One of the most important creative considerations and controls in photography is control of Depth of Field.
What is Depth of Field?
When you focus on something when making an image, a certain amount of the image behind and in front of the point of focus is also sharp. This is called the zone of “Depth of Field” (DoF). It is important to remember that most DoF falls behind the point of focus, not in front of it. A common mistake when taking a landscape image is to focus on the back of the image, blurring the foreground. It is always better to focus forward and let DoF focus transition do it's work for the background.
DoF is made up of three parts;
The fully sharp or focussed area, also known as the plane of best focus,
The transition zone, where fully focussed sharpness starts to become less sharp or softer, depending on the distance to the subject, their background and the aperture chosen (most fully sharp and transitional DoF falls behind the point of focus),
The out of focus area, where all sharpness is lost, becoming true blur.
How strongly a subject stands out from their background (or not) is called subject separation.
Good (strongly defined) separation is usually the result of shallow or very shallow DoF, but contrast and colour can also help to separate the subject from their surroundings.
How much depth of field sharpness, subject separation, focus transition and blur is shown in an image, depends on a few elements that are (usually) under your control;
The lens Aperture value you select.
The distance you are from your subject including “zooming in or out” with the lens. Increasing magnification either way reduces DoF and visa-versa.
The distance your subject is from it’s background. Increasing the distance the subject is from their background increases DoF and visa-versa.
1. Aperture Settings
Lens aperture settings are the most important, direct and practical DoF control.
Aperture settings come in full value settings of f1.4, 1.8, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, each one halving DOF (as you use smaller numerical values) and double the amount of light let through to the sensor (they are part of the exposure control triangle of Shutter Speed, ISO and Aperture). Thanks to modern electrical camera controls, there are also 1/2 or 1/3 settings available between these full settings, but it is generally best to just think in full value settings at this stage.
Below is an example of some of these settings in action.
(If you use a zoom lens you will probably not have these first two apertures available)
(All lenses will have the following settings)
Most depth of field falls behind the point of focus, so if the foreground does not look very sharp until apertures f5.6 or smaller, that is normal.
Looking at the images, you will see a clear pattern emerging.
When you use a wide aperture (small number value), the depth of field will be very shallow. At f1.8 it will look razor thin, making accurate focussing difficult, but important, especially at very close focussing distances.
As the aperture values become smaller (bigger numbers), the depth of field increases or becomes “deeper”. The terminology is confusing I know, but it is what it is.
An easy way of remembering the aperture values and how they effect depth of field is to think of them as measurements of DoF distance or of the number of bottles that are in focus when focussing down a line of them. A bigger number is more DoF or more distance in focus (f2.8 is 2.8 bottles sharp, f11 is 11 bottles etc). This actually has nothing to do with what the numbers mean, but it helps to remember which way they work.
2. Magnification Using Lenses or Changing Distance
If you increase magnification (get closer) by either increasing the strength of your lens or getting closer, you reduce DoF and the reverse is true. These two methods can help control DoF, but they will act differently in another way.
If you get physically closer to the subject, using the same lens focal length, your DoF will get shallower because you are increasing magnification by reducing distance and at the same time changing the relative distance between the subject and their background, increasing separation.
If you zoom in using a longer lens, the DoF will look shallower and the image more compressed (flatter), but the distance from subject to it's background will not change. using a wide angle lens will decrease magnification and expand perspective, making DoF look deeper.
Controlling perspective is as important as controlling DoF. The expanded look wide angle lenses give to landscape images add to their majesty, while longer lens compression makes an image more intimate and natural looking, ideal for portrait images.
As you have probably already worked out, most photographic situations will be a compromise of the above.
In a nutshell;
For the most dramatic shallow DoF looks, use a wide (small number) aperture value and increase magnification by using a long lens and/or getting very close.
For the strongest deep DoF effect, use wider angle lenses and small (large number) aperture values and don't get too close to the main subject.
The rules we looked at above have a great effect on astronomical photography.
Because the stars are so far away, it is possible to use a very wide aperture setting and still get everything in focus. This is really handy because time is your enemy with astro images. The more time you give the image, the better the colour and depth in the image, but because of the earth's rotation, (depending on the lens used- the wider the better), you will have from 1 to 30 seconds before movement blur ruins the image, unless you are after deliberately blurred star-trails that is.
The only things to keep in mind is the extra DoF needed to include foreground or earth bound elements in the image and possible lens aberrations* that appear when the widest apertures are used.Wider lenses give you more time, because they "push" things further away.
*Lenses used at their very widest apertures tend to show softness in the corners and chromatic aberration (colour fringing) or coma (smears) which may be obvious in your images. The newest batch of lenses are highly corrected, but older lenses will often show these flaws to some extent.
The opposite is true for close-up photography. When you get into the macro (technically this is images that are reproduced in "life size"* or smaller, DoF drops off severely in response to the strong magnification and very small relative distances used. At focussing distances of only a few cm, even f22 will only give you a few mm of DoF. There are a few trick you can use to increase DoF discussed in Tips and Tricks below.
*Life size means reproducing the subject on the sensor the same size as they actually are, so a 1" camera sensor would cover 1" of actual subject from edge to edge).
Tips and Tricks
1. Something you can do to increase DoF is change the angle by "tilting" the lens/camera forward, or downward (using vertical-portrait orientation helps) or sideways (along a fence line or wall) to match the angle of the focussing plane to the subject better. It is especially useful when you are placing a small subject in the foreground and still want the distant horizon sharp or you are photographing a group of objects on one plane such as a table top. Some photographers use "Tilt/Shift" lenses to allow them to change the focal plane without moving the camera.
2. Another trick is to take multiple images at different focussing points and combine them in post processing. This is called “Focus Stacking”, but it only works if your subject and camera stay still. Some cameras have focus stacking as a feature, but remember absolute stillness and consistency are critical. Just like panoramic style images, the programme used can only "stitch" the images together if they all match in framing.
3. If your subject is closer to their background than you are to them, it will be hard to separate them from that background. If shallow DoF with a blurred background is what you are after, move the main subject further away from their background. The opposite is true also. Decreasing the distance between your subject and their background will give a stronger sense of connection between subject and background environment.
4. For maximum possible DoF you can work out the "Hyper-focal Distance" focussing point. This can be done by using either the lens markings if there are any (increasingly rare) or there are Apps available. This will tell you the best distance to focus at each aperture to guarantee the most DoF for your lens focal length and camera format.
5. If you are finding it hard to achieve good DoF and quality in poor light or when doing close ups, use a tripod and/or flash. These will allow you to use f11-32 in combination with low ISO settings without slower shutter speed blur. Macro is also a good time to apply the tilting method to shoot side on or above your subject.
If f16 or smaller are used, be aware that they will be limited in sharpness by “diffraction”. Diffraction occurs around the edge of any hole light passes through, bending light waves and degrading the quality of that light. As the aperture gets smaller and smaller, diffraction becomes dominant, until eventually most of the light is degraded (diffracted). Extremely good lenses are sometimes called "Diffraction limited" lenses , meaning the only optical flaw they have is the only one that cannot be avoided.
When a “fast” or very wide aperture lens is at it’s widest settings, there may be some form of image flaw visible such as chromatic aberration (colour fringing- often green or purple and most obvious in high contrast images) or corner softness. Only the very best lenses avoid these issues (see "Diffraction limited" above).
The transition from sharp to soft areas is sometimes called “Bokeh” (Bo as in bone - Ke as in kettle, the H is there to help with pronunciation) or more correctly Boke-Aji which is literally "the flavour or character of blur".
There is no such thing as good or bad Bokeh (although everyone has personal preferences). Every lens will render the out of focus and transitional areas of an image differently, some lenses even varying at different focal lengths and distances.
Some lenses have messy or busy Bokeh, some feathery or smeary, others smooth and blotchy (there are more "official" terms like Ni-Sen or cross-eyed Bokeh, but these more general terms get the point across). Some have more or less coherent blurred areas than others and transition smoother or less smoothly. Get to know your lenses Bokeh characteristics as they can be as important as sharpness when used often.