One of the most important creative considerations in photography, if not the most important, is control of Depth of Field.
What is Depth of Field?
When making an image you must focus on something. A certain amount of the image behind and in front of the point of focus is also reasonably sharp until it becomes blurry. This is called the zone of “Depth of Field” (DoF). DoF is used to determine the most important point in the image, but also to control the amount of the image that is also sharp (and how much) and the amount of and speed of transition to blur.
DoF is made up of three parts;
1) The fully sharp zone, also known as the plane of best focus,
2) The zone of transition, where fully focussed sharpness starts to become less sharp (softer), and how quickly, depends on the distance to the subject, the subjects distance to their background and the aperture chosen (most transitional DoF falls behind the point of focus).
3) The out of focus area, where sharpness is lost, becoming true blur. This is often mistaken as the only place where “Bokeh” shows up in an image. Bokeh (see below) is actually the character of both the transitional and out of focus areas and how they relate.
How strongly a subject stands out from their background is called subject separation. Strong subject separation is what we usually define as “portrait style” photography. This makes a hero of the focussed subject and blurs the rest away into smooth softness.
Good (or strong) separation is usually the result of shallow or very shallow DoF, but light, texture, contrast and colour can also help to separate the subject from their surroundings.
How much depth of field sharpness, subject separation, focus transition and full blur is present in an image, depends on a few elements that are (usually) under your control;
The lens Aperture value you select is the first and often most powerful.
Changing magnification by changing the distance you are from your subject or/and “zooming in or out” with the lens. Increasing magnification either way reduces perceived DoF and visa-versa.
The distance your subject is from their background** is also important here. Increasing the distance the subject is from their background increases DoF and visa-versa*.
Perceived compression or expansion of perspective. This does not necessarily change the actual DoF of an image but it heavily influences how we perceive it.
*Relative distance control is really important. if you are closer to your subject than they are their background, you will increase perceived subject separation and the reverse is true.
**It is important to remember that the majority DoF falls behind the point of focus, not in front of it.
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 (these are only some of the full aperture settings, ignoring the third and half values also).
(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 nearest 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 values like f1.8 to f2.8), the depth of field will be very shallow. At f1.8 it will look razor thin, making accurate focussing difficult and very important, especially at very close focussing distances.
As the aperture values become smaller (bigger numbers like f8 to f22), the depth of field increases or becomes “deeper”. The terminology is confusing I know, but it is what it is.
Tip. 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/feet/fence posts/inches/trees that are in focus when focussing down a line of them. A bigger number is more DoF or more “in focus” distance (f2.8 could be 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.
If you increase magnification by either changing the strength of your lens or actually moving, you reduce DoF and the reverse is true. The more magnification/closer you get, the shallower the DoF will be. The further away you move or more you zoom out the greater the DoF. These methods can help control DoF, but they both act a little differently (see also point 3 below).
If you move physically closer to the subject, using the same lens focal length, your DoF will get shallower because you are increasing magnification and at the same time increasing the relative distance between the subject and their background. This increases subject separation. The opposite is also true, moving further away from the focussed subject reduces magnification and decreases the relative distance from the subject to their background (see relative distance below).
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 (see below), making effective DoF deeper, but again the actual distance from the subject to their background does not actually change (see relative distance below).
3. Relative Distance
If you are closer to your subject than it is to it’s background, you will be able to separate it from the background more easily. If the subject is close to it’s background (i.E. nearly on the same plane of focus), then separating it from the background, even with a very wide aperture, will be harder.
The two images above were taken using the same lens settings (f2.8, 40mm) and the same shooting distance. In the right image the second camera was moved back to show the effect of increasing the relative distance of the subject to their background and how it strengthens subject separation. The photographer to subject distance is now shorter than the distance between the subject and the rear camera.
4. Controlling the Perception of DoF using Lens Perspective
The last way of controlling the perception of DoF and separation (but not actual DoF) is by using different lens perspectives. Wide angle lenses expand perspective, increasing the perceived distances between objects. Longer lenses compress perspective seemingly “squashing” things together. This is not actually changing DoF, only the perception of it.
Portrait photographers use the compression of longer lenses to make their subject jump out. landscape photographers often use the wider angle lenses to increase the perception of deep depth of field.
If an object photographed to be the same size in any two images, taken on any lens and any format camera but using the same aperture, the DoF is basically the same (the math is not exact, but basically holds true). The perception of separation is however very different.
In the two images above, the same aperture was used (f5.6) and the same focussing point. Every effort was made to keep the main subject the same size in the view finder by moving in closer with the wide angle lens. The left Image was taken at 40mm and the right at 12mm. Even though the left image is more compressed, increasing the perception of shallower DoF and better subject separation (also changing how the blurring looks), the actual DoF is about the same. It is just stretched out, making the transition less obvious.
As you have probably already worked out, most photographic situations will be a compromise of all of the techniques discussed 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, especially if this exaggerates the distance from subject to it’s background.
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 relative to it’s background.
The rules we looked at above can be applied to 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 are usually used as they 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 tricks 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 it actually is, so a 1" camera sensor would cover 1" of actual subject.
Tips and Tricks
1. Something you can do to increase useful DoF is to 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.
This is especially useful when you are placing a small subject like the head of a flower 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, fence line or field of flowers. 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 or in camera with some models. This is called “Focus Stacking”, but it only works if your subject and camera stay still. Just like panoramic style images, the programme or camera used can only "stitch" the images together if they all match in framing.
3. 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.
4. 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).
A word on Bokeh
The transition from sharp to soft areas is often referred to as “Bokeh” (Bo as in bone - Ke as in kettle, the H is there to help with pronunciation, not that it always helps!) 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. Zoom lenses can be complicated here as they can vary with every focal length.
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 and you do not need to learn Japanese). Some lenses have more coherent transition to out of focus areas and others, often modern designs, drop off smoothly and quickly for better portrait applications. Get to know your lenses Bokeh characteristics as they can be as important as sharpness when used often.