Depth of field

For further photography-related information check out my compendium of tips.

I received this question during the week.

David,

I'm struggling with the concept of aperture used at long focal lengths. Close-up I understand how wide apertures demand accurate focusing, but what about at a long distance? It seems much more forgiving. Another layer to my question is how a long focal length will compress the background - does this have an impact on aperture use too?

Thank you!

Matt

This presents a good opportunity to reflect on the factors that affect depth of field (DOF), the distance range from the camera (or more accurately the focal plane, marked on your camera's body by a circle with a single line through it) where subjects appear to be in focus. I shall stick to the headlines in this blog post but, for a more in-depth view on this topic, check out Sean McHugh's fantastic Cambridge in Colour website.

The most important variables in controlling DOF are:

1. The aperture of the lens. As the aperture becomes bigger, the DOF becomes smaller.

2. The magnification of the subject. As the subject increases in size in the image, the DOF becomes smaller. You can easily illustrate this by picking up an object and moving it closer to your face - watch the background blur as your eyes' DOF shrinks. You can increase the magnification of your subject by either moving closer to it (reducing the focal distance) or by using a telephoto lens (increasing the focal length).

The magnification of the subject depends upon the focal distance (distance between subject and the focal plane of the camera) and the focal length of the lens. Note the key difference between focal distance and focal length! It is possible to keep a constant magnification of the subject, and hence DOF, whilst changing the focal length of the lens by making changes to the focal distance. This may seem like nit-picking but it's worth getting your head around.

You can convince yourself that DOF depends on magnification, rather than focal length, by taking a series of shots of a subject at various focal lengths and keeping it the same size in the frame. The DOF appears independent of focal length.

Let's say you're using Canon's 24-105mm f/4.0 L IS zoom lens. You focus on your model at a focal length of 105mm in aperture priority mode at f/4.0. You take your first shot, which with these settings will have a modest DOF. You zoom out 10mm to 95mm, making your subject smaller in the frame, which means you have to move in closer to restore your model to their original size. You repeat this until you're at a focal length of 25mm. If you were to estimate the DOF in each shot by eye you'd say it was constant.

It isin't exactly the same, however. At shorter focal lengths the DOF does increase minimally, even if the subject stays the same size, which means that DOF is not completely independent of focal lengths. The impact of this is small compared to the impact of magnification though.

The following is also worth noting. Most photographers are interested in a shallow DOF for blurring backgrounds. DOF extends in front of and behind the focal distance, but the ratio between the two changes with focal length. At shorter focal lengths the DOF extends approximately 1/3 in front of the focal distance and 2/3 behind. At longer focal lengths this changes to 1/2 in front and 1/2 behind. Therefore longer focal lengths have an advantage for blurring the background.

It's important to note that moving closer to a subject will affect its appearance due to changes in perspective. It is possible to have a blurred background when shooting a model at shorter focal lengths, but the reduced focal distance will distort their features due to the impact on perspective. If you want to shoot people with a blurred background then use a telephoto lens.

To address Matt's comments directly.

1. Using a wide aperture at a greater focal distance gives you a greater DOF and therefore makes accurate focusing easier.

2. A longer focal length will magnify the subject and therefore reduce the DOF. Wider apertures always give less DOF - this is not dependent upon focal distance or focal length.

Questions and comments (due to confusion or errors on my part) are most welcome!

Check out more of my photography here: wedding photographers in Kent