We know about how aperture affects exposure but how does aperture affect the depth of field? First of all, what is depth of field?

The depth of field (DOF) refers to the focal plane, or simply the part of a photo that is IN focus.

A narrow or shallow DOF can be acheived by using a large aperture (eg. f/1.8) and results in a lot of bokeh. A wide DOF is achieved by using a smaller aperture (eg. f/22) and results in most of the photograph being in focus.

Remember, aperture refers to the size of the opening in the lens. The smaller the f-stop, the bigger the opening. So, f/2.8 will be double the area size of f/4 and will let in double the light. See this article on Aperture with photos of the lens opening.

Most portraiture benefits from using a large aperture to achieve a shallow DOF in order to bring greater focus to your subject while the background is “blurred” by bokeh. Meanwhile, landscapes are usually photographed with a wide DOF so everything is sharp and visible.

To demonstrate the effect a changing aperture has on depth of field, please see the following series of shots where the aperture changes:

*click to enlarge

At f/2.8, only the leaves that I focused on are sharp while the background is blurred.

At f/8, you are able to see more detail in the blurred background.

At f/16, the leaves in the background are coming into focus but still not quite sharp. I would need a tripod to shoot this at f/22 (small lens opening) because my shutter speed would need to be 1/10s, which is difficult to do hand-held.

It’s clear to see the effect varied apertures have on the depth of field as well as on the overall look and feel of the photograph. When you want to make your subject stand out from the background, use a large aperture. For sweeping landscape shots, a small aperture will ensure everything is in focus.

Selective Focus

The wonderful thing about working with a DSLR is having the ability to select your focal point with ease. Be sure to check your manual to figure out which setting to use in order to be able to select your own point. Most cameras have a multi-directional pad or joystick, which make it easy to move your focus point around. Being able to do this is one of my favourite features of a DSLR. To show you what I mean, I used an aperture of f/2.8 (wide open) and selected three different areas as my focal point while the exposure setting remained the same for all three shots.

I aimed my focal point on the “A”, which is the only part that is sharp while the rest of the books in the background are out of focus.

DOF - Mid Focus
This time I aimed for the “C” so you can see the foreground and background are both blurred.

DOF - Back Focus
Lastly, I chose to focus on the “F” farthest away from me so that the foreground is out of focus.

You can use selective focusing to bring attention to your focal point, whether it be closest to you, surrounded by foreground and background, or perhaps farthest away from you. By choosing your focal point, you can utilize interesting foregrounds and backgrounds to add depth to your images.

Which Aperture Setting Should I Use?

Some cameras have a depth of field preview button so show you what your DOF will look like before you press the shutter release button. Personally, I rarely use that feature simply because I can just take a test shot and see it clearly on my LCD screen, which is what I do to determine which aperture setting I want to use for my purposes. How much of the scene do you want in focus and how much do you want out of focus? The decision is up to you but there are some guidelines you may want to keep in mind.

Most portraitists covet lenses with large apertures like f/1.4 but there is one caveat you must be careful of when using very large apertures. If you are shooting a subject very close up, be mindful of where you focus. It’s best to focus on the eye that is closest to you. If you use too large an aperture at a close range, then only one part of the face (the eye you selected) will be in focus.

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At f/1.8, Only the eye on the right is sharp and the other eye, farther away from the camera is a bit blurry.

Unless that is the effect you want, to prevent this there are 2 main methods:

  1. Use a smaller aperture (say f/2.8 or f/3.5) to get a wider DOF at close range, or
  2. Simply move back and shoot your subject from a farther distance while still using a large aperture. This way you can still take advantage of the amount of light you can get in by using a large aperture and more of your subject can be in focus.

This one was shot at f/1.8 as well but because I am farther away, most of her face is in focus.

Similarly, if you’re shooting more than one person and you wish for both of them to be in focus, your subjects should be more or less equidistant from your camera and you might want to use a smaller aperture like f/4. Group shots work well with f-stops above f/8. Remember, with smaller lens openings, you need to use a slower shutter speed or higher ISO to make sure enough light enters the lens, otherwise you may end up with blurry shots. Using a tripod at f-stops higher than f/8 will help as will using an external flash.

Depending on your intent, adjusting your aperture can give you more creative control over your images. Keep in mind that with each full stop you increase your f-stop to get a wider DOF, you must decrease your shutter speed by a stop to get the correct exposure. If you don’t want to fiddle with metering and shutter speeds, I highly recommend you use your camera’s Aperture Priority mode where you can set the aperture and ISO, while the camera will determine the shutter speed for you.

Aperture Markings on Lenses

Some lenses have an aperture denoted as a range. For example, the 18-55mm lens has an aperture of f3.5-f5.6. What does this mean? At 18mm, the largest aperture is f/3.5 and at 55mm, the largest aperture can only be f/5.6 (smaller). It’s more difficult to make a zoom lens with a constant aperture say f/4 at every focal length, or even f/2.8. This is certainly reflected by the price. Depending on the zoom range, lenses with constant apertures usually run over $1000 and beyond. Prime lenses have a fixed focal length, so it’s easier to have a larger aperture like f/1.4 or f/1.8 while keeping costs relatively low.

There is one more thing to know about lenses and aperture. Most lenses are at their sharpest 1 or 2 stops down from being wide open. For example, a 50mm f/1.8 lens may be sharpest at f/3.5 rather than wide open at f/1.8. So try out your lenses at different apertures and see if you can find it’s “sweet spot”.

In Summary

  • Depth of field (DOF) refers to the plane that is IN focus in a photograph.
  • A shallow depth of field can be achieved by using a large aperture (f/1.8) and is usually good for portraits.
  • A wider depth of field can be achieved by using a small aperture (f/22) and is usually used for landscapes.
  • You can use selective focusing with larger apertures for creative control.
  • Aperture priority mode is useful for when you want to concentrate on controlling the DOF rather than having to worry about the shutter speed.
  • When shooting close ups, be mindful of your aperture to get the DOF you want for your purposes.
  • Use a tripod or flash when shooting with small apertures (f/8 and higher) to prevent unwanted blur.
  • Find the “sweet spot” for your lens and try to shoot within range of it to prevent soft images.

If you have any questions, comments or suggestions, please leave a comment below. I’d be more than happy to address them. In the meantime, have fun controlling the aperture and DOF. I’d love to see some of your shots where you intentionally wanted a shallow or wide depth of field. Keep shooting!

Laura is a Vancouver wedding and portrait photographer. Visit laurahana.com.