Depth of Field

Scroll down for your free depth of field calculator

What do we mean by depth of field? Not surprisingly, the best way to get this point across is to show a couple of example photographs.

 

The first of these, on the left, is typical of the kind of results you'd get using a 'point-and-shoot' compact camera, which is exactly what I used to take it. These are simple to use, but offer few facilities for you, the photographer, to influence the outcome. The example on the right is the kind of thing you can produce with most SLR cameras, whether 35mm or digital. You've probably spotted the difference. In simple terms, the background is blurred. Two questions usually pop up here:- Why is this a good thing? How do you achieve it?

O.K., before everyone flames my email box, I admit I helped the second photo along a little using Paint Shop Pro. ;>) I thought the photo was a good example of what I was trying to illustrate, but I only had the one copy of it so I created a little depth of field!.

O.K., first things first. Why would you want a blurred background?

Being able to produce this effect at will is very handy and can turn a mediocre or boring photograph into something much more attractive. The first photograph above doesn't really have a focal point. It's rather flat and two dimensional, and you're not really sure of exactly what the photographer was aiming at when he took it.

The second example is very different. It's obvious immediately what the main subject is because it is separated from the background clutter. The main subject is the only thing you can focus on, so your eye is drawn straight to it. If you're taking a shot from a long distance it is possible to isolate the subject both from the foreground and the background using this technique known as depth of field.

Here's a diagram to explain depth of field a little better, then we'll look at how we achieve it.

 

Diag. 1

What we want to do is focus on the main subject in the diagram. The area between the two vertical lines is the area that will be in focus in the finished photograph. This is the 'field' in the term 'depth of field'. The 'depth' bit is the distance between the two lines. We can in fact move these two lines further apart or closer together as well as moving them individually closer to or further away from the main subject, by manipulating the camera controls.

 

How do we achieve this effect?

There are two things you have control over that affect depth of field when using an SLR camera.

On 35mm SLR's the 'standard' focal length was agreed to be 50mm. and zoom lenses were referred to by the focal lengths at either extreme of their range. A typical lens might be referred to as a 70-210mm zoom. In order to make digital cameras physically smaller and easier to handle and to keep down the cost of the ccd device used to capture the image, they use a smaller format than 35mm. This means that the 'standard' lens has become 37mm instead of 50mm. which has the effect of reducing the focal length required to produce photographs comparable to 35mm format. Manufacturers have also taken to referring to zoom lenses as 2x or 4x zoom which refers directly to their degree of magnification rather than their focal length.

• the focal length of the lens
• the aperture setting

I'll go into more detail about these two settings in another article, but put simply the focal length of a lens is the feature you are changing when moving a zoom lens within its zoom range. The aperture refers to the hole the light passes through when the shutter opens. This is changed by moving the aperture ring, which is the nearest one to the camera body, and is measured in units known as f-stops. When changing aperture settings you need to keep an eye on the shutter speed. Look through your camera's auto modes and set it on one which will allow you to change the aperture settings, but will automatically set the shutter speed for you. This is often referred to as 'aperture priority mode'.

Click here for an explanation of why you need to do this.

Aperture setting.

The most important item to control is the aperture setting. The larger the aperture used, the smaller the depth of field range.

Let me illustrate this. In the following table,

• f-stop refers to the selected aperture setting
• Subject distance refers to the distance from the camera to the main subject of the image
• Near Limit refers to the vertical line nearest the camera in Diag. 1 above
• Far Limit refers to the vertical line furthest from the camera in Diag. 1
• Total DoF is the distance between the vertical lines in Diag. 1

All measurements except focal length are in feet.

Effect of Focal Length and f-stop Settings

Focal Length	f-stop	Subject Distance	Near Limit	Far Limit	Total DoF
28mm	2.8	15	13.3	17.3	4
28mm	4	15	12.7	18.4	5.76
28mm	16	15	8.61	58.1	49.5
70mm	2.8	15	14.7	15.3	0.62
70mm	4	15	14.6	15.5	0.58
70mm	16	15	13.4	17	3.58
28mm	2.8	50	34.7	89.1	54.4
28mm	4	50	30.8	131.9	101
28mm	16	50	14.4	infinity	infinity
70mm	2.8	50	46.7	53.8	7.04
70mm	4	50	45.5	55.5	10
70mm	16	50	35.8	82.8	47

The above table makes it obvious that depth of field can be a very slippery character! There's no quick 'n easy way to estimate it's impact on an image other than experience. If you try experimenting by taking the same subject using different aperture settings you will soon pick up a feel for this very useful feature.

Free Depth of Field Calculator

At one time, lens manufacturers used to include depth of field guide marks on their lenses, but no longer do so. If you would like a handy little replacement for these marks, something that will let you quickly and easily guage the effects of different focal lengths and aperture settings, click here to download a handy little piece of free software that will allow you to calculate settings on your PC or print out depth of field scales to carry in your camera bag.

Practice makes perfect

You need to familiarise yourself with how this technique works and how it affects your photographs. Remember that, with a digital camera there is no expense involved in shooting practice photographs. You get the results straightaway and you don't have to pay for developing any film, so there is no excuse for not practicing. You can even practice indoors if the weather's poor. Standing a couple of items on your kitchen table and shooting them from a few feet away will soon give you an idea of how this works. A good way to develop your feel for the depth of field effect, as well as any other techniques you may wish to brush up on, is to use a technique known as 'bracketing' your exposures. Put simply, this just means taking a number of photographs with different camera settings so that you can compare the results.

Stand three coffee mugs on a diagonal line near the middle of your kitchen table then stand back a couple of paces. Set your camera on fully auto and focus on the mug in the centre of the line and take your first shot. It's good to have a pen and paper so that you can make notes of the camera settings for each shot otherwise you're likely to forget how things were set up for a particular shot, especially if you take quite few.

Now take another shot, but focus on one of the other mugs, followed by a third focusing on the last mug.

Set your camera to it's aperture prioriy mode. This will allow you to select an aperture setting and the camera will vary the shutter speed to get the correct exposure. Now take a series of three shots as you did before. Make a note of your camera settings for each one.

Open the aperture one stop and repeat. Do this with a variety of aperture settings, making notes as you go.

You can now load the photos onto your computer and compare the results. You should now have a good idea about how varying the aperture affects the resulting depth of field.

Go through this exercise again varying the focal length of your zoom lens if your camera has one.

 

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