Depth of field [DOF] refers to the area in front of the lens within which subject matter stays sharply focused. Understanding, manipulating and working with depth of field is one of the key skills you'll need to master in order to move your photographic abilities forward. Today we master it.
You might be surprised to learn that your eye focuses in a two-step process. The lens does most of the focusing while the iris fine tunes the image for presentation to the light sensitive retina at the back of your eye. Likewise, the glass lens elements used by a camera gather the light and shape it in a general sense for presentation to the light sensitive sensor. The fine tuning is accomplished through the aperture, a variable-sized hole through which light must pass as it travels down the barrel of the lens.
As a photographer, then, focusing is a two-step process. The centre of focus is determined by moving the lens elements in a relationship to one another. In modern, automatic cameras most of this is done behind the scenes, achieved by simply pointing the camera at an object and pushing the shutter release down halfway. For many amateurs, this is where focusing stops altogether. More advanced photographers however know that the aperture is required to selectively determine which parts of the scene in front of and behind the centre of focus will stay sharp and which parts will remain blurry.
When the hole is large, that is to say when the aperture is wide open, detail directly in front of and behind the subject will appear blurry, becoming more and more blurry as the distance away from the subject increases. When the hole is tiny, that is to say when the aperture is "stopped down", that detail in front of and behind the subject will remain sharp. Aperture setting then is an extremely valuable tool for manipulating focus.
To make matters more confusing though, smaller apertures have bigger numbers and extend the depth of field. For example, with an aperture setting of between ƒ16 ~ ƒ32 you can expect a very wide depth of field. Wider apertures conversely have smaller numbers and render narrower depths of field. Extremely fast -- and expensive -- lenses might have a maximum aperture setting of ƒ1.2 though consumer level zoom lenses typically start at around ƒ3.5 ~ ƒ4.5.
This terminology and the interplay between DOF and ƒ-stop, a measure of aperture size, gives newbies trouble so I'll put it in an easy to read chart:
|Aperture Size||Example ƒ-stop||DOF|
|Stopped down [tiny]||ƒ22||Wide|
In this assignment you'll need two or three pieces of fruit [or toys, or knickknacks], a flat surface with plenty of bright light to work on such as a table, the floor or even the ground. Place the chosen subjects in a line at different distances directly in front of the camera. How far away from the camera and how far apart your subjects should be will depend on the lens that you are using. Try setting the first one a meter from the camera and the other two at half meter intervals beyond it.
Check your camera manual to find out where your Depth of Field Preview button is. It's usually somewhere on the front of the camera to the right of the lens. You'll find this button quite useful.
Select a telephoto or zoom lens with a focal length of between 100 and 200 mm if you can. Manipulating aperture has little discernible effect with wide angle optics. The longer the focal length, the more pronounced the changes in DOF will be when aperture setting is changed.
Set your camera exposure mode on Aperture Priority [A] and use the command dial to stop down all the way so that the aperture is as tiny as possible. Next, line the subjects up and focus on the closest one. Push the Depth of Field Preview button and observe how dark everything becomes. That's because the tiny hole only lets through a limited amount of light. Also observe how focused the second and third subjects are.
Now, open the aperture up completely. Push the Depth of Field Preview button again and note that the second and third subjects have become brighter but blurrier.
Now take a series of pictures at these settings: wide open, ƒ5.6, ƒ8, ƒ11, ƒ16, ƒ22, ƒ32 and fully stopped down if your aperture gets any smaller. A tripod can be helpful here but it's not essential. When finished, load the photos onto your computer and take a look at the results. In each case the focus was on the first subject but notice how differently the second and third subjects appear depending on the aperture setting. This is an extremely useful tool.
Once you've grasped the techniques of manipulating depth of field take this new skill to the street to practice. A graveyard is an excellent place to try out this skill using headstones as a subject matter. Parking meters, fences, light poles and cars in a parking lot are all useful as subjects for practicing the manipulation of depth of field.
A narrow depth of field [wide aperture] is useful here for obliterating background spectators and separating the two Celtic dancers as they share a glance. Fully stopped down, this photo would have ended in the garbage heap, being too busy to clearly distinguish important subject elements.
Minimum depth of field, shot at ƒ5.6, separates the spider from the background yet provides enough information to distinguish the stalks of bamboo. Contrast between the dark spider and the bright bamboo is essential to make this photo work.
A modest depth of field -- aperture ƒ8 -- renders the juvenile eagle's beak, eye, body and left wing sharp while blurring the right wing. That the tips of the left wing exhibit motion blur even though this image was shot at 1/1000th of a second attests to the sheer power and speed of these raptors in flight.
Close cropping makes it clear that the farmhand is the main subject in this photograph of the cranberry harvest on Barnston Island. The uncluttered background allows a somewhat wider depth of field as achieved with the narrow aperture setting of ƒ11. This choice helps to delineate the tiny fruit in the immediate background while still tapering off to a blurry vermilion gradient.
Note here that manipulating depth of field with wide angle optics is not possible. From the verdant foreground to the distant horizon everything is sharply focused no matter which aperture setting we select. Taken at 1/320th of a second at ƒ11 with a 10.5mm full frame fisheye lens.
All photographs were taken by Brian Grover.