Depth of Field

We investigate why digital camera lenses have more depth of field than 35mm lenses – and how it affects your photography.

Most photographers see control over depth of field (the range of perceived ‘sharp’ focus in a picture) as a critical tool for picture creation. Depth of field is controlled by three main factors: the lens aperture, the distance between the camera and the subject and the relationship between the lens and the size of the imaging area. It is at its greatest with small lens apertures, distant subjects and small sensors and is minimised with large apertures, close subjects and large sensors. These facts are true for all types and sizes of film and digital cameras.

When adjusting camera settings, photographers normally recognise two basic depth of field techniques. Selective focusing, which is popular for portraiture, restricts depth of field to concentrate the viewer’s attention on the most important aspects of a subject, allowing distracting backgrounds to fall out of focus. The viewer’s eyes will be drawn to those parts of the photograph that are sharply focused. The opposite strategy aims for maximum depth of field to increase the apparent sharpness in all planes of the subject, from near the camera to the distant horizon. This technique is particularly popular for landscape shots.

To obtain the best results from any camera you need to understand how its controls work. This is particularly true if you want to control depth of field. While it’s possible to focus selectively with a 35mm camera by simply shooting at f2.8 and you will certainly obtain a very wide depth of field if you stop down to f22, the former may produce a plane of sharp focus that is too slim to produce engaging portraits, while the latter could lead to camera shake because of the long exposure times required.

The situation is nothing like as simple with a compact digital camera. Compared to a 35mm SLR, selective focusing is difficult and, occasionally, impossible because of the difference in size between the sensors of compact digicams and a 35mm frame (see diagram below). To understand why this occurs, it is necessary to understand how depth of field is defined and, in turn, how it can be controlled.

Calculating Sharpness Limits

In practice, depth of field depends on what we identify as “acceptably sharp” in a picture. This depends on a number of factors, including the degree of enlargement and the distance from which it will be viewed. Essentially, we are asking: how large can a point in the viewed image be made before it starts to look like a small disk and causes the picture to start looking unsharp? A practical limit, known as the circle of least confusion, has been set to define this position.

At a typical viewing distance for an A4-sized print – about 40 cm – people with normal eyesight will be hard-pressed to discern a dot smaller than about 0.25 mm in size. If we start with a 24 x 36 mm image from a 35 mm camera and enlarge it to 21 x 30 cm size, this involves a magnification of approximately 8.33x. The size of the spot on the negative would, therefore, need to be 0.25 øƒ · 8.33 = 0.03 mm.

In contrast, most digital camera sensors measure between 5.27 x 3.96 mm (1/2.7-inch) and 8.80 x 6.60 mm (2/3-inch). Enlarging a picture from the former to A4 size requires a magnification of 210 øƒ · 3.96 = 53.03 times, while a magnification of 210 øƒ · 6.60 = 31.82 times is required for the latter. This gives a circle of confusion for the former of around 0.25 øƒ · 53.03 = 0.0047 mm, while for the latter it’s 0.25 øƒ · 31.82 = 0.0079 mm. Note: In practice, the circle of least confusion value for a digital camera cannot be smaller than the smallest element the sensor can resolve; i.e. the pixel pitch of the sensor’s photosites. For a typical compact digicam, this is typically 3 to 4 microns (0.004 mm), while a digital SLR has a pixel pitch of between 7 and 11 microns.

Establishing Depth of Field

A useful definition of depth of field is: the range of distances reproduced in a print over which the image appears to be sharply reproduced. You can now see the importance of the circle of least confusion to this definition and why depth of field can be influenced by factors other than the lens aperture and focal length.

When you wish to shoot with maximum depth of field, you need to focus on a point known as the hyperfocal distance. When the camera is focused at this point, everything in the picture should be sharp from half way between it and the camera right out to infinity. (Note: from a practical viewpoint, this may be the only calculation you need to make.)

You can calculate the hyperfocal distance with the following formula:

F²

(f x c)

where f = the lens aperture (f-number) and c = the diameter of the circle of least confusion.

If we take an 80mm lens on a 35mm camera at f11, the equation is as follows:

(80²) øƒ · (11 x 0.03) = just over 19 thousand millimeters, which works out at roughly 19 metres. If you focus on this point, everything will be sharp between about 9.5 metres (halfway between the camera and the hyperfocal point) and infinity – for the defined lens aperture.

Carrying out the same calculation for a digital camera, where the 12.6mm lens with a 6.59 mm (1/2.7-inch) sensor is equivalent to an 80mm lens on a 35mm camera gives us the following equation:

(12.6²) øƒ · (11 x 0.0047) = 3071 mm, which is just over 3.7 metres. If you focus on this point, everything will be sharp between about 1.5 metres and infinity – provided you don’t change the lens aperture.

It’s easy to see which camera has the greater depth of field – and why selective focusing can be so difficult with compact digital cameras!

Near and Far Limits

Although portrait photographers usually prefer selective focusing, it can sometimes by advantageous to have a large depth of field and, in such situations compact digital cameras can produce better pictures (or at least good pictures more easily) than 35mm cameras. For close-up and macro photographs of small three-dimensional subjects, such as flowers and insects, the extended depth of field of a compact digital camera can provide sharp resolution over virtually all of the subject at reasonably large lens apertures and shutter speeds that allow the camera to be hand-held.

When you’re shooting close-ups it can be useful to establish precise depth of figures – especially when you’re using a 35mm camera. This involves calculating the near and far limits of the zone of acceptable sharpness. The calculation is easy once you’ve established the hyperfocal distance for the subject distance and lens focal length/aperture you are using.

The near limit (N) is derived with the following formula:

N = HD

H + D

And the far limit (F) is derived with:

F = HD

H – D

Where H = the hyperfocal distance and D = the distance at which the camera is focused.

Putting the above equations into practice, using the hyperfocal distance figures we’ve obtained for the 35mm and digital cameras above, we get the following results with a subject at 0.6 m from the camera:

For the 35mm system:

N = 19 x 0.6 = 0.58 m

19 + 0.6

F = 19 x 0.6 = 0.62 m

19-0.6

This gives a depth of field of 0.04 m (or 4 cm).

For the digital camera:

N = 3.7 x 0.6 = 0.52 m

3.7 + 0.6

F = 3.7 x 0.6 = 0.72 m

3.7 – 0.6

This gives a depth of field of 0.2 m, which is 5x greater than the 35mm camera delivers with the same effective lens aperture and subject distance. This is one area in which some compact digital cameras really shine as they offer closer focusing distances than their 35mm film equivalents. However it’s much more difficult to produce a portrait with a blurred-out background when you shoot with a compact digicam.

A ‘Quick & Dirty’ Method

Because the size of the imaging area has such a critical impact on depth of field, you can make a ‘quick and dirty’ comparison between the depth of field of your digital camera and a 35mm film camera by using the LMF x F rule. This simple calculation states that the depth of field for a digital camera at a given aperture (F) is the same as that of a 35mm camera with the lens closed down to the same aperture multiplied by the lens multiplier factor (LMF).

The following equation is used:

LMF = 35mm equivalent focal length
camera’s actual lens focal length

For example, a Nikon Coolpix 5400 camera, which has a 5.9-24mm zoom lens that is equivalent to a 28-116mm lens on a 35mm camera would have an LMF range from

28 = 4.7 to 116 = 4.8

5.9 24

Thus, if we set the lens aperture this camera to f2.8, it would provide acceptable sharpness for the same distance range as a 35mm camera with the lens closed down to

4.7 x f2.8 = f13.16 at the wide end of the zoom and

4.8 x f2.8 = f13.44 with the tele position.

In practice, most non-interchangeable lens digital cameras have LMFs between 3.8 and 8.2, with the larger, SLR-style models being at the lower end of the scale and the ultra-compact ‘visual note-taker’ types at the upper end (video camcorder LMFs are often as great as 10 to 12). Digital SLRs like the Nikon D100 and Canon EOS 10D have LMFs of 1.5 and 1.6 respectively. Interestingly, the Olympus E-1 digital SLR, which is due for release in October, has an LMF of 2.0.

Optical geometry also influences the smallest apertures a camera can support (which are defined by diffraction limits). As you might expect from the diagrams on these pages, the larger the format the smaller the minimum aperture (i.e. the larger the top lens f-number) that can be used before diffraction starts to degrade image sharpness. For this reason, although digital SLRs can usually support the same aperture ranges as 35mm cameras, few compact digital cameras offer minimum apertures smaller than f8 -and camcorders are often restricted to f4 or f5.6.