Controlling Depth Of Field
Introduction
Eye-catching photographs have a focal point – an area in the image that people are drawn to.
In portraiture, for example, a person’s face is usually the focal point. There are numerous ways to
create a focal point, but one of the most powerful is controlling sharpness.
Sharp vs Soft
Sharpness in a well-exposed image reveals detail that would be obscured by softness or blur.
Our eyes are naturally drawn to the sharp area of an otherwise soft image.
The precise point within the frame that you focus on is the point that should be ‘pin sharp’, if
you focus accurately and avoid camera shake. Everything the same distance from your camera
as that point will also be in focus. Anything closer or further away will become increasingly softer
or out of focus. The extent of this zone of sharpness is the depth of field of an image.
A Creative Decision
After exposure, depth of field is probably the most critical creative element you control.
There is no ‘correct’ depth of field. Some images are sharp front to back. Others are effective
because only a small area is sharp.
Generally, depth of field extends 2/3s behind the point of focus and 1/3 in front.
Depth of field is increased by narrowing the lens aperture and decreased by widening it.
Such changes affect exposure too.
Your choice of lens can influence depth of field. Wide-angle lenses can offer a generous
depth of field. Macro lenses frequently produce a very small depth of field, even at narrow
apertures such as f22.
The distance between camera and subject is also important. As you move further away
from an object at the point of focus, more of it will come into focus at the same aperture.
Stay Sharp
To achieve sharpness throughout an image choose a narrow aperture (low f-numbers such as f22)
or employ techniques such as hyper-focal focussing. Use a tripod (or some other support)
as small apertures generally necessitate slow shutter speeds to maintain correct exposure.
Be aware that some very small apertures (f32 and smaller) might produce lens distortion.
Softly, Softly
Selective focus is a technique whereby emphasis is given to a subject (or part of a subject)
by throwing everything else out of focus. It is commonly seen in macro photography.
These images of a socket set were created in near-identical conditions, only the aperture
and shutter speed changing. For the first, the aperture was f36, for the second f3.2.
Aperture priority mode delivered shutter speeds of 4” and 1/30 respectively.
The camera’s macro lens was focused on the tip of the spinner handle.
The first picture maintains reasonable sharpness throughout, getting a little softer at the edges.
The second image has a very narrow band of sharpness that does not extend much beyond the tip
of the spinner handle. The effect is more pronounced towards the top of the image because the
front sockets are closer to the tip of the spinner handle than the rear sockets are.
The second image is more engaging because the selective focus differentiates subject from
background, your gaze being drawn to the former. The first image is more detailed, but there
is no distinct area for your eyes to settle on.
Eye To Eye
Portrait photographers usually focus on their subject’s eyes, just as two people in conversation
make eye contact.
Macro lenses are often used to throw the background out of focus because of their intrinsically
shallow depth of field. Fixed focal length lenses are another alternative, as many offer wider
apertures than zoom lenses - this makes them great for low light photography as well.
Size Matters
Images look sharp on your camera’s screen because they are small, making it difficult to assess
depth of field. Check to see if your camera has a depth of field preview button; when pressed,
the bright area in the viewfinder is the area that will be sharp.
Size also matters when you are printing images. The larger the print, the more pronounced
any softness/blur will be.
The Choice Is Yours
When you compose a photograph, decide what the critical elements are, and where they are
in the image. Then consider the purpose of the photograph – who will look at it, and why?
The first of the previous socket images could show detail in an online auction.
The second is a bit ‘artier’ and could be used to create a more general idea of ‘tools’ or
‘fixing things’ on a DIY website. If you are not sure which to go for, you can always save
the decision for later, and shoot one of each.