Controlling what will appear sharp and what will be out of focus is one of the most basic skills for a photographer to master. Exact focus occurs only one plane in the photograph, and this is determined by the size of the sensor or the film format , the focal length of the lens, your aperture setting and the point on which the lens is focused. What appears to be sharp to the eye is considerably more than one point or plane. The range of acceptable sharpness is called 'depth of field' or depth of focus.
The apparent depth of field is possible because of an optical phenomena called the 'circle of confusion'.
When an object is at the exact point the lens is focused on, light coming from that object will be presented as a point on the film/sensor plane. In practice, no film or sensor can actually record a point - true points are a mathematical concept with position, but no size. So the camera sees a dot.
That means that the smallest point we can record is actually a circle (a sharp lens is one that can render a very, very small circle). Light reflected from objects not right on the focus point project larger circles. The more out of focus an object is, the larger these circles become.
Up to a certain point, the human eye can't distinguish these “circles of confusion” and the image appears to be in focus over a range of distances. It has 'depth of field'.
The definition of Depth of Field is "the range in front and behind the focus point that is still in focus and sharp".
Perceived sharpness depends on print size, viewing distance and lighting conditions as well as your eyesight. One of the benefits of owning a SLR camera is the control the photographer gains over depth of field. The general effect on depth of field are summarised below:
At any given focal length,
1. larger aperture (smaller f-number, e.g. f/2) offer shallower depth of field
2. Smaller aperture (larger f-number, e.g. f/11) provide a greater depth of field
3. Coming closer to the subject reduces depth of field
4. Moving away from the subject will increase depth of field
5. Lenses with shorter focal lengths produce images with larger DOF: a 28mm lens produces images with a greater depth of field than a 70mm lens at the same aperture.
Hyperfocal Distance
To get the greatest Depth of Field, you can utilise a concept called “hyperfocal distance”.
The Hyperfocal Distance is the nearest point to the camera that appears focused when the lens is focused at infinity.
If you focus on something at the hyperfocal distance, everything from one-half the hyperfocal distance to infinity will be in acceptably sharp.
The formula to calculate Hyperfocal Distance isn’t difficult, but you need some information to use it.
H = (L x L) / (f x d)
Where:
H = Hyperfocal Distance (in millimeters)
L = lens focal length (ie, 35mm, 105mm)
f = lens aperture f-stop
d = diameter of circle of least confusion (in millimeters)
for 35mm format d = 0.03
for 6x6cm format d = 0.06
for 4x5in format d = 0.15
If you have a digital camera, you have to apply conversion factors to the lens focal length and find the Circle of Confusion for your sensor. There are plenty of online calculators that make the task easier: do a search for Hyperfocal Calculator, put in the values for your lenses etc, and print out the results to carry with you.
Alternatively, for about $US40 you can buy a wheel type calculator for your camera bag, like the ExpoAperture2 Depth-of-Field Guide. You dial in the camera’s sensor or film size and desired focal length for a given shot. The Guide quickly provides the resulting depth-of-field for a given aperture, or, alternatively, determines the aperture necessary to achieve a desired depth-of-field.
Even without a calculator or a degree in math there are things you can do to maximise DOF.
Start by setting a very small aperture (large f-stop number). Place the camera on a tripod, since a small aperture usually results in a long exposure time, and use manual focusing.
SLRs usually have a Depth of Field Preview Button that is located somewhere near the lens, and Canon SLR cameras even have a setting that lets you point to the nearest and farthest objects that should be in focus: it does the calculations for you.
An easy way help control depth of focus is the 1/3 - 2/3 rule: for any f-stop, the depth of field extends about twice as far behind the focal plane as it does in front.
Consider doing a head shot of a child and another of the family’s pet dog: in both cases you want the eyes to be in sharp focus, but with the dog, that shiny nose is important, too.
The human head has a fairly flat profile compared with the dog’s, so with the eyes as the point of focus, the nose should be fine. But the dog’s long snout might defeat you: focus on the eyes and you lose the nose; get the nose sharp and the eyes blur. To get both, you should focus on a point 1/3 in from the nose with a small aperture...voila! Got both.
Intentionally Limiting Depth of Field
Now that you know how to get maximum Depth of Field, I want to consider time when you want to limit it. Shallow DOF can be used as a creative element to draw attention to a certain part of your photograph, to hide a distracting background or to add emphasis to a feature.
Putting your subject in focus and everything else out of focus is a good way of directing the viewers eye to what you want to ‘tell them with your image.
Choose a wide aperture (like 3.5 or 2.8) and focus just in front of the subject or on its closest point (remember the 1/3 -2/3 rule ?). Your aim is to limit the depth of field to just those elements that are crucial. If the aperture is too large, parts of your subject will be out of focus, too small and you will have distracting background elements in your picture.
Even when it is completely out of focus make sure you are aware of the background. Bright spots and splashes of colour can also spoil your composition; sometimes they are even more distracting because they are out of focus. The eye seems to be magically drawn to them.
Infinite Depth of Field?
Yes, it is possible. Tilt and Shift lens are the SLR equivalent of moveable backs on large format cameras. They let you control the angle of the focal plane puttingforeground and background into the same focal plane. This is probably the sharpest image you can shoot with a large DOF. Downside? Very expensive and only available for a limited range of cameras. I think they may only be available for Canon cameras, but I stand to be corrected.
The other option is to use multiple images taken at from the same position, exposure and focal length: definitely a tripod job. These images can then be loaded into an image editir as separate layers and merged after careful selective work with the eraser and other tools before merging the images. Tedious, painstaking work requiring an understanding of layer blend modes as well as general editing techniques.
HeliconFocus is a program that automates this process to achieve infinite depth of field. Helicon’s greatest strength is in macro and digital micro-photography (using optical microscopes). The program finds the focused areas in each of several images and combines these areas into one perfectly focused image
Understand and control depth of field and you are well on the way to mastering your camera.
