Aug 112011

I have decided to write several articles on depth of field. The articles will range from ‘What is depth of field?’ to ‘Problems and solutions associated with shallow and large depth of field’, the latter is detailed as it includes useful information on diffraction.

This week we’ll start with the basics. What is depth of field and how is it controlled?

Depth of field (DOF) is essentially how much of an image is in focus. Shallow DOF means that only the distance you have focused on will be in focus. A large DOF means that most of the photograph is in focus.

The combination of a telephoto lens (600mm), wide aperture (f4) and large, full frame sensor give this photograph a very shallow depth of field.

Depth of field is controlled by the following:

1. The size of the aperture in the lens: Small or shallow DOF = wide aperture = low f number (e.g. f2.8). Large DOF = small aperture = large f number (e.g. f16)

2. The focal length of the lens: Wide angle lenses give a much greater depth of field than a telephoto lens.

3. How close you are to the subject: Close = shallow depth of field. Far away = greater depth of field.

4. The size of the sensor: Small sensors = large depth of field (e.g. compact cameras), large sensors = shallower depth of field (e.g. medium format cameras)

The combination of a long focal length (105mm) and close distance to this flower means that it is very difficult to get a large depth of field despite using an aperture of f11.

Why use a shallow depth of field?

The advantage of using a shallow depth of field is that you can turn a distracting foreground and background in to a complete blur. This makes your subject really jump out, such as the photograph on the left. Therefore, a shallow depth of field is very useful when taking portraits and photographing animals.

A wide angle lens (20mm) and small aperture (f32) give this photograph a large depth of field.

Why use a large depth of field?

A large depth of field is used when you want both the foreground and background of the photograph to be sharp. This is traditionally the case in landscape and architectural photographs. This is typically achieved by using a combination of a wide angle lens and small aperture.

It is important to note that when you use a small aperture, the only area of the photograph that is truly ‘in focus’ is the part of the photograph that you have actually focused on. E.g. if you have focused on a point 10 feet away, then everything at 10 feet is critically sharp.

With digital cameras using a small aperture (usually smaller than f11 on dSLR’s) can cause a problem called diffraction. Diffraction can be a considerable problem if you create large prints. I will look at what diffraction is and to solve the problem in a future blog posting.

3 Responses to “Understanding depth of field”

  1. Jim Comfort says:

    Great Site!! Concise info easy to understand.

  2. Scott says:

    I am having a problem to understand how DOF works with TS lenses. Aperture set up does not seem to reflect DOF. Could you explain this?

  3. Richard says:

    Great question! Tilt-Shift and Perceptive Control lenses are difficult to focus but you can see the changes in depth of field as you change the aperture, however, those changes may not be where you expect them to be. Let me explain….

    When a tilt shift lens has no tilt, then it works exactly the same as a regular lens. I.e. the plane of sharpest focus is a vertical line at a distance determined by the focus distance on the lens. Increasing depth of field allows subject matter in front and behind the plane of sharpest focus to become sharper.

    In very simple terms; when you tilt a Tilt-Shift or Perceptive Control lens, the plane of sharpest focus becomes a diagonal line that runs fore-aft. Increasing depth of field allows subject matter in above and below the plane of sharpest focus to become sharper. The goal is to tilt the lens so that the plane of sharpest focus goes through your foreground and background and then have enough DOF so that everything above and below that line is also in focus.

    If you are using a wide angle (24mm) TS or PC lens then it can be very difficult to see all this in the viewfinder. When I’m using my Nikon Perceptive Control lenses then I use the live view mode to focus them which makes the whole process much easier.

    I hope that helps!

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