Basics: Depth of Field
One of those phrases you hear all the time with relation to photography is depth of Field (or DoF). This article offers a PHYSICS FREE explanation of what this term actually means in your photography, and how to control it, allowing the most basic of photographers the chance to deliberately improve their work.
As with all explanatory blog entries on this website, you can read along, or you can follow it with your camera in hand (which is always better), and so to follow this article fully you will need:
- A cup of tea ( or suitably tasty beverage)
- Three small objects to shoot such as a soft toy or ball (or two…. and a cup of tea)…
- Your dSLR/bridge/ hybrid camera (see here for an explanation of the types of camera)
- A little bit of space (e.g. a couple of metres)
Ready? OK, so let’s get into it…
Depth of Field: A Definition (of sorts)
I promised a physics free explanation, so I wont be talking about points of transition and lens correction, or even the “circle of confusion”. Instead I’m going to focus on what it really means to a tog and his or her kit.
When you focus on an object to take a photo, other items in the image are in focus. These objects are related in the distance from the lens – that is, those things that are all pin sharp are of similar distances from the lens. The Depth of Field, refers to the distance from the closest object, to the furthest object, that are in focus for any given image.
A small depth of field is one where only items of very similar distances from your lens are in focus. We call this a “shallow” depth of field. Conversely a large depth of field refers to a situation where objects some distance apart on the Z axis (the axis from the camera through the lens to infinity) are in focus (see image below).
A geeky aside: you would imagine that your point of focus is in the exact middle of your DoF, but actually its not, it’s about 1/3rd of the way from the front of your DoF.
OK that’s about as close to science as I’m going to go (if you want to get more into the actual physics and maths, then there are plenty of textbooks and websites on focus theory, basic light and so on – but I find these to be less important if you just want to understand it for your average photography session). If you kept up, pat yourself on the back, take a sip of tea, and lets talk about how to control DoF.
You’ll be pleased to know that controlling the DoF in your image is simple: it relies on just three things:
- Aperture – if you can control aperture on your camera, you can directly control depth of field (e.g. make it shallower or deeper)
- Distance ratio from you to the subject and the subject to other objects. Even if you cant control aperture, you can control this.
- Focal length of the lens when you shoot an image.
If you want to calculate your DoF reasonably accurately before you shoot something (or just want to hash out rough figures for a shoot, and don’t want to trial and error it, there are a couple of decent DoF calculators online. My favourite is this one (link opens in a new tab).
OK, lets take a look at this one at a time
I spoke before a crucial part of understanding creative lighting is to understand that” Aperture Controls Flash Exposure“. Aperture also controls DoF.
The wider the aperture (e.g. the lower the f/ number) the shallower the depth of field is for a given image. Below shows how changing the aperture size effects DoF in the real world.
In Figure 3 the scene was set up with 3 balls and a pepakura Yoshi that I was working on as a prop (more on this later this month). The closest ball was 2 feet away from the lens, and then each object was 2 feet further away from the one preceding it. The door frame was 10 feet from the lens.
The images were shot with varying aperture size (1: f/1.7 | 2: f/6.3 | 3: f/18 | 4: f/22). As you can see, the wider the aperture the shallower the depth of field and vice versa.
OK now over to you: Get your camera in aperture priority or manual mode and place your objects on a surface at various distances from you in a row (slightly diagonally, so that the objects don’t eclipse each other when you set up your shot. See if you can get one of the objects in focus but not the others Remember that you can focus on an object further from you and then set a shallow DoF so that closer objects are out of focus too. Try not to move the camera closer to the objects in your comparison… we’ll discuss that next.
When shooting “wide open” to get the shallowest depth of field (which people who shoot portrait or macro like to do) be aware that most lenses look very “soft” when the aperture is wide open (that is, even the subjects in focus are a touch less sharp). For example, My Sigma 50 mm f/1.7 is a great portrait lens. It allows me to select very shallow DoF when shooting subjects however the focus is very slightly soft even in the focal “sweet spot” up until about f/2.8. This is not really noticeable to the average eye, but when you work at full crop or larger (e.g. when blowing up images for posters) you can see. The difference between the softer lenses and those that remain tack sharp right down to their widest aperture is one of the factors that makes the lens entry level (e.g. for this particular lens class, about £100) and pro (£900+).
Another problem with shooting wide open, in low light conditions (which is often when you want to open up the aperture to get more light in), is that auto-focus’s may not work so well. If you set your DoF too shallow, and you miss-focus, then you risk ruining your whole image (where your subject is not in focus at all).
Distance Ratio between you, the subject and other items
Depth of field increases over distance between the camera and your subject (focus point) –
Let’s consider a 50 mm lens at F/2.8:
- subject is at 1 m, DoF is about 4 cm – too short for portrait, but great for macro images.
- subject is at 10 m, DoF is about 4.5 m
- subject is at 25 m, DoF is about 38 m
- subject is at 50 m, DoF is effectively infinite, but the closest object in focus would be about 24 m.
this varies slightly from lens to lens, camera to camera and format to format.
The distance ratio between the camera and the subject (that you want in focus), and the distance between the subject background or other objects (which you don’t want in focus), alters the ease in which you can “pull out” the subject from the surroundings. If your subject is say 100 m from you, and the background is 10 m further back, it will be difficult to successfully separate the subject from the background by simple DoF control (Distance ratio 10:1). If the subject is 10 m from you, and the background is 10 m further back, it will be markedly easier to separate the target out by using simple DoF control (ratio 1:1). This means if you want to separate out your target from its background more easily you can either decrease the distance between yourself and the subject, or increase the distance between the subject and the background. Realistically you will want to do both – and in the studio often just means asking your model to take a step or two towards the camera (away from the background).
Back to you: Get your camera, and set the aperture to f/3.5 or so. Set up some objects on a surface about 2 feet apart in a row as before, and then focus on the closest object. Move yourself closer to the focal object as you take images. See how close you need to get to the object before the other objects are no longer in your focal field. Don’t adjust the focal length just yet, we’ll talk about this next…
All things being equal, when shooting a subject, increasing the focal length of the lens, decreases the depth of field. (e.g. as you ” zoom in”, DoF drops, for any given aperture and distance set up)
for example lets consider a subject at 10 m from the camera, with an aperture of f/5:
- with a focal length of 35 mm, the DoF will be about 40 m
- with a focal length of 50 mm, the DoF will be about 9 m
- with a focal length of 70 mm, the DoF will be about 4 m
This means for really shallow depth of field you will want to use a large focal length, which will make your subject larger in the image. Realistically this will not always be easy to do, as when you get to really high focal lengths you may not be able to fit your subject into your image in the composition you desire, and in doing so you will step back a bit to make it fit – thus reducing the effect on your DoF, as you have increased he distance to your subject. Wildlife photographers will often buy expensive 300-600 mm lenses with a nice big aperture) to get the best of both the aperture and the focal length DoF factors, allowing nice shallow depth of field with largely magnified birds and insects and the like. (it also means you dont have to get right up close to what you want to shoot).
The effect of Focal Length on DoF also rings true the important lesson that it is best to compose the image in camera rather than just crop it out in post production – shooting everything on a wide angle and then cropping down to what you should have composed properly often means that your DoF is much larger than you might have wanted ( e.g. for a subject at 10 m, f/5 and a wide angle focal length of say 20 mm, your DoF effectively close to infinite (with the closest point in focus being about 3 m from the lens)). Properly composing a tighter angle image would allow you much better control of DoF.
Your turn: Taking the same set up of objects spaced about 2 ft apart that you used in the previous exercise, sit about 4 feet from the closest object, and set you camera to an aperture of about F/5. Now, without moving closer, shoot the subject at 20 mm, 30 mm, 40 mm, and 50 mm (your kit lens should be able to do this range – don’t change lenses mid way through). As you zoom in by increasing the focal length, try and frame it so that the the other objects are still just about visible over the shoulder of your subject, and compare their sharpness as you go.
Why would you want to control DoF
There are plenty of reasons why you will want to purposefully control DoF, some creative and some are just the unwritten rules of photography. Below is a couple of examples of both.
Portrait photography usually uses shallow DoF to bring out the subject from the background. Typically you will aim to focus on the eyes, with a DoF calculated to give a sharpness from the tip of the nose to the ear lobe. this means that everything on the face is sharp, and the hair begins to lose focus
Macro photography is often set to bring out nature’s smallest detail in a flower or insect with a completely unfocussed background. As insects are very small, DoF is often set to a few centimetres, which means either using a telephoto lens (e.g. a 300 mm), with a small aperture, as close to the subject as you can get (which is often the lens’s minimum focus distance)
Sports Journalism photography like the images in the back of your tabloid, are set to capture victorious sportsperson’s facial expressions whilst blowing the crowds behind them completely out of focus. One of the real trials with this kind of photography is – you only get one chance to shoot a given goal or action, they are often across the pitch, and are often very fast. Lenses for this kind of photography are large aperture, heavy weight, and can cost thousands of pounds (e.g. Nikon 800 mm f/5.6 is about £15,500).
Landscape photography often warrants a deep depth of field in order to capture detail in way off points in the horizon when focussing on closer points of interest. This is especially true when shooting things like mountainous horizons
Large Group Photography, such as a wedding party photo or a school photo with multiple layers of people. You want to be able to capture detail in all the people regardless of their distance from the camera.
Depth of Field refers to the amount of image in the Z-axis that is in focus when you take a photo. This can be altered by using your lens’ aperture, and also by altering the distance to your subject, and controlling the ratio of distance between your subject and you, and your subject and other background elements. Certain styles of photography prefer shallow, and others prefer deep DoF.
You don’t necessarily require super large aperture lenses, and they needn’t cost the earth – taking your standard kit lens out with you will still enable you to explore DoF fully, using a bit of know-how, and manipulation of subject distances. Photography is hands on, and the best way to learn how to shave the angles and get the DoF just right is to get out there and practice.