Aperture, focal length or distance? Decisions, decisions…
"Ah, grasshopper..." (taken at f/8 and 800mm from 20 yards away)
Which would give you a shallower depth of field: a 400mm lens at f/2.8 or a 600mm lens at f/4? It’s actually the 600mm lens at f/4. Surprised? Well, let’s hope you didn’t go out and buy the 400mm rather than the 600mm lens for better bokeh!
Most photographers probably know that aperture, focal length or distance are the three drivers of depth of field, but, as you can see, you also need to know which matters more. What’s the best way to change your depth of field? Is it by opening your aperture, zooming in or moving closer to your subject?
Let’s take a look…
What is Depth of field?
"Behind you!"
Depth of field is simply the area in a scene from front to back where objects are ‘acceptably sharp’. This depends on the distance between the camera and the subject. Whatever the focus distance, roughly a third of the depth of field will come before it and two-thirds beyond it.
Given the laws of physics, there’s only ever going to be one distance at which light rays meet exactly on the sensor to create a perfectly sharp image. All the points in the
scene that match that distance will be sharp. They form what’s called the focus plane.
Imagine a cinema screen 20 yards from a member of the audience. If it has a gentle curve, it’ll be roughly the same distance away at the edges as in the middle. That means the image will be sharp across the whole screen, and you won’t get any pincushion effect. It’s the same with the focus plane. Everything on that plane will be sharp, but everything nearer or further away will be increasingly blurred.
Bokeh
Photographers often refer to these out-of-focus areas as ‘bokeh’ (pronounced ‘boh-kay’, but with a shorter second syllable than ‘bouquet’).
The word bokeh originally comes from the Japanese words ‘bokeru’, which means ‘to become befuddled’, and ‘pinboke’, which means ‘the state of being out of focus’. The Japanese spell it ‘boke’ in the Roman alphabet, but the English word mysteriously gained an ‘h’ during the 1990s!
Hyperfocal Distance
Another useful term is the hyperfocal distance. This is the focus distance that gives you the maximum possible depth of field, making sure everything is ‘acceptably sharp’ from a certain point in the foreground to infinity. If you use a smartphone app like SetMyCamera, you can calculate it for any combination of aperture and focal length. For example, at 50mm and f/5.6, the hyperfocal distance would be 14.97m (49 feet). That means everything from 7.49 metres (25 feet) to infinity would be sharp.
Why is it important?
Green on Green
The reason why depth of field is important is that the human eye is attracted to things that are in focus. If you want people to notice important parts of your images, you need to make them sharper.
Wildlife photographers often try to make their subjects stand out by separating them from the background. One way of doing this is to limit the depth of field. Any objects in the foreground or background outside this area will be blurred, which ‘separates’ them visually from the sharp subject.
Whether you take pictures of humans or animals, people are naturally drawn to the eyes. That means photographers can get away with a very shallow depth of field as long as the eyes are sharp.
However, there might be times when you don’t want the depth of field to be too shallow. If, for example, you’re photographing an animal or bird facing the camera, you might want its nose or beak to be in focus as well as its eyes. Alternatively, you might be taking an environmental portrait of an animal in a beautiful landscape, so you might want the trees, hills and/or mountains to be sharp, too.
The point is that you need to have control over your depth of field so that you can draw attention towards certain parts of the frame and away from others.
In addition, the quality of the bokeh is important. Some people rave about the ‘buttery’ or ‘creamy’ smooth blur created by certain lenses, and that can become very important when you have specular highlights in the background, such as Christmas tree lights.
What you want is nice, smooth, round circles of light, but low-quality glass might give you squashed ellipses with pointy ends, and a mirror lens might give you doughnuts instead!
How do you control Depth of field?
Dust Bird (taken at f/4 and 600mm from about 10 feet away!)
There are three ways to control your depth of field:
Focal length. The longer the lens, the lower the depth of field.
Aperture setting (ie f-stop). The wider the aperture, the lower the depth of field.
Distance to the subject. The shorter the distance to the subject, the lower the depth of field.
All these variables are in your control, but your options may be limited by your equipment and the situation:
If your lens doesn’t have a wide maximum aperture, even shooting wide open might limit you to f/6.3 or even f/8.
If you only have prime lenses, you can’t change your focal length by zooming in and out.
If you’re in a fixed position such as a hide or blind, you can’t change the distance to your subject—except by waiting for it to move!
In addition, you can make the background appear more blurred by making sure it’s as far away as possible. One way of doing this is to get down to eye level with your subject. The depth of field doesn’t change, but as your shooting angle gets shallower, the horizon is pushed beyond the distance at which objects are ‘acceptably sharp’.
It should also be said that sensor size does affect depth of field—but only indirectly. All other things being equal (ie at the same aperture, focal length and distance), shots taken with full-frame and crop sensor cameras will have the same depth of field. However, if you crop the full-frame shot to give you the same framing as the crop sensor one, the full-frame version will appear to have a more blurred background.
Aperture
Most wildlife photographers shoot wide open most of the time. I know that’s a massive generalisation, but it doesn’t make it wrong! The advantage is that your default shot has a shallow depth of field, which helps separate your subject from the foreground and background.
Given the convenience of widening your aperture to create a shallow depth of field (as well as letting you shoot at lower ISOs in low light), it’s not surprising that professional photographers often spend thousands on ‘fast primes’, ie lenses with fixed focal lengths that have a wide maximum aperture.
Typically, these lenses will be long telephotos with maximum apertures of f/2.8 at 400mm, f/4 at 600mm and f/5.6 at 800mm. In my own case, I had an AF-S NIKKOR 800mm f/5.6E FL ED VR lens and then a Sony FE 600mm F4 GM OSS before buying a NIKKOR Z 600mm f/4 TC VR S with a built-in 1.4x teleconverter.
The f-stop is calculated by dividing the focal length by the diameter of the entrance pupil (ie the aperture when looking through the lens from the front). The denominator effect means it’s much easier (and cheaper) to produce wide-angle lenses with wide maximum apertures, such as f/2.8, f/2 or even f/1.8. However, wildlife tends to be small and skittish, so wide-angles are generally less useful.
Focal Length
If you have a zoom lens, it’s easy to change the depth of field by zooming in or out. However, there are a couple of problems with that. First of all, unless you own a bridge camera, the zoom range will probably be limited to 2-4x. That means you probably won’t be able to make do with just a single lens.
If you have a 180-600mm telephoto, you might still need something shorter for subjects that are large and/or nearby. Equally, a 12-24mm lens might be great for wide-angle elephant close-ups, but it won’t be much use for anything else!
If you need more than one lens, it’s very inconvenient because you might have the shorter lens on your camera when you need the longer one (or vice versa). That means you have to change lenses, which is a time-consuming pain that might make you miss shots and risks getting dust or other impurities in your camera that cause sensor spots. And, of course, you might end up dropping your camera or lens!
It helps to have two camera bodies, and that’s the route I took as soon as I could afford it. I now go on most game drives carrying a Nikon Z8 with my 600mm lens and a Sony ⍺1 with my 70-200mm lens. The big advantage of the telephoto is that it has a built-in teleconverter, which lets me switch between 600mm and 840mm. That’s a real help when photographing both animals and birds.
Finally, there’s a bit of a trade-off between image quality and flexibility. There are exceptions, such as the NIKKOR Z 180-600mm f/5.6-6.3 VR Super-Tele Zoom Lens, but zoom lenses don’t generally offer the optical quality available from primes. In addition, they tend to have narrower maximum apertures—especially at long focal lengths. That makes it harder to photograph in low light or achieve a shallow depth of field.
Distance
It’s easy to forget that you can change the distance to your subject by simply changing position. That’s easy if you’re on foot, obviously, but it’s still possible if you’re in a vehicle. On game drives in Africa, your guides will probably park as close as possible to the birds or animals to give you the best view, but you can always ask them to move.
The other advantage to changing position is that you can avoid any distractions in the background. You might have to move anywhere from a few inches to a few yards, but it gives you the chance to frame your shot perfectly. In addition, you can choose between backlit, front-lit or side-lit shots.
Which is best?
Tortoise Crossing (taken at f/8 and 400mm from about 20 yards away)
So what’s the answer? Should you change your depth of field by altering the aperture, focal length or distance to the subject? Like most things in photography, it’s a trade-off:
If you change your aperture, you’ll have to change your shutter speed or ISO to keep the same exposure.
If you change your focal length, you’ll have to change your distance to the subject to keep the same framing.
If you change your distance to the subject, you’ll have to change your focal length to keep the same framing!
In addition, there are various constraints:
Your aperture is limited by the maximum value (f/2.8, f/4, f/5.6 etc).
Your focal length is limited by the lenses you own or have with you.
Your distance to the subject is limited by your location, the skittishness of your subject and safety!
So where does that leave you? Well, the maths might get a little complicated, but the least you can do is try to find out the relative effect of each method. How much ‘bang for your buck’ do you get by changing your aperture, focal length or distance to the subject? To double your depth of field, can you just double your f-stop, halve your focal length or double your distance to the subject—or is it more complicated than that?
Let’s find out with the aid of a few charts…
As you can see, the curves on these charts are exponential, and almost all the changes in depth of field happen within a very small range of values. Here are a few key takeaways:
Aperture: at 50mm and from a distance of 20 metres (66 feet), the depth of field goes from 48.34 metres (159 feet) at f/2.8 to infinity at f/5.6.
Focal length: at f/5.6 and from a distance of 20 metres (66 feet), the depth of field goes from infinity at 50mm to 3.34 metres (11 feet) at 200mm.
Distance to subject: at f/5.6 and 50mm, the depth of field goes from 2.28 metres (159 feet) at 4 metres (13 feet) to infinity at 15 metres (49 feet).
These charts are obviously not much help in the field (!), so let’s put it in more realistic terms. If you were 20 metres (66 feet) away from an animal or bird and using a 400mm lens at f/5.6, your depth of field would be 0.82 metres (nearly 3 feet). What would you need to do to halve your depth of field?
You could increase your aperture by two stops to f/2.8.
You could increase your focal length to ~560mm.
You could reduce the distance to your subject to ~14 metres (46 feet).
If we look at all three options, increasing your aperture is the one you’re least likely to be able to do. Unless you’ve shelled out over 10 grand on an f/2.8 400mm prime, you’re likely to be stuck at f/5.6 or at best f/4.
Admittedly, not everyone has a zoom lens capable of reaching 560mm, but a 1.4x teleconverter is not that expensive, and an animal or bird that’s happy to stand still at 20 metres is probably still going to be there at 14 metres—on a good day!
Let’s have a quick look at the opposite case. What would you need to do to double your depth of field?
You could reduce your aperture by two stops to f/11.
You could reduce your focal length to ~275mm.
You could increase the distance to your subject to ~28 metres (92 feet).
In this case, it’s easy enough to reduce your aperture or move further away, so the ‘difficult’ option is reducing your focal length. Yes, it’s obviously possible if you’re using an 80-400mm or 100-400mm zoom, but not if you’re using a prime.
If you’re any good at Maths, you might have spotted a pattern: if you want to halve or double your depth of field, you have three options:
halve or double your f-stop or f-value (NOT the size of your aperture)
multiply or divide your focal length by √2 (~1.4)
divide or multiply the distance to your subject by √2 (~1.4).
Overall, then, aperture is the least powerful way of changing your depth of field. To get the same effect as changing your focal length or distance to the subject by ~40%, you have to change your aperture by two whole stops—which means changing the light levels by a factor of four!
Another way of looking at it is to calculate the impact of halving or doubling each variable:
Halving the size of the aperture (ie removing one stop of light) increases the depth of field by ~40% (ie multiplying it by √2), and doubling it reduces it by 30%.
Halving the focal length increases the depth of field by a factor of four (+300%), and doubling it reduces it by 75%.
Doubling the distance to your subject increases the depth of field by a factor of four (+300%), and halving it reduces it by 75%.
Let’s take a practical example. If you wanted to make sure two lions were both in focus, narrowing your aperture by just one stop would probably not be enough. At 400mm and f/5.6 and shooting from a distance of 20 metres, adding one stop to f/8 would only increase your depth of field by ~40% from 0.82 metres (nearly 3 feet) to 1.17 metres (nearly 4 feet)—which is only an extra 35 centimetres (just over a foot)!
Verdict
Black and Blue (taken at f/5.6 and 400mm from about 5 feet away!)
Depth of field is a common topic in wildlife photography, so it’s important to understand what it is and how best to control it. In theory, you can alter your aperture, focal length or distance to the subject, but, in practice, the aperture has the least powerful effect.
Whether you rely on your WYSIWYG electronic viewfinder, test shots or a smartphone app, it doesn’t take much to master depth of field. Once you know roughly how much you need to change any of the three variables, you’ll be in a better position to separate your subject from the background and make sure nothing that’s supposed to be pin-sharp ends up as a blurry mess!
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