How to adjust depth of field to improve your images. This article was originally published in The Blue Bill, the journal of the Kingston Field Naturalists.
In my early days as a wildlife photographer I was happy just to get a shot of the birds, beasts and insects I came across. But it wasn’t long before I started wanting to take better photographs. Looking back critically at images from a few years ago I found that some of my photos, particularly of birds and butterflies, were not as fully in focus as I wanted them to be. It was time to get a better handle on depth of field.
Depth of field is a relatively simple concept. Basically, it’s the area in a photograph where objects are acceptably sharp. Aside from exotic specialist equipment, cameras can only focus on one point. That point, and anything else at the same distance from the camera, can be precisely in focus, but anything nearer or farther will be less than optimally focused.
That’s where “acceptably sharp” comes in. Because the reduction in sharpness happens gradually, there is a range within which objects are sharp enough that they appear to be in focus. The range between the nearest and farthest objects that are acceptably sharp is called the depth of field (DOF). So as long as your subject is within that range all will be well.
(Note that apparent sharpness changes when an image is printed in a larger format, or when the observer is closer to the image, so an image that is acceptable sharp in 5×7” format may not be when blown up to 16×20”).
Why should I care about depth of field?
Landscape photographers obsess about getting maximum depth of field, and they use highly technical concepts such calculating hyperfocal distance to work out the optimal DOF. Depth of field preview functions, available on some camera models, are also used primarily in landscape photography. Fortunately we as wildlife photographers don’t need or have time to delve into those issues because our targets are constantly moving.
For us there are two main reasons to consider depth of field: to ensure that the whole target bird, turtle or butterfly is in focus, and to make an artistic choice about how much of the background should be in focus.
So back to the challenge of getting better images. I noticed that some of my photos of birds and butterflies had insufficient depth of field: typically in images of birds taken at close range the tails would be out of focus, and for butterflies one antenna was in focus but the other one was not. (See examples below).
To address this issue, we need to understand the two main factors that influence depth of field: aperture size and proximity to the subject. Larger apertures reduce depth of field, as does moving closer to the subject.
If you are interested in the technical explanation for why this is so, a search of the internet will bring up multiple sources. I recommend you start with Wikipedia or Cambridge in Colour. But I believe it is not necessary to understand the physics as long as you understand the effect.
Before we begin this section let’s refresh our memories about apertures. The aperture governs the amount of light passing through the lens. Larger apertures (bigger openings) are expressed by smaller numbers. Thus f/2.8 is a large aperture, and f/11 is a small aperture. Again, you can read up on the technical reasons for this or you can just remember the differences and move on.
The images below show the differences in depth of field as aperture size changes. Notice that the point of focus (the cocktail glass) remains constant, but the objects behind it start to become fuzzy as the aperture increases (i.e. the aperture number becomes smaller).
While increasing the aperture (changing to a lower f stop) reduces DOF in a linear manner; increasing your proximity to the subject reduces DOF as an inverse square law. So as you get closer to your subject DOF decreases radically – a major challenge for macro photography.
Butterflies and odonates present a special challenge. Because they are small the temptation is to get as close as possible. But that is where the inverse square law comes into play – get too close and your DOF will be so shallow that parts of the insect will be outside the acceptably sharp range.
So how do I fix this?
The Canada Jay photos below show DOF in action. The birds were close (the images are uncropped) and there was not a lot of light available. Image 8 is taken at an aperture of f/6.3 and the tail is not acceptably sharp. Image 9, taken one half stop up at f/7.1 is noticeably better. So in principle, when taking photos of close-in subjects a higher than normal f stop (i.e. a smaller than normal aperture) is recommended. For more distant subjects a mid-range aperture should suffice.
As a rule of thumb if there is enough good light available, apertures in the f/7.1 to f/8 level should give you a good chance of capturing all the details of a bird that is relatively close.
For butterflies and odonates, a search through my files shows that in general I got better images from remaining bit farther away, using a smaller aperture and letting my telephoto lens do its job.
If your subject is cooperative, remember that a key advantage of digital cameras is that you can check your images on the camera’s monitor and see immediately whether the depth of field is correct.
DOF and artistic composition
Having made the case that ensuring adequate depth of field is important, let’s now look at a situation where you may want to limit DOF. Many sports and wildlife photographers subscribe to a fetish that background detail is to be avoided at all costs as they claim it detracts from the subject. In general I believe that wildlife is best depicted in its environment, and that means there should be background detail – an animal is not an icon to be shown detached from the ecosystem it inhabits.
However there are situations where the background detail would not add any value – perhaps it’s too far away to be sharp regardless of the aperture setting, or perhaps the background is an unattractive pile of random scrub. In those cases choosing a field deep enough to just cover the subject can create an attractive effect. The Savannah Sparrow in the image below was perched on a fence with nothing behind it but long grass. In this case an aperture of f/5.6 was enough to ensure that the bird, the wire and the one leaf below it are sharp, while the background is a sea of formless colour.
Camera management for depth of field
So… let’s imagine that I have convinced you that depth of field is a thing you should consider. How should you go about controlling it?
The first step is to confirm how your camera displays the critical information: aperture, shutter speed and film speed. This information is probably displayed in your viewfinder and/or on an information panel. Check your manual to be sure, and then make a habit of keeping an eye on the aperture setting.
If you use the Programme or Automatic mode it suggests that you are relatively new to photography and need some help from the camera so you can concentrate on the subject. There is no shame here – everyone starts out using an automatic mode and those who aren’t intent on getting the best possible images can happily stay in those modes. Just be aware that by leaving all the decisions to the camera you will have no control over depth of field.
Many of the more experienced photographers use one of the semi-automatic modes: Aperture Priority (shown on the mode selector as A for Nikon cameras and Av for Canons), or Shutter Priority (S for Nikon, Tv for Canon).
In Aperture Priority mode you control the aperture setting manually. The camera will make what it thinks are necessary adjustments by changing shutter speed and (if you enable Auto ISO) film speed. If you use this mode keep a close eye on the shutter speed. For wildlife (or plants if there is a breeze) you should use shutter speeds below 1/500 sec with extreme caution. Motion blur will ruin any image regardless of how well you have judged the depth of field. If you are in Aperture Priority mode and need more shutter speed you can select higher film speeds (ISO) until you reach a point where the camera boosts shutter speed to compensate.
In Shutter Priority mode you essentially give up control of the aperture setting. If there is not enough available light your camera will default to a wide open aperture setting regardless of what you might want to see from a depth of field perspective, though in fairness most of the affordable telephoto lenses have base apertures of f/5.6 or more so even wide open there will still be some depth to the image. Again, increasing film speed will eventually cause the camera to compensate by stopping down the aperture.
There is a way to balance all elements of the light triangle – aperture, shutter speed and ISO – yourself to ensure that you can make the best decision under the circumstances. It involves taking the plunge into Manual mode – a topic for a future post.
Some random final notes
- If macro photography is your thing, one way to get around the issue of very shallow depth of field is to invest in a camera that allows focus stacking. This process involves taking a large number of images of the subject with the focus point moved slightly between each image. These images are then “stacked” using software to yield a single image that is in crisp focus from one end to the other. I have seen some amazing insect and flower images taken using focus stacking. One consideration, though, is that the subject has to remain completely immobile (which usually means it needs to be dead).
- Smaller apertures lead to greater depth of field, but only up to a point. Using apertures of f/11 and above can bring diffraction into play. Without delving into the technical explanation, the bottom line is that diffraction can seriously degrade the sharpness of your image. So more isn’t always better.
- The notion that telephoto lenses have inherently shallow depth of field is a common myth that is repeated by many supposedly expert sites. A more accurate statement is that telephoto lenses appear to have a shallow depth of field because of the distribution of sharpness. Telephoto lenses tend to have an even distribution of acceptable sharpness in front of and behind the focus point, whereas for wide angle lenses the bias is tilted to the areas behind the focus point. In landscape photography this is an advantage because it creates a more gradual fading away of sharpness towards the horizon. But the bottom line is that a given aperture (e.g. f/5.6) will give the same depth of field with any focal length of lens.
- Finally, it is a fact that the size of the sensor on your camera affects DOF. Counterintuitively, the large sensor of a full frame camera will develop a shallower depth of field at a given f stop than a camera with a cropped sensor. This is an interesting factoid, but one that’s not particularly relevant to wildlife photography. If I were in the market for a new camera body I can think of a lot of factors that I would consider before I got down to that one.