Depth of Field

Depth of field (or DOF) is the zone of sharp focus in front of, behind, and around a subject. A photographer decides on the ‘look’ of his photograph by choosing this depth of field consciously. For example, a shallow depth of field results from the subject of interest being in much sharper focus than all other elements.

So what are the advantages of utilizing a shallow Depth of field? Generally speaking, the human eye does not percieve the world in this manner, unless plagued by sight problems? So why use a shallow DOF? Well for starters, viewing your subject in a shallow DOF makes for a fascinating and unique experience.

Changes to the Depth of field (DOF) depend on following parameters:

• Aperture,
• Subject magnification,
• Shooting distance
• Sensor size

The larger the aperture, the shallower the DOF and vice versa. The Depth of field is much shallower at f/2.8 than at f/16, keeping all other parameters constant.

The larger the magnification of the subject, the shallower the depth of field. Subject magnification can be altered in two ways, either by approaching the subject closely or by using a larger focal length lens. So, a 20mm focal length wide angle lens will produce a larger DOF as compared to 200mm focal length lens, if you keep the aperture and the shooting distance constant as the magnification provided by a 200mm lens will be more than that of a 20mm lens.

Having said that, the focal length, per se, does not influence the depth of field, contrary to popular belief. Even though telephoto lenses appear to create a much shallower depth of field, this is primarily due to the fact that they are often used to magnify the subject when one is unable to get closer. Also, when a subject occupies the same fraction of an image, when viewed through either a telephoto or a wide angle lens, the depth of field is virtually constant with the focal length.

A 20 mm wide angle lens at 1 meter produces a DOF of 42 cm. A 200 mm Tele lens at 10 meter also produces an equivalent DOF. Thus, the DOF is the same as the magnification.

The closer the shooting distance, the shallower the depth of field. As we near the subject, the resultant magnification cause the depth of field to shrink. Similarly, the further we move from the subject, the more the DOF increases, finally reaching a point where it becomes infinite.

A parameter that cannot be changed (easily) without changing the camera, is the sensor. A full frame camera sensor produces a shallow DOF as compared to a cropped frame. Point and shoot Digital cameras & mobile phone cameras possessing a tiny sensor, in turn, produce a very deep DOF.

As a photographer creating imagery, depth of field is a vital component in most genres of the art form. While visualising an image, you’ll need to decide upon the kind of DOF you wish for your subject to possess, often by highlighting a particular aspect and limiting the DOF away from a confusing and often boring background. By using a wider aperture, lesser distance from the subject and a larger focal length, you can create an ultrathin DOF, just like what you see in my photo of young cat Rafiki, who prowls our garden, here.

Even though the lens focuses on a plane, the depth of field is not a single plane, for it spreads evenly across a small area. Since there is no critical point of transition, a more rigorous term called the “circle of confusion” is used to define how much a point needs to be blurred in order to be perceived as ‘unsharp’. When this ‘circle of confusion’ becomes perceptible to our eyes, the ‘unsharp’ region, also known as ‘bokeh’, is said to be outside the depth of field.

Thus, the subject should ideally be situated within this ‘circle of confusion’, in order to appear sharp. Also, any background should be outside it and well into the region called the “bokeh”. The ‘Circle of confusion’ and ‘bokeh’, deserving of a more comprehensive treatment, will be expanded upon in subsequent posts.

I thank Javed Ahmad for helping me in editing this blog.