Depth Of Field: An Overview

Depth of field is one of the oldest tricks in photography. It is a techniques used by photographers to bring everything in the frame right from the closest object to the farthest object in clear focus. This means that both the foreground and background are in clear focus. This technique is used most commonly in landscape photography and it can result in stunning photos. This article will delve into the intricacies of this technique.

The depth of field or DOF is dependent on three basic factors:

1.  The focal length

2.  The aperture size

3.  The object on which you focus in your composition

Let us discuss each of this in detail.

Focal Length

As a general rule, wider the angle of focus, greater is the depth of field. So it is appropriate to use a wide angle lens if you have one. Otherwise set the zoom lens to its widest range. The ideal focal length to get the perfect DOF is around 24 mm. This figure has been arrived at after lot of experimentation with the 35 mm format. This would mean that you will require a lens which has a focal length of 18 at its widest range. This is because the DSLR's (unless you are using the full format DSLR's) has a sensor size smaller that the 35mm film format. This alters the relationship between the camera and the lens. A regular DSLR like Nikon D70 or the Canon EOS 400 has a sensor surface area of around 22 sqmm. Therefore the focal length of the lens will have to be multiplied by about 1.5 to get the concordant focal length of a 35mm format. For eg, a 18 mm lens in the DSLR will translate into a 27mm lens in 35mm format.

Since most of us use the normal DLSR's the 18-55mm kit lens that accompanies most cameras will do. If you have a 10-22mm lens it is better.

However if you are using a digital point and shoot then there is a problem. The range of focal length provided by most point and shoot is in the range of 34-114mm. This focal length is a little narrow angle to get a good DOF. Therefore you will need a wide angle adapter for point and shoot cameras. Most cameras have a wide angle adaptor which can be used to remedy this problem. Therefore no matter what camera you use, remember this figure of 24 mm. This is the ideal focal length required for the DOF experience.

Lens Aperture

As a general rule, larger the aperture lesser is the depth of field. So as a corollary if you want a greater DOF you should set your camera's aperture to the smallest possible value. I personally prefer to use whatever is the smallest aperture which the lens would allow. Generally most cameras will allow a f stop of f/22. There are two methods of doing this. You can either use the aperture priority or manual mode. In aperture priority mode, you set the aperture and the camera automatically selects the shutter speed depending on the light conditions. I personally prefer to use the manual mode as this gives me a greater control over the camera. In the manual mode, you have the option to set both the aperture and shutter speed and hence I prefer to use this mode.

Since the aperture is going to be small, the shutter speed will have to be slow. Generally speaking if the f stop is f/22 then you will need to keep the shutter speed at 1/8 or even less. Therefore it is prudent to use a tripod to prevent camera shake.

Set focus manually

This is the last and most important aspect of getting greater DOF. Turn off the autofocus and set the focus manually. Now focus on a spot that is one third the distance between the foreground and background. For eg, if you were taking a photo of a flower with a mountain in the background, focus on a point which is one third the distance between the flower and the mountain. It may be difficult if you do not have an object to focus on, but mostly you should be able to find something to focus on. Some serious photographer will go to the extent of carrying many props for this specific effect; some even carry indigenous plants for this purpose.

However when you initially look through the view finder, both the foreground and background will look out of focus. This is because the aperture is wide open and closes to the smaller value only when you release the shutter. Therefore most DSLR's come equipped with a DOF preview button. When you press this button, the aperture becomes small and you will find both the foreground and background to be in focus. Mind you that when you press the DOF preview button, the image will become darker, because less light is entering the lens.

By using the above three methods you should be able to get the perfect landscape shots.