WHAT IS DEPTH OF FIELD?

 
The following notes are an extract from part of a lesson in our  100 hour Photojournalism Practice Course
To see an outline, go to http://www.acs.edu.au/courses/photojournalism-practice-i-580.aspx
 
Depth of field is the amount of the picture that is actually in focus. It is the amount of distance between the nearest and farthest objects that appear in sharp focus in a photograph. A shallow (narrow) depth of field is created by using a larger aperture (smaller F-stop).  A picture that has a clear picture of a person with the background blurred would be regarded as having a shallow depth of field (larger aperture). When an entire image is in focus, it is regarded as having a wide depth of field (smaller aperture).

Depth of focus reduces proportional to the width or size of the aperture used to admit light during exposure.  In other words – the bigger the hole which lets light in to the lens – the shorter the depth of field.

Aperture (the hole / F-stop) controls the volume of light that is let into the lens. Aperture can be adjusted to compensate for light conditions and control depth of field.  By adjusting the aperture, the photographer controls the depth of field and thus what is communicated to the viewer.

Controlling aperture enables the photographer to keep the subject in sharp focus while blurring the background. In this way, a photographer can use the depth of field to place emphasis on the subject.


For example:

If a photo of an object (for example, a person) is taken at a focussed distance of 10 ft. then at f16 everything from 3 ft. from the camera to 50 ft. behind it will be in focus.

But at f2.8, again focussed at 10 ft., only objects from 9 ft. and 12 ft. will be in focus. This fact allows us to effectively isolate a subject from its background by using a large aperture and rendering the background as a blur.  

When the aperture (the hole that lets the light into the camera) is at its largest (low f-stop number), the colours that are out of focus actually blend and blur together.

A wide depth of field would be when everything in the picture is totally in focus.  A shallow depth of field may be when the foreground and background are blurred, but the object in the picture is in clear focus.

Using a small aperture, several different planes of the image (each of which can contain compositional elements), can be drawn together in perfect focus and great depth.

Here is a table of widest to smallest apertures on a given camera:

Largest aperture     f1.4   f2   f2.8   f4   f5.6   f8   f11   f16   f22    Smallest aperture
(Shallowest depth of field)                                                             (Widest depth of field)

Remember that the largest aperture refers to the smallest f numbers and vice versa.

EXAMPLE
A hypothetical photograph taken at F16 with the main subject at 3 metres from the camera, the effective depth of field could be 1.8m in front of the subject and 12m behind the subject. Anything within this range would be in focus.

If the same subject was photographed with the F stop on 2.8, the depth of field would be greatly reduced, with only about 0.7m in front of the subject to 2.5m behind the subject being in focus.

This phenomenon has both advantages and disadvantages.

Disadvantages

  • If we raise shutter speed and open aperture to get better camera stability, we lose depth of field.
  • Focussing must be much more accurate on the main subject with a reduced depth of field. Focusing in dim light can be a problem. You may need to use a tape measure to check focus if it is difficult to see.

Advantages
• We can choose to either retain the background or make it blur if it detracts from the picture.

DIGITAL VS CONVENTIONAL

Conventional and digital photography have their similarities and differences. Both have their advantages, so in the foreseeable future, there will remain applications for each.
Conventional photography using chemically photo-sensitive film is a well known and highly developed quantity – very close to a perfected technology.
We know how to use it, how to get the best out of it, and how its life span can be optimised because it has been around for so long, used so much and had so much effort and expense spent on its development.

Digital photography is, on the other hand, a relatively new and radically different technique which records images in the form of digital (ie. 2 digit or binary) codes. In simple terms digital codes are similar to Morse code. One number or digit is indicated by a pulse of electricity, a second digit is indicated by no electrical pulse. By combining these pulses and lack of pulses into codes, we can, for example, create representations for letters of the alphabet; allowing us to write language or text on a computer. When we combine these electrical "pulses" and "no pulses" (or ‘ones’ and ‘zeros’) in more complex combinations, we can create more complex representations. These can include the colour, and degree of darkness or brightness in a single spot on a picture. When huge quantities of such dots are combined together, into a grid or array, we can then create a digital picture. (This is basically how digital photography works!) Each dot is referred to as a pixel (PICTURE ELEMENT) and is represented by ‘bits’ of data – thus the digital image array is often referred to as a ‘bitmap’.

As time passes, digital photography is improving and its imagery now rivals that of traditional silver halide based photography. However, due to its nature, it is unclear at this point whether it will ever make traditional photography totally redundant, particularly in situations where extreme resolution or detail is required. Current levels of technology suggest digital will eventually become the technology we rely on for creating still images. However, in the shorter term silver image systems will actually be cheaper to use in many applications.


Digital Depth of Field

Digital cameras typically don’t have as wide a range of aperture settings as standard film based cameras, therefore you cannot alter depth of field to the same degree as you can with an SLR film camera. However, it is still possible to produce a distinct shift in depth of field.
 

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