Bokeh is Japanese for ‘blur’…

Introduction

The aperture is simply the size of the hole in the lens through which light passes on its way to the sensor, and the principle is similar to that of the shutter speed. 

The bigger the aperture, the more light reaches the sensor and therefore the brighter the image (all other things being equal). The smaller the aperture, the less light reaches the sensor and therefore the darker the image.

The only difficult thing about it is the numbers, which often have a decimal point in them like f/5.6 or f/7.1.

The reason the aperture is not always a nice round number is because it is what you get when you divide the focal length of the lens by the diameter of the hole.

It would be nice if the aperture numbers doubled or halved with every stop like the shutter speed and ISO, but that’s never going to happen because the diameter of the aperture is only a measure of distance rather than area.

The formula for the area of a circle is πr², so if we double the diameter, the area gets multiplied by four.

Anyway, leaving aside the awkward Maths behind it, the aperture is measured in f-stops, which typically start at f/2.8, f/4 or f/5.6 and continue up to f/22 and beyond.

A 'fast' lens is one that has a wide maximum aperture such as f/1.4. Photographers like fast lenses as they allow pictures to be taken in low light and offer great flexibility. 

The reason why the aperture is such an important setting is that it controls the depth of field, which is the amount of the subject that is ‘acceptably sharp’.

The laws of Physics dictate that lenses will only focus on one particular point, so the words ‘acceptably sharp’ are used to refer to what is good enough for most people.

This is based on an arbitrary value for what’s called the Circle of Confusion (CoC), which is the maximum diameter in millimetres that a blurry spot on the camera sensor will appear as a point in the final printed image, but I wouldn’t worry about it unless you need a very large format print…!

(If you’re really interested, the Photopills depth of field calculator assumes a print size of 8’’×10’’ (20cm x 25cm), a viewing distance of 10” (25cm) and the manufacturer’s standard visual acuity, but you can always change these settings if you wish.)

The depth of field extends from the focal point in both directions - both towards and away from the camera - but a half to two-thirds of it will be beyond the focal point.

If you want to take a ‘traditional’ portrait with a narrow depth of field to isolate your subject, then you have a number of ways of doing it:

• Use a wide aperture (eg f/4).

• Make sure your camera has a large sensor (ie full frame 35mm)

• Get as close as possible to your subject (bearing in mind the minimum focusing distance of your lens).

• Maximise the distance from your subject to the background.

• Use a long focal length (eg 400mm)

The human eye is drawn to things it can see clearly, so making sure the subject is sharper than the background is an ideal way to focus the viewer's attention on an animal, say, but landscape photographers might want their images to be sharp all the way from the boat in the foreground to the mountains on the horizon.

For situations like that, most people would use a wide-angle lens and a narrow aperture such as f/16, but there’s a danger in going much further than that because you’ll start to get diffraction effects.

This is a lack of sharpness (or ‘Airy Disk’) due by light dispersed by the edges of the diaphragm within the lens, and it starts to show at f-stops greater than f/16.

Diffraction varies with each lens, so if you want to know more, you’ll have to shoot a few test shots at different apertures and compare the results at 100%.

For practical purposes, though, you can simply use a diffraction calculator to work out the aperture at which diffraction is likely to occur, and you can again find one on the Photopills site, together with a more detailed explanation.

As a result, the only times that my aperture gets that narrow are when I’m taking a slow pan (when it really doesn’t matter) or when I want to turn the sun into a pointed star (using f/22).

If you really want to maximise front-to-back sharpness in your images, there is also a useful concept called the ‘hyperfocal distance’.

Simply put, if you focus on a point at the hyperfocal distance (using the values printed on your lens), everything from half that distance to the horizon will be acceptably sharp.

If you still find that objects at infinity such as the stars at night are blurred, then you can either choose a slightly narrower aperture or focus the lens at a slightly longer distance.

It’s very difficult to calculate the hyperfocal distance manually, but there are plenty of photography apps that will handle that for you easily enough.

Again, there is no right answer when it comes to depth of field; the important thing is to experiment and find what works for you.

Depth of field options

Low

Traditionally, wildlife photographers tend to shoot ‘wide open’, which means using the widest possible aperture.

There are a number of reasons for this:

• The lower depth of field means that the object is nicely ‘separated’ from the background, which might otherwise be a distraction.

• More light falls on the sensor, which reduces noise and allows a faster shutter speed for action shots or in low light.

• The background blur - or ‘bokeh’ - will be softer, and any blurred highlights will be rounder rather than pentagonal or octagonal.

As a result, it’s a big advantage to have a ‘fast’ lens as the maximum aperture will be wider than normal.

A typical long lens will have a maximum aperture of f/5.6, but you can pay a few thousand more to get it down to f/4 or even f/2.8.

Just be careful, though, as the depth of field will often be tiny.

For example, using a 400mm lens at f/5.6 to shoot a subject 10 metres away, your depth of field will only be 20cm!

That’s fine if you’re taking a picture of a European bee-eater like the one at the top of this article, but it gets a bit risky if it’s a group of lions, say.

The key is to be aware of the likely depth of field before you start shooting, and you can do that in two ways:

• Use your depth of field preview button.

• download an app (or use an online depth of field calculator like the one from Photopills).

The advantage of using the depth of field preview button is that it gives you an immediate visual answer that you can see with your own eyes through the viewfinder, but it works by closing the aperture to the one you’ve chosen, so that might make it tricky to work out if you’re shooting in low light.

The alternative is to use an app, and there are a few free ones such as SetMyCamera that do the job perfectly well.

All you need to do is to enter the camera model, focal length, aperture and distance to the subject, and the app will calculate the depth of field for you.

It might also show a few other related figures:

• Hyperfocal distance

• Hyperfocal near limit

• Depth of field: near limit

• Depth of field: far limit

• Depth of field in front of the subject

• Depth of field behind the subject

Here are a few examples of shots that rely on a narrow depth of field.

Note that the aperture is only one element that goes into determining the depth of field, so not all of these were shot wide open.

Medium

There are a couple of reasons why wildlife photographers might want a little more depth of field in their images:

• To capture more than one subject at different distances from the camera (ie on different ‘planes of focus’).

• To give themselves more leeway when taking action shots in order to be sure of getting a sharp result.

Again, it’s important to have a vague idea of what the numbers are because you might find that you need a pretty narrow aperture to get the depth of field you need.

For example, in the same imaginary situation in which you’re using a 400mm lens to shoot a subject 10 metres away, switching from f/5.6 to f/8 would only increase your depth of field from 20cm to 29cm!

In fact, you’d need to shoot at f/14 just to get 50cm of depth of field, and even f/22 would only give you 82cm.

If you feel you’d have to stop down too far in order to get all your subjects sharp, then you can always try focus stacking

The basic principle is to take a group of photos of the same scene, each taken with the focus at a slightly different point.

For example, if you’re shooting two lion cubs, you might focus on the eyes of one, take a shot and then focus on the eyes of the other and take another shot.

These can then be easily combined in Lightroom.

If you’re interested in finding out more about focus stacking, I cover it in more detail in my article on Depth of Field.

Here are a few examples of shots that have a medium depth of field (both group portraits and action shots).

High

There would obviously be plenty of occasions when you’d want a high depth of field if you were a landscape photographer, but this article is meant to be about wildlife photography, so the rules are slightly different.

The most likely time when you might need lots of depth of field is when taking an ‘environmental portrait’.

This is just a fancy name for a shot in which the animal only occupies a small part of the frame.

The idea behind this kind of shot is that it puts wildlife in context and shows it in its natural surroundings.

Obviously, that would be a bit tricky shooting wide open as the entire background would be out of focus, so you’ll probably have to stop down to f/16 or f/22, depending on how far away you were and the length of your lens.

It’s also true, of course, that your aperture will probably end up quite narrow when taking a slow pan, but that’s really just a result of the slow shutter speed.

Anyway, here are a few examples of both situations.

And finally…

Just one final thought: this whole article has been based on the assumption that you’re focusing accurately.

If you accidentally focus on the wrong part of the animal (either too near or too far), then your depth of field will be in the wrong place, and the eyes of the animal or bird won’t necessarily be sharp.

To make sure, you’re focusing correctly, it’s a good idea to have a look at your images on the back of your camera at 100%.

Nikon users can simply press the centre of the Multi Selector Button when viewing an image to jump to 100% magnification, and this is easy enough to set up in the menu system.

If you’re really obsessed with sharpness, you should view your shots at 200% just to be absolutely sure…

If you’d like to order a framed print of one of my wildlife photographs, please visit the Prints page.

If you’d like to book a lesson or order an online photography course, please visit my Lessons and Courses pages.