Aperture – Everything you wanted to know but thought it was uncool to ask

Camera aperture

If you were to devise a system that was intended to be confusing, you’d have a hard time coming up with anything more confusing than photographic aperture. I’ve so often seen people glaze over within five seconds of the start of an explanation, their minds a whole galaxy away – anywhere, so long as it’s not here listening to f/stops, depth of field and so forth. But when you split it up, it’s pretty easy.

So, what is aperture?

The basic idea is that light reaches your camera’s sensor (or film) through a hole.

With pinhole cameras, it’s literally that: a hole in a light-tight box projects an image on the inside. With cameras, we put some glass around the hole to make the image sharper. But essentially, it’s still a hole. History would have been different if photographers talked about ‘hole numbers’ or adjusting the size of their ‘lens hole’ but somehow that did not sound cool – even in the 1870s.


So photographic aperture is the hole in the camera lens which lets light in.

Why size matters

As you know from general experience, the bigger a hole, the more can go through it. Think about turning on a tap (water faucet): open it a little and the flow is only a trickle, open it up and more water flows through.

It’s the same with lens aperture: the larger the aperture, the more light gets through to the sensor. Obviously this affects the exposure of your image.

Now, giving the film or sensor the proper exposure is like filling a cup of water: if the water flow is slow (from a small aperture), it takes longer to fill (the exposure time is longer).

And obviously, if the flow is faster (we turn the tap on to make the aperture larger), it takes less time to fill the cup (exposure time is shorter).

What do the numbers mean?

Now, it’s easy to measure exposure time – directly in seconds or fractions of a second. With the aperture, it was realised early on that simply measuring the size of the hole was not enough. That’s because holes of the same size in different lenses of different designs or focal length will look different to the film or sensor.

A good way to see this is to pick up a pair of binoculars or SLR lens if you have one handy). Look down one end, and turn over and look down the other: the hole will look different sizes – but it’s the same hole. What has changed is the effective focal length. This shot shows a 24mm lens: the aperture is much larger on the sensor side than from the subject side.

Tree at night

Basically we need a measure that relates the size of the hole to the focal length. At the same time the measure needs to show that as the size of the hole becomes smaller, less light flows through (and vice versa: bigger aperture lets in more light).

The answer is the f/number: we divide the focal length by the effective diameter of the hole.

Why are they like that?

Suppose we have a 50mm focal length lens. If we have a big size hole – a big aperture, it might measure 25mm. So 50 divided by 25 gives us 2: the f/number is 2, which we write as f/2.

If the aperture is smaller, say, 3mm in diameter, 50 divided by 3 gives us about 16: the f/number reads f/16. As the hole is smaller, less light gets through. So f/16 is said to be a small aperture or small f/number.

That’s why you could get confused if you read about an aperture of 16 being smaller than 2: that does not make sense and is, in fact, wrong. A photographic aperture is written as ‘f/number’: it means the focal length divided by the aperture diameter. So f/16 is indeed smaller than f/2. (Microscopists talk about numerical aperture, but that’s a different thing.)

f/number sequence

The basic f/number sequence is 1.4, 2, 2.8, 4, 5.6, 8 – it’s a doubling every other step – then it falters a bit: 11, 16, 22, 32, 45 but is essentially still a doubling every other step.

Each step to a lower f/number represents a doubling in the area of the aperture, which means a doubling in the amount of light passing i.e. a one-stop increase in exposure. Conversely, each step to a higher f/number means a halving in the area of the aperture, which means reducing exposure by one stop.

Don’t worry about why this is (or ask a friendly mathematician if you really want to know): just remember that the sequence means that if you change aperture setting from, say, f/4 to f/8, then the exposure time needs to increase by two stops, and vice versa.

What are stops?

Here’s another source of confusion. The word ‘stops’ is used in two senses. One goes back to the days when lens aperture was changed by dropping in a metal plate with a hole cut in it. You changed aperture by taking out one plate and dropping in another one with a different sized hole. These were called stops (actually Waterhouse stops, after the inventor). From that we get the term ‘stopping down’.

Now, these stops were arranged so that each smaller hole halved the exposure (and conversely, each larger hold doubled exposure). From that we get the term ‘f/stops’. From this you still hear photographers talk about ‘one stop’ meaning a halving or doubling of exposure. Goes all the way back to late nineteenth century!

Carting sets of metal plates with holes in them is a bore, not to mention really slow to use and before long the aperture diaphragm was invented. This was a set of leaves which were pivoted on the rim so that they fanned across the gap – the more they overlapped, the smaller the central hole. And that’s what we still use now.

Aperture and depth of field

So much for aperture and exposure. What complicates the whole subject further is that aperture affects two quite different things independently. Just as shutter setting contributes to exposure but also influences motion blur, aperture setting contributes to exposure but also influences something else altogether.

Aperture is one of the factors controlling depth of field. In fact aperture is the single most powerful and easiest way to control depth of field.

What you need to know

Use a small aperture like f/16 if you want as much as possible of the image to look sharp. Use a large aperture like f/2.8 to make just your main subject sharp against a blurred background i.e. for minimum depth of field. In between, an aperture like f/5.6 is good for general uses as it produces an average depth of field. It’s real simple.

f/16 f/2.8
16 aperture 2.8 aperture
f/5.6
5.6 aperture

Here’s another example: at f/3 the nearest pink flowers are blurred while the next set of flowers are sharp but beyond that the trees are very blurred. At f/14, the near pink flowers are sharper and the trees beyond are more detailed. However, there is still blur, which shows that when objects are widely separated, aperture alone may not be enough to make everything sharp.

f/3 f/14
3 aperture 14 aperture

In fact, aperture also affects another complex of interactions – very subtle, but vital for advanced photography: aberration correction.

Aperture and corrections

The image quality of lenses change, usually quite subtly but substantially, with different apertures – the details are really technical, but the up-shot is cleaner, clearer and sharper images. The usual pattern is that there’s an improvement as you choose apertures smaller than maximum (photographers talking about ‘stopping down’), there’s a peak and then quality drops as you stop down to the minimum aperture.

Full and minimum aperture

That’s why I left explaining what full and minimum aperture are until now. Full or maximum aperture is the largest hole in the lens that lets the light through. It is calculated from the size of the front element, not from the size of the actual hole. So if the front element is 25mm in diameter and the focal length is 50mm, the full aperture f/number is taken as: 50 divided by 25 equals f/2. In all but rare instances, at maximum aperture, the iris diaphragm does not cut into the hole.

The minimum aperture is, naturally, the smallest hole. But it’s not the smallest hole possible, only the smallest hole the manufacturers allow you to set. That’s why minimum aperture can vary from f/8 all the way to f/45. The reason for this is made up of equal parts mechanics and image quality. We’ve learnt that image quality drops with small apertures – in fact, at really small apertures it can be disastrous. To prevent this, manufacturers limit minimum aperture according to the lens design.

Even so, you’d be advised to avoid the smallest apertures on your lenses: back off by one stop e.g. if the minimum is f/22, use no smaller than f/16. On a typical SLR lens you can see that even at minimum aperture, one could make the hole smaller.

Tree at night

Aperture shape and bokeh

Bokeh is the word for the quality of the out-of-focus blur. There are several discussions about this on the Web, which affect mainly SLR users with fast lenses.

http://www.vanwalree.com/optics/bokeh.html
http://www.kenrockwell.com/tech/bokeh.htm
http://www.luminous-landscape.com/columns/sm-04-04-04.shtml

The first is quite technical, the second is more chatty, the third is well illustrated.

The debate is about the quality of the blur image – if you think about it, the majority of any image is actually out-of-focus. And if you work a lot with blur – in portraiture, weddings, wild-life – its quality matters a lot.

Depth of feeling

It was the great humanist and photographer Eugene Smith who asked “What use is having a great depth of field, if there is not an adequate depth of feeling?”

He has a point: some photographs are so perfectly sharp from corner to corner and beautifully lit, yet rather easily forgotten. For such a great photographer who taught us just about every trick in the photojournalist’s book, Smith is disappointingly represented on the Web.

But check out:
http://www.photo-seminars.com/Fame/eugesmith.htm
http://en.wikipedia.org/wiki/W._Eugene_Smith
www.magnumphotos.com

Search for Eugene Smith on the Magnum website which has high-quality images.

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Comments

1  

Michael

August 24, 2008 @ 11:52 pm

They way I seem to remember Aperture is

Small number : F2.8 = small background
Big Number : f22 = Big or more background

2  

Judy

September 30, 2008 @ 12:50 am

This is the best explanation of aperture that I have read. Thank you for taking the time to share your knowledge with us.

3  

Anthea

November 6, 2007 @ 10:56 pm

Thank you for such an informative post on a subject that can be so confusing! As an amateur photographer still learning the ropes, I find time and again that I forget which way is which with the aperture settings. You have explained it very clearly and I have made a list of the basics to store in my camera case and help keep me on track:

* long exposure = small aperture (and vice versa)
* the higher the f/- the smaller the aperture and less light will get through
* a smaller aperture = a maximum depth of field and a sharper picture

4  

Michael

October 18, 2008 @ 3:47 pm

I am getting ready to step up to my first digital slr(Canon 50D). This kind of info will be like gold to me as I start to absorb the needed materials. Thank you very much!

5  

Drew

November 11, 2008 @ 11:42 pm

Awesome! Thank you so much for explaining this so simply.

6  

Alison

December 2, 2008 @ 4:55 pm

Great, interesting and informative read – It has refreshed a lot of my ‘lost’ photographic knowledge !
Thanks…

7  

Millard

December 5, 2008 @ 6:11 am

Wonderful explanations to a lot of stuff that I never really fully understood.

8  

Ashleigh

January 16, 2009 @ 9:18 pm

I have just discovered that photography is not only my passion but now my newfound road to a new career. I have decided to change my life and these articles have really helped. I hope to learn more!

9  

Barbi

February 11, 2009 @ 10:45 pm

I am new to all of this (DSLR)and just purchased a Canon XSi several days ago. I must add that I’m very pleased with the camera although there is still a lot to learn. I basically know nothing about lenses as well as what the numbers mean but I’m slowly learning. This page was explained wonderfully. Of course I still haveso much to learn but then again, that’s the enjoyment of photography… Do you have any suggestion on books that may also explain the lenses, especially for beginners such as myself as well as many others?

Thank Again for sharing with us.

[...] is? Probably. But do you really know what aperture is? Why is it numbered the way it is? Check out this post from iDigitalPhoto to learn [...]

11  

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August 26, 2009 @ 10:01 am

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12  

Jeni

September 26, 2009 @ 6:16 pm

Wow… This is the best aperture explanation in the whole world wide web! Thank you!

13  

evemc

November 4, 2009 @ 8:09 am

I have just started a photography course and had a 10 min lecture on aperture and was expected to understand and be an expert on the subject.
Thanks this has been a great help and will recommend your article to a few of my class mates who were in a similar state of mind tome. Brilliant easy to understand article.

14  

jon

November 21, 2009 @ 12:44 pm

What a superb explanation. Not so much detail was covered when gaining a couple of city and guilds in photography some 15 years ago.
Brilliant

15  

Jetzky

February 17, 2010 @ 8:40 pm

I just recently bought a Canon Rebel-500D and I should say that this article will be such a good way for me to learn what aperture is.

16  

Sunil Sherekar

April 19, 2010 @ 11:29 am

Thanks for such informative explanation of F number & Aperture. You explained it in so easy language that amateur like me will remember it forever. Thanks once again for sharing such a nice information in your post.
- Sunil

17  

Jim Packer

June 14, 2010 @ 12:45 pm

Thank you so much for the amazing explanation of what looked such a confusing subject. I spent the whole day yesterday trying to work this out using my new Canon T1i. I can’t wait to get home and start again with this new found knowledge. Thank you, thank you, Thank you.

Jim.

18  

Christian Balfour

August 4, 2010 @ 8:16 am

Thanks so much. I bought my first DSLR (Nikon D5000) 3 days ago and have been trying to get my head around what F numbers mean and how to set aperture. I have made a few notes on paper and will use them when I get stuck. Such a great guide. I really can’t thank you enough.

Chris.

19  

Jan Timmons

October 3, 2010 @ 6:15 pm

Thanks for including “aberration correction”. My spouse and I have argued over this a bit ;) When I push my macro lens to f/37 or even f/52, Nikon calls that an “apparent aperture”, since the Nikkor 105mm lens isn’t designed for that. Yet at 200%, the few little doohickies that appear can easily be cleaned up.

But I digress. Good, clear explanation of the way one can increase past the manufactured aperture. What should I say to adamant photographers who insist that a landscape shot must use f/11, at most?
Thanks, Jan

20  

David Humphrey

December 1, 2010 @ 4:26 pm

All that is discussed is f/stop and I understand the basics but, what is confusing to me is to understand completly the relationship between f/stop.shutter speed and depth of field.

21  

Miri

January 3, 2011 @ 3:49 pm

thanks for the article. it refreshed me the forgotten material…

22  

Kelly

February 10, 2011 @ 4:54 am

So well explained, I get it now :0) and looking back its much more simple than my tiny brain was working out… I have spent years glazed over as soon as someone starts mentioning F numbers.

Many thanks

23  

Sunando

May 12, 2011 @ 11:43 am

Thank you very much for writing such a simple and elegant discussion about apertures. For a newbie like me, it’s a life-saver.. Thanks again..

24  

Heidi

September 4, 2011 @ 1:09 pm

I am starting a food blog, an important part of which are beautiful, distinct pictures. I have been fretting over learning to play with depth of field to get the “perfect” picture, and this article has helped so much!

Thank you for your clear, easy to understand explanation. I also love learning the history and the whys behind f-stops, etc. It really helps me to understand, retain and put the information to use when I know the background behind something, so thank you for that.

~Heidi

25  

Max

September 17, 2011 @ 6:27 am

This is really easy to understand! I’m studying Photography and the one thing I am struggling with is Aperture etc. Thanks! :)

26  

Soham

September 26, 2011 @ 1:24 am

Awesome information about Aperture .Realy helpful!!

27  

Peter

February 10, 2012 @ 5:56 pm

As one struggling with the ropes using a 35mm Rangefinder for the firat time. This sort of thing is invaluable and has (after reading it a couple of times given me an insight in a way I find refreshing. Thanks for taking the time to write it up.

28  

James

April 9, 2012 @ 6:17 pm

For a fixed focal length, halving the f/number would mean doubling the diameter of the aperture. Double the diameter for a circle means quadrupling the area and hence the amount of light. Please correct me if I’ve misunderstood something here.

29  

Julie

May 23, 2012 @ 1:40 pm

Thank you, I really did glaze over during this part in graphic design school. of course I now regret it but hey, if it were explained this way…

30  

Mike

June 20, 2012 @ 6:41 am

Great article, however I am confused by the part explaining why the aperture appears to be different size in the same lens?

31  

Stephen

November 6, 2012 @ 11:41 am

Thanks for the explanation. I was having a hard time with the depth of field.

32  

Munirajarathinavel poovan

December 19, 2012 @ 7:30 am

I just bought one nikon d5100 and am a novice to the field of photography. I was searching for the best explanation on aperture & shutter speed and this is the best one i came accross. Thanks for taking interest on educating people like me.

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