameras to above ISO 3200 before they start losing image quality.

In summary: the higher the ISO setting, the brighter your picture will be, but also the more noise there will be.

So how do I use this?

Each setting increases or decreases the “exposure” of the image. For a correct exposure, you need to balance each of these settings depending on your desired result and to filter out as much of the side effects as possible. Here’s a few examples:

Here we’ve got my dog Shandy drying herself off after a quick swim in the lake. She’s moving exceptionally fast and I want her body and face to be defined, but I’m not too bothered about anything in the background. To do this, I increased the shutter speed to 1/3200th of a second. So at this point, the shot is going to be exceptionally dark, so thanks to the lens I’m using, I increase the aperture to f/1.8. This means there’s a lot more light getting in to compensate for the loss of light from the fast shutter speed, resulting in an evenly exposed picture.

This is Fistral Beach in Cornwall. It also happens to be one of my favourite pictures. I wanted every element of the shot to be defined and in focus, so this meant I’d have to use a narrower aperture of f/11 and the lowest ISO setting of 100. Since I’m not too bothered about anything in the shot moving too quickly, I can compensate for this by lowering the shutter speed to 1/80th of a second. This gives me enough light to ensure that the shot is still properly exposed.

That's all pretty straightforward, but what if life doesn't want to play ball?

Sometimes you’re going to find yourself in a situation where you aren’t going to be able to get away with simply twiddling the settings. For example, you’ll find that in darkened rooms like nightclubs or church halls that the shutter speed required to get the proper exposure will be too low for you to get a clear image. Generally speaking, once you hit 1/100th of a second you’ll start seeing blurring on an image even without anything actually moving, and that’s because you’re holding the camera. No matter how hard you try, there will always be some element of movement in your body, which means the camera itself will move and distort the image. Don’t even try to get something with 1 second shutter speed while handholding, it’s just not going to end well. You could pump up the ISO setting

This is where extra equipment comes into it. Not enough light in the picture to freeze movement? Use a flash to create more. A landscape shot at night with no movement? Use a tripod to stablise the camera and prevent any shaking ruining the details.

With time and practice, you will grasp these rules and eventually start learning when to break them. Sometimes you want the motion blur a fast moving object with a low shutter speed brings, or maybe you’ll want to experiment with different apertures and what effects you can get out of them.

Next time: that cheeky swine that is light and how to control it’s infinite machinations!