DOF: What is it?
Depth of field is the nothing but the field behind and in front of the subject that is in focus. It is controlled by the aperture of the lens (larger the aperture, smaller the DOF).
Remember: The smaller the aperture number, the larger the aperture is and hence more light falls on the film/sensor.
DOF: Explained with example
The most popular example to explain DOF is tap and paint. Consider a tap which is supplied with paint source and directly below it is a plate or paper where you want the paint to be applied. Now open the tap slowly such that only one droplet comes out of it. You will notice that the paint falls on the paper/plate and forms an even circle. Now turn the tap to pour the paint fully onto the paper. It fills up the entire plate quickly but not evenly. Now consider the same anamoly for light falling on the film/sensor through the tap called lens. The larger the opening on the lens, the more the lights get scattered and more distortion in the image. If the aperture is small, the light falls evenly and less the distortion.
This image from Wikipedia explains DOF diagrammatically
So an object which is focused by the lens, passes the light to the sensor through the aperture setting of the lens falls on the sensor it exposes the picture. But the light from the surrounding objects near the subject falls evenly if the aperture is low thereby making them also appear in focus. If the aperture has been increased, it would have just focused the subject and distorted the surroundings.
DOF: Usage
DOF does not necessarily mean bad always. It is sometimes used to add more attention to the subject when focus alone cannot make the subject stand out. On the other side, if we want to keep everything in focus, we need greater DOF and hence smaller aperture. But how small is too small? That is explained in the Hyper focal Distance section.
Below are some pictures taken at different aperture setting by me. You can see the difference in how the DOF varies by aperture.
Taken at f1.8 - Shallow depth:
Taken at f4.5 - Shallow depth better than f1.8:
Taken at f8.0 - Medium depth:
Taken at f22 - Greater depth:
DOF: Hyper-focal Distance
A Hyper focal distance is the minimum distance that when focused by the lens with a particular aperture and focal length of a lens will have everything from 1/2 of the minimum distance to infinity in acceptable sharpness and yield the maximum depth of field.
There is a mathematical formula attached to this which is:
Hyper focal distance in millimeter = (focal length in mm) ^2/(Aperture number * 0.03mm) + focal length
But calculating this while in the field is not only tedious but you would miss the photo opportunity. So there is a cool tool available called DOF Master which you can download and get print out of charts. The chart gives you the distance where you need to set your lens to focus at a given aperture and focal length to achieve the hyper focal setting. Click here to download the chart.
Here is a photograph taken with Hyper focal setting.
