My ramblings, my photos, photography, image editing, etc.

Aperture & Depth of Field

The aperture of a lens is one of the things that can be controlled automatically by the camera or can be controlled by the photographer. Why would you bother to control it? Why not let the camera look after it? Here are a couple of photos where I try to say why you might want to take control.

In this shot I have used the largest aperture available on the lens f:2.8 and focused on the nearest flower. You can see that the others on the same stem very quickly go out of focus. The further away they are the more out of focus/blurred. I think it is quite a pleasant photo of Bleeding Heart, Lady in the Bath, Dicentra Spectabilis.

In contrast this photo is taken with the same exposure value i.e. using the same light levels and image brightness, but with an aperture of f:22 – the smallest the lens will go. As you can see there is a lot more that can be identified in this photo. The shape, texture and veining of the leaf is now visible.

For both these photos I set the sensitivity to ISO 1600 so that at the smallest aperture of f:22 I still had a sensible shutter speed (1/64s) to avoid camera shake or the wind causing motion blur. At the larger aperture of f:2.8 the shutter speed was 1/4000s.

The camera was set to program mode (P) and I adjusted the shutter speed/aperture combination with the finger wheel to give the aperture I wanted without having to worry about exposure or shutter speed.

Large aperture
Small f: number

I am not saying that one photo is better than the other, or that either of them is a good photo, but you can see that using a large aperture (small f: number) you can force attention on a particular area by reducing the distraction of other areas. Perhaps you have a view with a scruffy building or distracting road signs, or you don’t want the surroundings to be identifiable? Perhaps you want to concentrate/focus on a particular person in a crowd? Use a large aperture.

Small aperture
Large f: number

Equally there are reasons to go for a smaller aperture (larger f: number). If I want to send the photo to a plant ID website it needs to have flowers, stems and leaves visible. (But in this case I can ask my wife.) Perhaps you want the background to be in focus, or at least reasonably clear/identifiable – a once in a lifetime trip to the pyramids or the Great Wall of China. Use a small aperture.

Halves and quarters

For those that want to know why the f: numbers used are as they are read on. It may help with understanding but can be conveniently forgotten. It only means anything to those with one foot still in the film world. My suggestion would be to not read this section.

The f: number is a measure of the size of hole through which the light has to pass to get to the sensor. The number is based on the ratio between the focal length of the lens and the diameter of the hole. If a 50mm lens has a 50mm diameter aperture then the f: number is f:1. If the same lens has a 25mm diameter aperture, i.e. 1/2 the diameter, then that is f:2. If it is 12.5mm diameter then it is f:4. However the amount of light that a given aperture lets through is proportional to the AREA of the aperture. Therefore if you want to halve the amount of light let through then you need to halve the area. Doing the sums at f:1 on a 50mm lens the area is π*50*50/4 = 1963. If we halve that to 961 and work out the diameter we get 35. The f: number is therefore 50/35=1.4.

Proportional Areaf: number (full stop intervals)Diameter if f=50mm

This chart starts with an area of 1 and then halves it each time. The second column is the f: number that this gives. These are the numbers that are sometimes quoted as full f:stops

If we switch to a 200mm lens then we would need a 200mm aperture to give f:1 – enormous. The 200mm lens takes the light from a much reduced image area when compared to 50mm but if the aperture was 200mm then the amount of light let through would be the same as a 50mm lens with a 50mm aperture. As a result the f: numbers become independent. At f:5.6 the 200mm lens lets the same amount of light in as a 50mm lens. Therefore a given aperture, shutter speed and ISO that gives a correctly exposed photo using a 50mm lens will give a correctly exposed photo when using a 200mm lens.

With older more mechanical cameras the shutter speeds available were quite discrete – 1/30s, 1/60s, 1/125s. 1/250s, 1/500s, 1/1000s whereas now it is possible to set intermediate values or for the camera to set whatever speed it likes between the slowest and fastest. When going along the numbers mentioned the amount of light allowed through is either half or double the previous setting.

A similar condition applies to sensor speed as defined by ISO 12232:2006. In this standard the numbering is arithmetic – doubling the number doubles the speed. ISO 200 is twice the speed of ISO 100.

In photography terms this doubling or halving is referred to as a “stop”. This most likely comes from the fact that a lot of these settings were done by turning something between click stops.

Note that film speed can also be quoted using a logarithmic scale. ISO 100 is the same as ISO 21° using the log scale and ISO 200 is ISO 24° on the log scale.

In many of these settings the number is irrelevant. With the variety of sensor sizes and advent of image stabilisation the main thing is to get an understanding of how changing the different settings can change the image you get. Get an idea of where along the spread of minimum to maximum you set things so you can do the same thing later if it worked. Whose bothered what the numbers actually are unless you have the same camera?


One response to “Aperture & Depth of Field”

  1. […] of the background. Using the maximum aperture (smallest number) of your lens gives the minimum depth of field. To make the best use of this minimum depth of field it is then necessary to make the distance from […]

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