17/11/09

So what is Depth of Field, why is it important, and how do you control it ?

Depth of field (DOF) is a term which refers to the areas of the photograph both in front and behind the main focus point which remain "sharp" (in focus). Depth of field is affected by the aperture, subject distance, focal length, and film or sensor format.

APERTURE

A larger aperture (smaller f-number, e.g. f/2) has a shallow depth of field. Anything behind or in front of the main focus point will appear blurred. A smaller aperture (larger f-number, e.g. f/11) has a greater depth of field. Objects within a certain range behind or in front of the main focus point will also appear sharp.

DISTANCE & FOCAL LENGTH

Coming closer to the subject (reducing subject distance) will reduce depth of field, while moving away from the subject will increase depth of field. In simple terms, that is why images taken in Macro mode (very close up) have a shallow DOF, accentuated if shot at f/2, and landscapes for example have a greater DOF, again accentuated if shot at the other end of the Aperture scale f/11 and above.

Lenses with shorter focal lengths produce images with larger DOF. For instance, a 28mm lens at f/5.6 produces images with a greater depth of field than a 70mm lens at the same aperture.

AUTOMATIC APERTURE MODE

In "Aperture Priority" mode, the camera allows you to select the aperture over the available range and have the camera calculate the best shutter speed to expose the image correctly. This is important if you want to control depth of field or for special effects. Note that because of their high focal length multiplier, a shallow depth of field is often very hard to achieve with digital compact cameras, even at the largest aperture.

Selecting Aperture Priority mode on your camera can be simply done, depending on which camera you own, it will either be an option in one of the menus, or alternatively it can be by the turning of one of the dials, normally positioned on the top of the camera. The relevant position will be shown generally with either an " A "or "Ap", although some brands do use different symbols . Unfortunately you will have to check for the control position within your manual

Below l have reproduced one image taken with three different Apertures in an effort to show the simple effects of Aperture settings. The first is at f/2, the second at f/4 and the third at f/11.
















































































The differences can be clearly seen, especially in the last image, that taken at f/11

In addition below is a photograph taken in Macro mode at f/2, and from which you can see that the background is well and truly blurred out. This particular effect is sometimes referred to in magazines and such like as " Boketh " and which can in certain circumstances be pleasing to the eye :














































To assist further, l reproduce below the text of a workshop undertaken by a member of the Society, Simon Lutter, on this very subject, which may assist in understanding further the topic :

Depth of Field

Depth of Field is the acceptable amount of sharpness – or focus in a picture.

When you take a photograph, there are a number of manual mode controls that you can use in a camera.

• Aperture Priority
• Shutter Priority
• Manual Mode
• Manual focus
• ISO settings.

Without going into too much detail there is a relationship between the size of the aperture, the ISO Film Speed and the speed of the shutter as it crosses the film plane or digital sensor on a camera.
There is also a lot more to Depth of Field than time allows here so I am going to keep this brief and simple.

By the end of this session you will find out the advantages of Aperture Priority and why there is a good creative reason to start using it.Basically an Aperture is a hole in a lens that you can regulate the size of.

The basic principal is the wider the hole the more light gets to the film or sensor, and the smaller the hole the less light gets to the or film sensor.

Aperture on lenses is measured in f stops. If you stop (turn the f number higher) the lens down you are reducing the size of the hole and if you open the lens up you are increasing the size of the hole. Some lenses will allow you to vary the size of the hole in ½ stops. The measurements can go from f1.2 to f33 or more. f1.2 being the widest hole and f 33 being the smallest hole.
It all depends on the type of lens you have.

Some lenses are referred to as Fast lenses meaning that they have a wide aperture. For the moment all you need to know is the smaller the aperture the more you will get in focus and the wider the aperture the less you will get in focus. Changing the aperture will change the way you want your photograph to look in terms of how much of the picture will be in focus.

A rule of thumb is for pictures of faces (portraits) a large aperture is desirable because creatively the eye wishes to concentrate on the facial features with the background out of focus and for landscapes a large amount of focus is desirable so that all the detail in the scene can be admired. To use aperture priority you will need to also control the shutter speed. The reason for this is to get the correct exposure and a satisfactory exposed image.
The wider the aperture the faster the shutter needs to be and the smaller the aperture the slower the shutter needs to be. However this will depend on the prevailing light conditions.

In older cameras aperture is controlled on the lens by means of a ring on the lenses, modern cameras this is usually done with a dial on the camera body. Some cameras may have a Depth of Field Preview Button By depressing this button you can check your depth of field. The distance that your subject is away from the camera will also dictate the degree of focus in your picture.
By altering the size of the aperture you control the Depth of Field and therefore you are deciding the amount of focus in your image.
If you set a modern camera to Aperture Priority it will automatically set the shutter speed for you.

Some lenses on the focus ring have numbers printed on them indicating the focus area in feet or meters with an infinity sign-max depth of field.( no 8 on its side)