In the beginning was A, and the A was for Aperture. And as starting points go for a series all about photography, it’s not a bad one. When we take photographs, whether we are advanced photographers or newbies, we usually want our images to be well exposed, and to be acceptably in focus in the right places. And of all the buttons and dials, bells and whistles that our cameras might offer us, the ones that control the aperture are the ones that play the biggest role in helping us achieve this aim. So if we are to take photographs that we are happy with, it seems logical to understand the topic of aperture. So let’s begin.
A warning in advance: This is quite a technical topic, so it requires quite a technical write up, but don’t let that put you off the entire series, as I promise it is not all going to be as technical as this! If you find this a bit daunting on the first reading, the headings will hopefully help you to zone in on the bits that might be useful. For those who are experienced photographers, some of the more technical parts of this might still be useful to help you understand why aperture works as it does. And, this being part 1 of this series, all feedback is welcome (both good and bad), so leave your comments below.
An hole by any other name
Every camera has an aperture. It is simply an opening, a hole, a gap – usually midway between the front and the back of the lens. [As an aside, from here on I'll talk about sensors, but what I say is valid for film photography too.] Without an opening somewhere between the very front of your camera’s lens, and the sensor (somewhere near the back of the camera), light can’t reach the sensor. And if no light reaches the sensor you can’t take a photo. That opening (or aperture) is usually circular, or close enough as makes no difference.
Size matters
What a photographer refers to as the aperture is not so much the opening itself, but rather the size of the opening. Most cameras, but not all, give you some (or total) control over how big or small the opening is for each photograph you capture. So one of the things that a photographer needs to decide before taking a photograph is how big or small to let the aperture be. In order to decide that you need to understand what effect the size of the aperture has on your final image, why you might want it to be large, and why you might want it to be small. And in order to understand that you need to know a little bit about how a lens works.
The science bit
A lens is a piece of glass (usually) that focuses light from the subject onto the camera’s sensor. By adjusting the distance between the lens and the sensor, we can adjust the point in space from which the light that will be focused on the sensor originates. This is the basic concept behind focusing a lens. We move the lens closer to or further from the sensor, so that objects further from or closer to the sensor are in focus when we photograph them.
As an example, we could focus our lens so that everything 1 metre from the camera is in focus:
Or we could move it a little closer to the senosr so that everything 1.1 metres from the camera is in focus:
But we cannot, with a regular lens, focus perfectly on both areas at the same time:
Wherever we focus our lens objects, closer than or further away than that point will not truly be in focus unless we reposition the lens. This is not an issue if your subject is a two dimensional object parallel to the sensor of the camera, as in that case once you get one point of the subject in focus, the whole of the subject will be in focus. However, most subjects are three dimensional objects. So that means they have depth. And the way a lens behaves means that we can’t, all at once, focus perfectly on all of an subject that has some depth.
Perfection, though, is a funny thing and our eyes are far from perfect too so it turns out that even if objects aren’t perfectly in focus they can be acceptably in focus. It is this fact that allows photographers the world over to photograph things that are three dimensional subjects and have them look as if they are in focus – they pick a distance (or, more usually, part of the subject itself) to focus perfectly on, and then they ensure that, at that distance, the rest of the subject is acceptably in focus.
What dictates acceptably sharp? Well that’s down to the human eye, and it varies from person to person but if you photograph a point source (think something very very small) it will look acceptably sharp in a reasonably sized print or on a screen if it is represented as a point no bigger than about 0.2mm in size, as a general guide. If it is bigger than this, it is blurred, and so not acceptably in focus.
Looking at this diagram again, we might accept that the blue dot in the photo, although larger than the point which we are photographing, is still close enough to its original size to be deemed to be acceptably in focus:
But points even closer to (or further from) the lens may be deemed to be too big on the sensor, and so not acceptably in focus (they are focused in front of – as per the green dot – or behind – as per the purple one – the sensor):
Aperture and depth of field
So we can see now that in every perfectly focused photograph, only part of the subject – a certain distance from the sensor – is perfectly in focus, and then a bigger area of the subject – both in front of and behind that area that is perfectly in focus – is acceptably in focus. In front of and behind this acceptably focused region, the rest of the image is, basically, not in focus. The size of this acceptably focused region is known as the depth of field:
What has this got to do with the size of the aperture? Well, if we make the aperture smaller, look what happens the out of focus dots:
They get smaller – and possibly even become small enough to be considered acceptably in focus. Similarly if we make the aperture bigger, the reverse happens:
Now the blue dot, which was previously acceptably in focus gets even bigger, and so becomes out of focus. Previously out of focus dots (the green and purple ones) get bigger too, making them even more out of focus. Note that the red dot, which is the one we are actually focusing on, remains sharp, because we haven’t moved the lens – just changed the size of the opening.
So by making the aperture bigger or smaller, we can change how objects away from our focal point appear on the sensor, and we effectively change whether they are considered acceptably in focus or not. Therefore, as we change the aperture, we change the depth of field – the area considered to be acceptably in focus. If the aperture is small, the depth of field will be large, and so more of the scene will be acceptably in focus, which, from now on, we refer to simply as “in focus”. If the aperture is large, the depth of field will be small, and so less of the scene will be in focus.
That, in a nutshell, is the aesthetic effect that aperture has on your image. Assuming something in your image is in focus, then aperture dictates how much of the image is in focus, and how much is blurred. We’ll come back to that in a moment, but first, there’s another effect of aperture you need to know about. And if you don’t get this right, the aesthetic aspect won’t matter a damn.
Aperture and exposure
I’ve already mentioned that the aperture dictates how big or small the opening that leaves light through our lens onto our sensor is, so lets follow this a little bit further. If the opening is big (a large aperture), then more light will get through than if it is small.
To take a photo – any photo – you need light to fall on the sensor, but more than that, you need just enough light to fall on the sensor. That’s what we call exposure, and a perfect exposure has just the right amount of light falling on the sensor for just the right amount of time to render dark parts of the scene dark and bright parts bright. Too much exposure (too much light) and the dark parts get bright, and the bright parts get too bright. Too little, and the reverse happens.
There are two fundamental ways to control how much light falls on the sensor. One is how big opening through which the light enters the camera is, which, as we know now, is the size of the aperture. The other is how long the light is allowed to fall on the sensor for, which is controlled by shutter speed (we’ll come back to that later in the series). So if you want to let more light fall on the sensor, one way is to use a wider aperture. If you want to let less light fall on the sensor, you can use a narrower aperture.
Some examples
Now we know that a small aperture will give a large depth of field, meaning that more of the image will appear to be in focus, and it will also allow less light to reach the sensor, meaning we need to keep the sensor exposed for longer. Conversely a larger aperture will give a small depth of field, but will leave more light in, allowing us to expose the sensor for a shorter period of time.
Here is a photo taken with a small aperture:
Notice how it is in focus all the way through the image. Objects a few metres from the camera and objects hundreds of metres from the camera are acceptably sharp. This is because the image has a large depth of field, thanks to a very small aperture.
Here’s a photo taken with a large aperture:
This time only the nearest parts of the scene to the camera are in focus. The near end of the skewers are in focus, but the middle of the skewers are blurred, and the far end of the skewers are even more blurred. That’s because the large aperture has resulted in a very shallow depth of field.
Aperture and f/ stops
So far we’ve only talked about “large” and “small” apertures, but we haven’t tried to quantify them. Because everyone uses different lenses, and because each lens has a different aperture to begin with, a standard notation has been developed to refer to aperture, to make life easier for photographers in determining exposure and depth of field – the two things affected by the aperture. This standard notation is the f/ notation, and it’s not the most intuitive thing in the world, but it can be understood relatively easily if you know a little about it.
To unify aperture between different lenses, the notation basically relates the size of the opening to another characteristic of the lens – its focal length. This is the f in the notation, and the aperture is expressed as a fraction of that the focal length. I’m not going to go into the details of this f- notation other than to hightlight this fact, and to explain how we can understand it. An aperture of f/4 means that the diameter of the opening is the focal length of the lens divided by 4. An aperture of f/8 means the diameter of the opening is 1/8th of the focal length of the lens. Knowing that, hopefully you can see that an aperture of f/4 is bigger than an aperture of f/8, which is itself bigger than an aperture of, say, f/16. Dividing by a smaller number gives a bigger result, so the smaller the number after the f/, the bigger the aperture.
Also as we’ve discussed before the bigger the aperture, the smaller the depth of field, and the bigger the aperture, the more light left into the sensor.
The normal f/ stops you might come across are:
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and f/32
From left to right, the aperture gets smaller, the amount of light let into the sensor gets reduced, and the depth of field gets bigger.
The mathetmatically minded amongst you might notice that each number in that sequence is ~1.4 times the previous number (or, more accurately, the square root of two). This is because this series is chosen so that as you go from the left to the right, the amount of light that’s let into the sensor is halved. Why is this?
Well, you might recall from maths class, that the area of a circle is proportional to its radius squared. If we change our aperture from f/2 to f/2.8, we’ve reduced the diameter of the opening (and so the radius of the opening) by a factor of 1.4. That means the area reduces by 1.4 x 1.4 = 2. We’ve halved the area. Which itself means we’ve halved the amount of light that gets in. So we need to double the length of time we expose the sensor for.
This is what is known in photography as a stop of exposure. Each stop corresponds to a doubling (or halving) of the amount of light that falls on the sensor during the exposure. And so the aperture is sometimes known as the f-stop.
The f-number notation means that every lens, set to the same number, and focused on the same distant object, gives matching image brightness irrespective of the focal length of the lens, or indeed the camera.You can change lenses and/or cameras, but as long as you set the same f-number as you had, the image brightness is the same, and so the exposure is the same.
Later on in this series we’ll talk about another aspect to f-numbers, and how they can help you choose what lenses you might want to buy for your camera bag.
When to use a small aperture
All that said, to get started in photography all you need to know is when to use a small aperture, and when to use a big one. And, as we’re about to see, some cameras even help you with the decision.
A small aperture is useful in two situations: where the scene is very bright, or where you want to have lots of the scene in focus. On a bright sunny day, therefore, you might want to use a small aperture to reduce the amount of light hitting the sensor, and keep the image from being over exposed. This shot is at f/16:
Or if you’re taking a landscape shot, or a scene which has a lot of depth that you want to keep in the final image, a small aperture will help keep it all sharp. This shot is at f/11:
When to use a large aperture
A large aperture is often required if the scene is dark (and if you don’t want to use a long shutter speed), or is also useful if you deliberately want to let the background go out of focus. For instance, you might want to isolate the subject from the background – something that is quite common (and quite pleasing) when photographing people… or snowmen. This shot is at f/4:
Alternatively you might want to isolate a detail in the photo by using a large aperture – this shot is at f/2.8:
Aperture on cameras that don’t have an aperture control
Most high-end cameras – such as SLRs – allow you to choose your aperture, either via buttons on the camera, a menu item in the software, or a mechanical aperture ring on the lens itself. If you have a camera that offers this level of control, you can play with aperture and see the effects of changing it. One quick and easy way to play with the more interesting side of aperture – the depth of field variation – is to put your camera in A-mode (which is Aperture Priority mode) and it, then, will adjust the shutter speed as you adjust the aperture to give you a good exposure. Of course, if you want to play with the aperture to see it’s effect on exposure, you’ll need to switch into M-mode (which isManual mode).
However lots of cameras don’t offer the photographer the ability to choose an aperture, though many do still give you some influence over it. If your camera, for instance, has scene modes, the chances are it is using these to change aperture in many cases. For instance, a mode designed for portraits will choose a large aperture, to blur the background and give a more pleasing effect. The mode for landscapes will choose a small aperture, to keep as much of these scene as possible in focus. Sport mode is going to be trying to get you the fastest possible shutter speed, so it too might use a large aperture so that lots of light falls on the sensor, and the shutter can close after a short space of time. Night mode, which is for low light photography, will similarly be using a large aperture to maximise the amount of light hitting the sensor. And macro mode, meant for close up shots, will more than likely use a small aperture to give maximum depth of field when photographing things near very near the lens.
Knowing this, there’s nothing to stop you selecting one of these modes even if not photographing a subject traditionally suited to it. For instance, you might want a landscape with some detail in the foreground isolated. If so, use the portrait mode and see what happens:
Other considerations
A few things, briefly, that we haven’t yet considered about aperture, and specifically about depth of field. One is that the closer the subject is to the lens, the shallower the depth of field becomes (even if the f-stop remains the same). This might seem to contradict the diagrams above, but remember that when a subject is moved closer to (or further from) the lens, the lens needs to be re-focused, and so the whole area of perfect focus and the larger area of acceptable focus change. If the area of perfect focus is close to the lens, the area of acceptable focus will be smaller than if the area of perfect focus is far away.
Another consideration is that the longer the focal length of your lens (the more zoomed in it is, effectively), the less depth of field it gives, even at the same aperture and same subject distance. I won’t go into the optical reasons behind this, but this is one reason why portrait photographers often prefer a longer (or more zoomed in) lens. You may need to stand further back than you might otherwise need to, in order to frame your image with the subject contained within the frame, but that shallower depth of field means the background blurs more pleasingly. For instance, the following two images are taken from the same position, but the first has a wide angle lens and the second uses a longer lens:
Look at the trees behind the couple. In the photo on the left, they are reasonably well in focus. In the photo on the right, they are blurred. Both photos use the same aperture, are taken from the same position, and are focused on the same point. All that is different is the focal length of the lens – or how zoomed in it is in other words. That’s quite a dramatic effect.
How to choose aperture?
So, with all that said, choosing aperture comes down to two things. How bright do you need your image to be, and how much of it should be in focus. For me, I tend to think that the depth of field is the more important of these two, so like choosing an aperture that suits the subject matter and the depth of field I want – perhaps shallow depth of field for a portrait, large depth of field for a landscape, or maybe on occasion breaking these rules for whatever reason. So I usually let aesthetics dictate what aperture to aim for. If, say, I choose an aperture of f/4, I then need to check if I can get a good exposure at that aperture by adjusting the other elements that control exposure – namely shutter speed and ISO (both will be dealt with later in this series). If I can get a good exposure, that’s my decision made. If I can’t, I need to decide whether to change aperture, whether to compromise on exposure, or whether to come back when the light is different and try again.
On occasion, the shutter speed will be more important to me than the aperture, in which case I might be forced into using a particular aperture to get a good exposure. If I find that at that aperture, the depth of field isn’t what I would like it to be – perhaps there’s too much depth of field, and the background is very cluttered – then I need to decide whether to change shutter speed, whether to compromise on depth of field, or whether to come back when the light is different and try again.
Lastly, it is probably worth throwing it out there that if depth of field is not important to you at all, then for most lenses an aperture of f/8 gives best image quality, because most lenses are sharpest at this point. So if you have no particular reason to choose otherwise, f/8 is a good aperture to use.
Aperture and camera phones
This story has a sad ending for those of you who live in iPhone land, or generally use your phone as your camera, because iPhones and most other camera phones have fixed apertures, meaning that they don’t give you the option (even via scene modes) of changing or controlling the aperture in any way. For the record, they are optimized to give a pretty large depth of field. Consider it an excuse to buy a nice new camera!
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