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Photography Basics – Aperture

by · December 15, 2011

Aperture – “ap-er-ture”

Noun:

  1. An opening, hole or gap.
  2. A space through which light passes in an optical or photographic instrument, esp. the variable opening by which light enters a camera.

Aperture is simply a hole which allows light though

What is aperture?

Aperture in both amateur/professional photography and videography is simply the hole in the camera lens which allows light to pass though to reach the camera’s sensor or film stock. This hole can be controlled to vary the amount of light entering the camera, and thus (along with shutter speed) allowing you to control the exposure of an image.

The size of this hole is measured in f-stops – which stands for focal ratio.  You do not need to get bogged down in all the maths and history behind how and why we call them “f numbers”, but you just need to know what people are talking about when they say “oh, yeah, that shot was an f/8 dude” or “if you shot it wider than f-6 you would have been able to freeze the action better man”…

In fact, as you can see above, you can write the f value in different ways – f/6, f-6, f6.  It is up to you, but I prefer to write it f/NUMBER as that is the more traditional way it was written, and it helps you understand the backwards thinking needed to understand that large is small, and small is large… confused yet?!

Understanding what the f number means for a lens

If you think of the f number as f/NUMBER it is a fraction and therefore, the larger the denominator, the small the fraction.  A small aperture value, say f/2 = ½ is larger in size than f/16 = 1/16.  So if you take this principle into photography and lenses, the larger the f number gets, the smaller the value gets, and so the small the hole gets.  A lens set at f/2 has a larger opening than a lens set at f/16.  This can be seen in the graphic below:

Aperture open and closed

When you buy lenses, they will always provide an aperture value.  This is the smallest value that the lens can be set at, and if you work out from the image above – the widest open that it can be, which will allow the most amount of light though the lens.

banner of lots of lens aperture values

In terms of lens design, the wider a lens can “open” the more light it can get through to the camera, and so less time is needed to take a correct exposure.  This is also known as the speed of the lens.  If you have a fast lens, your f number will be low, such as f/1.4, f/1.8.

Aperture size and its effect on exposure

If you have a constant shutter speed and static ISO value – as you close up the lens aperture the amount of light entering the camera is reduced.  This is the same as being in room with only light coming in from an open door.  Start to close the door and the light level in the room drops.  What was a lit room is then becomes darker and darker.

Using this principle and the graphic below, you should be able to understand this as a basic rule for controlling your exposure.

Chart showing aperture value and exposure

Aperture size and its effect on depth of field

Depth of field is the distance between the nearest and farthest objects in your scene which appear acceptably sharp.  When taking landscape images, it would be advisable to maximise the depth of field so every aspect from your foreground interest to the horizon is sharp and in focus.  When taking portraits it is advisable to have a low depth of field, which will have your subject in focus and sharp, and the surrounding background out of focus and in blur. The primary control of this aspect in photography is by varying the aperture value.  A lens with a low f-stop value (i.e. f/1.8) which is wide open will give you a shallow depth of field – so your focal plane will be limited from anything from a few centimetres to a couple of metres (this is also dependant on your distance to your object in focus).  Increase the aperture value to around f/16 and the depth of field will increase – objects which appeared out of focus in your previous image will now be more sharp and in focus.

How it controls sharpness

As mentioned above, the depth of field is the distance between two objects in your scene which appear in focus.  Objects which are in focus are considered to be sharp.  You would think that if you wanted everything to be sharp you would set your lens to its maximum aperture value.  However, as you reduce your aperture size, i.e. the hole in which the light can pass you will eventually get to a size which will stop producing a sharp image and in fact start making the whole image “soft”.  This is due to a phenomenon within the lens as the light passed through the ever reducing hole it starts to spread due to diffraction.  The light rays are no longer travelling straight and start to spread.  Unless a high aperture is needed, it is best to not go further than about f/16.

How it controls “blurriness” or bokah

The out of focus area of your image, outside the depth of field area is the “blurry bit”. This blur can in fact have a shape to it, and depending on your aperture shape it can change in character and appearance.  This “shape” of blurriness is known as bokeh (definition coming soon!). Any point of light which lies within the out-of-focus area will take on the shape of your aperture hole.  More expensive lenses have aperture holes which appear round at most values.  But cheaper lenses have less diaphragms making up the iris, and the hole is in fact polygon in shape.  This principle can been seen in the following image which was taken by one of our Manchester based photographer – which shows out of focus fairy lights in the same shape as my aperture hole which is made up of 5 diaphragm elements

Example of aperture hole shape visible in blur bokeh

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