Understanding (and Using) ISO, Aperture, and Shutter Speed

When I was first getting into photography, two of the most burning questions I had were:

• what are ISO, aperture, and shutter speed?
• how can I use them to improve my photography?

So I did the sensible thing and read a few articles that explained them. Unfortunately, most of these articles were either confusing or incomplete.

Some didn’t make things simple enough for a beginner, while others didn’t explain how I could use the knowledge they were giving me. None of them answered all my questions in a way I could understand and implement.

So now, a few years on from this, I’m writing for you the article I wish existed when I was starting out. If you have your camera with you to play with as we go, it’ll help you to understand the points as they’re made.

But before we get into it all, I want you to internalise the following idea:

ISO works alone, whereas aperture and shutter speed work together.

The following statements elaborate on or reinforce that, and it’d be good to internalise these too. Read them more than once if you have to:

• ISO sets how much light you need for your photograph, whereas aperture and shutter speed allow you to get that amount
• ISO is the solid foundation that we set, whereas aperture and shutter speed are fluid variables that can alter
• ISO generally stands alone when we’re shooting, whereas aperture and shutter speed affect each other

We’ll explain this again as we go, but I believe it’s useful to have that mental separation of ISO from aperture and shutter speed before we begin.

What are ISO, aperture, and shutter speed?

ISO

ISO controls the camera sensor’s sensitivity to light, with a lower ISO value meaning more light is needed to take a well-exposed shot.

Using a higher ISO will result in a slightly worse image quality, so the ideal practice is to use a value as low as you can in the light you’re shooting in.

In film photography, the values relate to the film itself. Different conditions mean needing different film.

When shooting digital though, we can simply change the ISO setting.

Key point: the lower the ISO value, the more light you need to take a well-exposed photograph

On a sunny day in the park, use your lowest ISO (on my camera, this is 100).

On cloudy days, indoors, or at night, you will probably need to choose a higher ISO.

A typical range of ISO values available on a camera might look like this:

ISO 100 – 200 – 400 – 800 – 1600 – 3200 – 6400 – 12800

Notice how the values double as they increase.

This will help you to remember the sequence.

Changing the ISO value on my Sony NEX

Aperture

Aperture controls the size of the hole – the aperture – in your camera’s lens.

Your camera lens has a diaphragm mechanism that can open or close to change the size of the hole that lets light in.

When we change the f-number – the aperture value – we change the size of this hole.

A bigger hole means more light is entering the lens over a given period of time, while a smaller hole means less light can enter over that time span.

The smaller the f-number, the bigger the hole.

Key point: the lower the f-number, the larger the hole, and the more light can get in

A side effect of changing the aperture value is to change the depth of field in our photographs. Lower f-numbers allow us to blur backgrounds while larger f-numbers will keep more of the photograph in focus.

The f-numbers on your lens will probably be somewhere in the following range:

f1.8 – f2 – f2.8 – f4 – f5.6 – f8 – f11 – f16 – f22 – f32

This is a list of full f-stops.

You may have half stops too which will give you numbers in between these. For this article, we’ll only consider these full stops.

Notice how the list of full f-stops also double, albeit every other value (f5.6 –> f11 –> f22 or f8 –> f16 –> f32 etc).

Again, this helps you to remember the sequence, which is important.

Demonstrating f1.7, f5.6, and f16 on my Yashica Yashinon-DX 45mm f1.7

Shutter Speed

Shutter speed controls how long the shutter stays open when we take a photograph.

Varying the amount of time the shutter stays open can alter our photographs in two main ways.

The first is to ensure the exposure is right by allowing only the right amount of light in. The second is to freeze a subject (with a fast shutter speed) or to allow motion blur (with a slow shutter speed).

Key point: the lower the shutter speed, the longer the shutter stays open, and the more light can get in

The shutter speeds available on your camera will probably be somewhere in the following range:

30″ (seconds) – 15″ – 8″ – 4″ – 2″ – 1″ – 1/2 (of a second) – 1/4 – 1/8 – 1/15 – 1/30 – 1/60 – 1/125 – 1/250 – 1/500 – 1/1000 – 1/2000 – 1/4000 – 1/8000

Notice how the values double as they increase.

Yet again, this helps you to remember the sequence, which is again important.

Shutter Priority mode (which I never use) on my Sony NEX

How do ISO, aperture, and shutter speed work together?

Remember these three statements from earlier?

• ISO sets how much light you need for your photograph, whereas aperture and shutter speed allow you to get it
• ISO is the solid foundation, whereas aperture and shutter speed are fluid variables
• ISO generally works alone, whereas aperture and shutter speed affect each other

Again, we need to mentally separate ISO from aperture and shutter speed.

Imagine ISO as a solid standalone value (of how much light is needed), whereas aperture and shutter speed can be thought of as fluid values working together (to allow the right amount of light in).

A simple explanation might go like this:

If our selected ISO means we need 10 arbitrary units of light for a well-exposed image, we could allow these 10 units through the lens in a number of different ways.

One method would be to open the aperture wide and allow all 10 units in at once.

Or we could close it a little and allow 5 units in at once but leave the shutter open for 2x as long.

Or we could close it a little more and allow 2 units in at once but leave the shutter open for 5x as long.

Or we could close it right up and allow 1 unit in at once but leave the shutter open for 10 times as long.

All of these examples would give a well-exposed image if 10 units of light are what is needed.

This is simply the aperture (how many units are allowed in at once) working with the shutter speed (how long the shutter is open for).

Remember, once you’ve set the ISO, the amount of light needed for a well-exposed photograph remains the same.

However, changing the aperture means the shutter speed has to change also to compensate for the new hole size.

If you have your camera with you, you can try this yourself by setting it to the Aperture Priority mode (A or Av) and following the guide below.

Shallow depth of field achieved with a low aperture value – probably f2.8 but I can’t remember exactly

Demonstrating how aperture affects shutter speed

When we set the ISO, the camera figures out how much light we’ll need for a well-exposed photograph.

When we’re in Aperture Priority mode, it makes sure we get the right amount by changing the shutter speed to suit whatever we set the aperture (f-number) at – hence, the aperture has priority.

You control that aperture value. You choose whether it’s f1.8, f4, f8, or anything else.

The camera then chooses a shutter speed to match.

By learning how aperture affects shutter speed, you can then use Aperture Priority mode for the vast majority of your photography.

But first we need to set our foundation: our ISO.

You can use the following table as a guide to starting points, depending on what the light is like wherever you’re reading this.

As long as you then stay in the same light, you shouldn’t need to change your ISO value once it’s set.

Demonstrate to yourself how aperture affects shutter speed

1. Set your ISO value according to the table above.

2. Now set your aperture to something around the middle of the scale on your lens; f8 should do.

3. Take a look at your shutter speed, and remember what it is.

4. Change your aperture down one full stop to f5.6 and see what happens to your shutter speed. It should double.

5. Change your aperture down one full stop again to f4 and see what happens to your shutter speed. It should double again.

6. Change you aperture up to f11 – which is one full stop above where we started – and see what happens to your shutter speed. It should be half the original speed at f8.

7. Change you aperture up one full stop to f16 and see what happens to your shutter speed. It should be half again.

This is how changing the aperture affects – or works together with – the shutter speed while the ISO value remains the same.

As with the simplified 10 x 1, 5 x 2, 2 x 5, and 1 x 10 examples earlier, the camera adjusts how long the light has to enter the lens based on how big or small we make the aperture.

Now you know this, you can use it to get more creative with your photography.

How to *actually* use ISO, aperture, and shutter speed

Playing with aperture values and observing the shutter speed changing is all well and good but you’re not going to have any photographs to show if you just sit there and do that.

So now we need to use this knowledge in the real world.

We mentioned earlier that a lower f-number results in a shallower depth of field and a higher f-number leaves more of the image in focus.

So:

• shooting at f2.8, for example, means you can achieve a blurred foreground and / or background, as in the first image here
• shooting at f11, for example, means you can leave more of the image in focus, as in the second one

Here’s how you can practice this for yourself:

1. With your camera in Aperture Priority mode, set your ISO according to the light you’re in right now.

2. Change your aperture to the lowest it can be. This number will depend on your lens, but will probably be between f1.4 and f3.5.

3. Notice the shutter speed. As the lens is wide open it should be quite high, and so cause no problem.

4. Take a photograph of something with a blurred background and / or foreground. Easy as that.

5. Change your aperture to a higher number. Somewhere around f8 and f11 should be fine for this exercise.

6. Take the same picture and notice the difference in depth of field.

Of course, the shutter speed will have been a lot slower when taking the second image. If it’s now so slow that your photograph is blurry, you should try raising the ISO value.

But by how much?

If you remember that changing the aperture by one stop doubled or halved the shutter speed, then you might have guessed the answer here.

Raising or lowering the ISO value by one step – and leaving the aperture as it is – will have the same effect: it will double or halve the shutter speed.

This is how you know when you should shoot with a higher ISO than the one you have set; when you can’t get the shutter speed you want with the f-number you want at the ISO value you’re set at.

We also mentioned how a fast shutter speed can freeze a subject while a slow shutter speed can allow motion blur.

So:

• shooting at a speed of 1/1000, for example, means you can freeze a moving subject, as in the first image here
• shooting at a speed of 1/30, for example, means you can achieve motion blur, as in the second

Here’s how you can practice this for yourself:

1. Head somewhere with a good number of moving objects. The side of a road is probably a good spot. Then, with your camera still in Aperture Priority mode, set your ISO according to the light you’re going to shoot in.

2. Change your aperture to around f5.6. This should give you a balance between enough depth of field and a fast enough shutter speed (which may be around 1/400, but depends on how fast the subject is moving). If not, raise your ISO value by one.

3. Hold your camera still and wait until your subject – perhaps a passing car or bicycle – is where you want it in the frame.

4. Shoot. If your shutter speed is fast enough, your subject will be frozen. If your speed is too low, you could change your aperture to f4.

5. If you don’t want to lose depth of field, increase your ISO value until you get a fast enough speed and try again until you get a frozen subject.

6. To achieve motion blur, we need a slower shutter speed. This is reached by selecting a higher f-number, so change your aperture to around f11 and see what the speed is. You may need to change the ISO again, but aim for something around 1/100.

7. For the sake of this exercise, find a static subject to focus on. Something like a tree or a bin or a friend or family member.

8. Hold your camera very steady or rest it on a nearby object. Compose a shot of the static object and wait until a moving vehicle enters the frame.

9. Shoot. If the amount of motion blur is too high or too low, you will need to alter your shutter speed to suit.

Playing with shutter speed is harder than playing with depth of field, so don’t get disheartened if this doesn’t work right away.

However, once you do get it, you can then start moving the camera too, to achieve shots like the one below.

Explaining that, however, is for a different post.

For now, practice the above exercises as much as you can, every time you shoot, until shooting in Aperture Priority mode becomes second nature.

Because no matter your level now, given enough time and effort, it will.

Why use Aperture Priority mode?

Every exercise in this post has been done in Aperture Priority mode.

The question you may have is why.

The answer really is because that’s what I use and I’m explaining to you how I shoot.

Other people will have different opinions on how they like to do things, but I believe Aperture Priority mode is the most efficient way to get the most creative control over how your images look.

Your camera most likely has a whole range of settings you could use, from fully automatic to fully manual.

On the automatic side of things, you’ll probably also have things like Sport, Night, Portrait, and Landscape.

You could use these, but all they do for the most part is alter the ISO, aperture, and shutter speed anyway. Unfortunately, they can’t know exactly what you’re trying to achieve.

A Sport mode that freezes motion is no good if you actually want some motion on the race car you’re shooting.

A Portrait mode that completely blurs the background is no good if you want to use part of it to complement the subject.

Learning how to control the ISO, aperture, and shutter speed yourself allows you to make the photographs you want to make, without having to hope the camera guesses correctly.

At the other end of the scale is the fully manual mode.

The reason I don’t like using this is that you have almost too much control. In a nutshell, it’s too easy to mess up your shots when using Manual mode.

Remember how the camera changes your shutter speed to suit your f-number when shooting in Aperture Priority mode?

It doesn’t do that in Manual mode. When you change one, you have to change the other too.

If you’re shooting in a studio with time to set every shot up and with consistent light, then you may prefer this level of control.

However, I shoot travel and street photography. I often don’t have time to set up a shot and the light is often changing.

Using Aperture Priority mode means I control as much as I have to and no more, and leave one thing – the shutter speed, which keeps the exposure right – to the camera.

It’s just the most efficient way I’ve found to have complete control over how my photographs look.

That efficiency manifests in the following ways:

• it’s quicker to focus on what I want and not have to re-focus every time an Automatic mode doesn’t do what I want it to
• it’s quicker to just change the aperture than it is to cycle through Sport, Portrait, Night, etc. every time
• it’s quicker to let the camera deal with shutter speed after I change the aperture
• it’s quicker to pick up a new camera and start using it as Aperture Priority mode is Aperture Priority mode whereas Automatic settings may vary between brands and / or models
• it’s quicker to control the shutter speed with Aperture Priority mode than it is to keep changing between that and Shutter Priority mode

If you’ve been paying attention or are familiar with your camera settings, that last point may answer a question you had.

Let’s elaborate.

Motion blur achieved with slow shutter speed – 1/80

Why use Aperture Priority to control shutter speed?

As well as Aperture Priority mode, your camera will also have a Shutter Priority mode.

This does the inverse of the Aperture Priority mode that I always use; it allows you to control the shutter speed and let the camera decide the aperture based on that.

So in situations where shutter speed is more important than aperture, like in the second set of exercises above, why wouldn’t I use Shutter Priority mode?

There are a few reasons but the main one is to keep a consistent line of thinking when I’m out shooting.

When I shoot photographs where the aperture is more important than the shutter speed, I still have to make sure the shutter speed isn’t too slow.

So even with these shots there’s an element of how will the aperture affect the shutter speed?

Controlling the shutter speed with the aperture even when the shutter speed is more important means I’m always asking myself the same question – what will the shutter speed be at this aperture?

Switching to Shutter Priority would mean reversing my thinking.

If I change the speed to 1/500, what will my aperture be?

That would be less efficient for me and lead me to missing shots.

It would also mean having to physically change the camera setting from Aperture Priority to Shutter Priority and back again every time.

So when I say it’s quicker to control the shutter speed with Aperture Priority mode than it is to keep changing between that and Shutter Priority mode, I mean in my head as well as on the camera.

A final point is that your lens probably has fewer f-numbers than your camera has shutter speeds.

The chart below uses the f-numbers from my Yashica Yashinon-DX 45mm f1.7 and the shutter speeds from my Sony NEX.

The amounts and ranges of each mean there’ll always be a speed for the aperture you choose.

However, if you go to an extreme shutter speed, there may not be an aperture that works with it, which means changing the ISO, which means less efficiency and maybe more shots missed.

Do I *have* to learn ISO, aperture, and shutter speed and shoot in Aperture Priority mode?

No, you don’t.

You can shoot in whatever way you like. Ideally, it’ll be the one that helps you make the best photographs you can.

For some people, that’ll be some fully automatic mode.

For me it’s Aperture Priority mode.

I’m just telling you how I shoot and get the results I get.

If you want to try this method, my advice is this:

• learn how ISO, aperture, and shutter speed work together
• practise so much that you know what effect changing the aperture will have before you even change it
• always shoot in Aperture Priority mode, even when the shutter speed is more important

Do all this until it becomes second nature and you’ll have mastered a very efficient way to have complete creative control over all your photographs.

Let me know in the comments or on Twitter how it goes.  😀

… p.s. if you’ve found this post on understanding and using ISO, aperture, and shutter speed useful and think others will too, why not share or pin it?