Exposure - Part 4: Aperture, DoF, and Bokeh

Controlling Depth of Field

A wider aperture (small f number) will produce a small Depth of Field (DoF) while letting more light pass through the lens.

A small aperture will produce a deeper DoF but let in less light.

Note: all measurements and f-stops examples mentioned are based on an assumed 35mm format. Adjust as required.

So, what is Depth of Field?

When you focus your lens on an object you are actually focusing on a point a specific distance from the camera's film plane/sensor plane. You are creating what is known as a plane of focus. Anything that is the same distance from the film plane to the object on which you focused will also be in focus. This is easy to test. Manually focus on an object that is say 3m away now, turn around and point the camera at some other object that is around the same distance. It will also be in focus.

There is an area, both in front of and behind your focus point that will also be in sharp focus. This is the Depth of Field - the distance from the front to the back of the plane of focus in which objects remain acceptably sharp. Objects outside the DoF will appear progressively more blurry as the distance from the plane of focus increases.

While aperture has the most affect on DoF, there are other factors that will also influence the perceived DoF. The focal length of the lens, distance to subject, the camera's film/sensor size, and Circle of Confusion (CoC) all play a part. I won't be discussing film size or CoC in this tutorial as they are not variables that can be adjusted. Both could be interesting topics for possible future articles though.

You will often hear the DoF described as shallow/narrow or deep/wide to indicate how much of the scene is in acceptable focus.

Subject Distance: The closer you are to your subject the more shallow the DoF.

Focal Length: You may hear that the longer the focal length the more shallow the DoF. This is true, to a point... For example:

- A 50mm lens at f/4 with a distance to the subject of 3m will have a DoF of around 870mm.

- A 100mm lens at f/4 at the same distance will have a DoF of only 210mm.

However, if you took the 100mm lens back to a distance of 6m, so the subject appears to be the same size (field of view) as the 50mm lens at 3m, the DoF will be 850mm - only slightly less DOF at the same field of view.

DoF at different f-stops indicated in grey

DoF at different f-stops indicated in grey

Note: any recommended settings detailed below should be considered as a guide and not a rule. The idea is to experiment and eventually find your own preferences.

Landscapes - maximum DoF

For landscape images we usually need to maximise the DoF to ensure that the entire frame is in sharp focus. Landscape photos are most commonly made with a wide angle lens; between 14 and 35mm are popular choices. Personally, I find that I tend to like the lens to be in the 24 - 30mm range. Choose a smaller f-stop, around f/8 to f/11 is a good starting point, and focus on a point 1/3 of the distance into the frame. This focusing method works because the DoF will extend from about 1/3 of the distance toward you and 2/3 of the distance behind the plane of focus. With a wide lens and a small aperture the depth of field should cover just about all of the frame.

A more accurate focusing method would be to calculate the hyperfocal distance or use the double-the-distance method. Check my blog post titled "Three Ways to Focus Landscape Photos" for more information.

Note that a loss of sharpness due to diffraction will occur at smaller apertures and will start becoming noticeable from f/16, regardless of focal length or how much your lens cost. Diffraction is caused when light waves passing through a small opening begin to diverge and interfere with each other. So, there is a trade-off to be had... Maximum DoF vs maximum sharpness. Diffraction is more noticeable on digital sensors with high pixel density and so it will be a slightly bigger issue for high megapixel cameras and camera sensors smaller than full frame.

Just how noticeable the effects of diffraction will be depends very much on your subject matter. With some subjects a small loss of sharpness will not be noticeable, even if the lens is stopped down well below f/16. Some other subjects, such as architectural or close up photographs may exhibit a slight loss of sharpness relatively quickly.

While there are other factors that can and will affect image sharpness, diffraction limitation is something that needs to be considered in all landscape work. Enter the term "Airy Disc" into your preferred search engine to find out more about why diffraction occurs in all lenses.

Environmental portraits/objects - minimum DoF

When photographing people, or pets, or things it is often desirable to separate the subject from the background by making the subject sharp and the background out of focus.

To minimise the DoF choose a lens that will allow you to get reasonably close to your subject, ensure that there is space between the subject and the background, and open the aperture. An f-stop of f/4 or larger will be enough in most cases to produce a blurry background. The wider the aperture the more blurry the out of focus area will become. The blurriness is referred to as "bokeh". See below for more information about bokeh.

Note that very wide apertures, such as f/1.8, f/1.4 etc, will produce an extremely thin DoF. You may find that your subject's nose is in focus but their ears are blurry (which can look very effective). Generally avoid using wide angle lenses for portraits as they tend to make closer objects (noses for example) appear larger and so do not produce flattering images.

Studio portraits/objects

Usually when photographing in a studio environment it is not necessary to separate the subject from the background as the background is likely to be a solid colour and not distracting. A wider DoF is not a problem in this situation. Try stopping your lens down to around f/5.6 or f/8 for more detail and sharpness.

Architecture and highly detailed objects - maximum sharpness

When it is more important to consider the detail in an image than the DoF...

All lenses have a sweet spot, an aperture setting at which the lens will be at its sharpest and will produce the most detailed reproduction. This sweet spot is often three to four stops closed from the lens' maximum aperture. For example, if your maximum aperture is f/2.8 you may find that its performance is sharpest at or around f/8. Of course, when it come to sharpness f-stop is just one consideration. Also use a low speed film/low ISO, mount the camera on a tripod, use a cable release, and shoot with the mirror up.

Creating starbursts - small apertures

When capturing an image that contains a strong point of light (such as the sun or street lights) it is possible to produce an attractive "starburst" from that point of light. To do this simply close your aperture down. The smaller the aperture the more distinct the burst but, very small apertures will lead to a softening of the image due to diffraction.

The shape and number of aperture blades in the lens has a direct effect on the appearance of the starburst.

- Lenses with an even number of blades will produce a starburst with the same number of spikes. I.e. a lens with eight aperture blades will produce a star with eight spikes.

- Lenses with an odd number of blades will produce two spikes for each blade. I.e. a nine blade aperture will produce 18 spikes.

- Due to the shape of the blades, some lenses will produce split spikes - each spike will consist of two lines, one more prominent than the other.

- Lenses with very round edged blades may not produce starbursts at all, at any aperture.

- A very bright light source, such as the sun, may also introduce unwanted lens flare or ghosts. Generally prime lenses handle flare better than zooms and multi-coated lenses perform much better. UV filters, smudges and grime on your lens can make flaring much worse. It's a good idea to remove any filters and clean your lens before making such a photo.

Starbursts at f/22

Starbursts at f/22

A bit more about bokeh

The aesthetic quality of blur, in the out of focus area of a photo, is known as "bokeh". The term comes from the Japanese phrase boke-aji - meaning "blur quality".

Bokeh is not the amount of blur in an image but rather a description of the quality of the blur. Your will hear adjectives such as creamy, smooth, jagged, jittery used to describe bokeh.

To create "creamy" bokeh, pick a long lens, choose a large aperture and get close to your subject. With a large enough aperture you will get some bokeh using just about any lens - as long as you are not too far from your subject.

A cool feature of bokeh is that out of focus light points may render as circular blobs. The actual shape of the blobs will be the same shape as the opening in the diaphragm of your lens. A lens with more blades (and more rounded blades) will produce rounder, more pleasing shapes than a lens with say seven hard-edged blades.

The image below is three shots of my pet wombat, Mr Wilson, some wine glasses, and some candles. Notice how the wine glasses become more blurry the further they are from the plane of focus and the way that the candle flames become blobs of light.

Bokeh at three large apertures

Bokeh at three large apertures

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