Editors Note: Depth of Field | From Lastolite School of Photography was originally written by Mark Cleghorn for www.lastoliteschoolofphotography.com and has been reproduced with permission.
Depth of Field
When shooting a photograph, you will need to choose a focus point within the scene whether it is a landscape or a close up subject like a portrait. When the point of focus is selected, the rest of the scene is deemed out of focus unless on the same plain as the focus point.
To control the depth of focus we use a combination of aperture, focal length and the subject’s distance to the camera. It takes all 3 elements to fully control the depth of field of focus.
Depth of field can either be used to control the sharpness int he image especially when fine details are required or it can be used creatively to throw a background or foreground subject out of focus. Depth of field alone will make an image stand out from the crowd by either significant detail due to focus or dramatic blur due to out of focus.
Lens Focal Length – The lens selection is an important factor in the equation as this alone can exaggerate the loss or increase int he focus area. A longer lens like 200mm will drop the field of focus suddenly due to its magnification and produce a shallow depth of focus making it perfect for portraiture.
A wide angled lens such as a 24mm will help maintain a greater depth of focus especially when used at a distance to the subject making it perfect for group photography and landscapes.
Aperture – The chose f-=stop is the secret here, as a small number like f/2.8 will exaggerate the loss of the depth in focus, whist an f-stop of f/22 will help contain more focus depth in the image.
The focal length of the lens and the aperture will have different results when combined together depending on how close the subject of focus is to the camera position and the set aperture.
Camera to subject distance – As a rule of thumb, the closer the camera is to the subject, the less the natural depth of field with the combined aperture and lens. This of course is the reverse when the subject is a long distance from the camera as more natural depth of focus is maintained especially with a wider angled lens.
Creative or Captured? – When shooting a portrait session there will be a variety of full-length images and head & shoulders, so to make sure I have enough variety for the client, I basically use 2 different focal length lenses:
24-105mm – Is my all round lens as it allows me to shoot everything from full length to three quarter portraits, whist still allowing me to be close to the subjects so I can interact with them for expression. The only problem is that as soon as the lens is zoomed back to its 24-55mm focal length, the background becomes more visible.
70-200mm – Is my favourite lens as it quickly separates the subject from the background due to its increased focal length. It throws a background out of focus, even when shooting a three quarter or full length portrait, especially when shooting with a wide open aperture.