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

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.

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
f/5.6

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

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.

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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