Photography Explained: Focus
You've noticed something interesting or funny or beautiful and you want to take a photo of it. Whenever someone looks at your picture you want to make sure their eye is immediately drawn to whatever it was that grabbed your attention in the first place. It has to be the stand out element. It has to be in focus.
That could be a single thing in the image, a famous landmark for example. Or a couple of things, maybe a friend standing in front of the landmark. Or it can include everything that's in the frame; a landscape made up of flowers in the foreground, a lake behind them and mountains in the distance. Anything in focus will be sharp and clear, while everything else will be blurred to some extent. Your job is to decide what should be in focus and what, if anything, shouldn't.
Two main factors influence focus in a photograph:
- Depth Of Field
- Motion blur
Depth Of Field (DoF)
This term describes how much of an image is in focus. By that I mean the distance between the nearest point to the camera that's in focus, up to the point that's furthest away while still in focus. This can range from just a few millimetres to hundreds of metres and indeed to infinity.
The setting on your camera that determines how large the depth of field will be is aperture size.
In the exposure tutorial we learnt that a large aperture, for example f2.8, will allow more exposure than a smaller one of f16. In addition to this a bigger aperture gives you a shallower depth of field. As the aperture becomes smaller the depth of field increases.
By deciding what parts of a scene should be in focus and working out which aperture size will give you the depth of field to cover that, you can make the key elements of your photo, such as the eyes in a portrait, stand out. They will be sharp while everything outside the depth of field will be blurred.
In the images below the tulip is obviously the subject but the background in the first example is distracting and contributes nothing to the photo. That problem is eliminated in the second picture because the shallow depth of field has served to blur the foliage, making the tulip stand out a lot better. Your eyes go straight to it and stay there instead of wandering around the rest of the frame.
In fact, I'd say the DoF is too shallow in the second image. Even parts of the flower are starting to fall out of focus. Maybe a smaller aperture would have been better for this shot. It's a matter of experimenting with the options until you find a winner.
Once you've got a feeling for how this works you can get mathematical about it if you want and calculate the exact depth of field you'll get with the camera's current settings and how far it is from your subject. That's the topic of the bonus section at the end of this tutorial.
Motion Blur
What does a statue, a parked car and a vase of flowers standing on a table have in common? And what about an energetic toddler, a car on the motorway and wild flowers blowing in the wind? No chocolate buttons for correct answers here. Obviously the first group are stationary subjects whereas those in the second group are likely to be moving.
Focusing on a moving subject is slightly more challenging. The first thing to consider is whether you want to portray the movement or freeze the action. You've probably seen closeup action shots of sports stars in newspapers; a footballer's face contorted with the effort of straining to win a header. He didn't stop to pose. This is an example of motion freezing where shutter speed comes into play.
We know from the exposure tutorial that increasing the shutter speed decreases exposure because there's less time for light to hit the sensor. It has the additional effect of freezing motion. A waterfall is a good subject to demonstrate this.
The image on the left was taken with a shutter speed of 1/400, the one on the right with a much slower 1/30. On the left the water looks as if it's stopped for that moment in time. You can see the droplets in sharper focus. On the right it's become blurred suggesting a sense of movement. Both are different but equally valid ways in which you as a photographer can portray the waterfall.
What shutter speed to select depends on how fast your subject is moving across the frame. It needs to be much faster for a passing train than a jogger plodding towards you but it also depends on the degree of motion you want to capture. Do you want the train to look as if it's rushing by or as if it's not moving at all? If the first choice is your goal, select a slower shutter speed. If the opportunity allows, take a lot of shots at different shutter speeds until you find the one that nails the effect you're after.
At the beginning of this tutorial I said the key elements of an image should be in focus but now you know that doesn't always have to be true. The photo above demonstrates the exact opposite but hopefully you understand what I mean. How you want to make those key elements stand out by playing with the focus is your creative choice.
Avoiding camera shake
Unless you're observing one from the ISS, mountains don't move, right? Right. So if you want to take a picture of one you don't need to worry about what shutter speed to use because there'll be no motion blur, right? Not quite.
Even if everything in your scene is fixed to the spot, something is still moving. Your camera, no matter how steady you think you're holding it. Look through the viewfinder with your lens at its maximum zoom and you'll probably see the scene wobbling around slightly. It's virtually impossible to hold a camera completely still.
Increasing shutter speed helps to minimise blur caused by camera shake. You might be thinking 'I've taken tons of pictures holding my camera and they're not blurry', and that'll be true. With a fast enough shutter speed and a wide enough angle it'll be fine. But blur becomes more noticeable as you zoom in or slow down the shutter speed.
Pick a stationary test subject, put the camera into shutter speed priority mode and set it to 1/250. Zoom in as much as you can, focus and take a shot. Then take the same picture, at the same zoom, with a shutter speed of 1/20. Unless you really do have a talent for holding the camera steady, you'll notice the difference.
Berlin is the city of bears and Bruno is the king, holding court in the Nikolaiviertel. The picture above was taken with a shutter speed of 1/20, far too slow to avoid the signs of camera shake and consequently his fur looks blurred.
Problem solved in the next shot. The shutter speed was increased to 1/250, fast enough for handheld shooting. The exposure is the same because I widened the aperture to compensate for the faster shutter speed. This of course means the depth of field is shallower so the background is slightly out of focus, but Bruno is the sole subject and he is nice and sharp.
So why would you use a shutter speed of 1/20 when you can use 1/250 and eradicate any blur caused by camera shake? We're back to exposure again. Suppose you want to capture a beautiful, snow-capped mountain peak against the dark blue sky just after sunset. 1/250 will give you a disappointingly underexposed result. What can you do? There's the option of increasing the ISO or opening the aperture but let's say in this case you don't want any of the noise that comes with a higher ISO and you need an aperture small enough to keep the foreground features in focus.
You know what I'm getting at. Your best option is to keep the ISO as low as possible and the aperture at the size you've already determined gives the required depth of field. Slowing down the shutter speed is the only way forward and for that you need what I consider to be the most important photography accessory. A tripod.
Once you have your camera on a tripod the issue of camera shake is banished and you can set the the shutter speed to be as slow as you like: 1/10 second, 1 second, 30 seconds, 30 minutes and upwards, opening up a whole new wardrobe of creative possibilities.
Another way of saying 'my shutter speed is 30 seconds' is 'I'm taking a 30 second exposure.' These long exposure shots in low light can yield some spectacular results and is one of my favourite types of photography.
Bonus Section: Precise Depth Of Field
Aperture size isn't the only variable to affect DoF. It also depends on the focal length, how far the camera is from your subject and the size of the sensor in your camera. Before explaining how you can accurately calculate DoF, I'll briefly go over how these factors come into play.
First of all, focal length. What's that?
Technically speaking, it's the distance between the camera's sensor and the optical center of the lens but I'm doing my best not to speak too technically in these tutorials so for now think of it as how far you zoom in or out. Focal length is measured in mm and is a property of the lens, not the camera. It becomes longer as you zoom in, creating a narrower angle of view and a smaller depth of field.
Next is the distance between camera and subject. This could be just a few centimetres if your lens is capable of focusing on something that close, or hundreds of meters. Or even more, you can focus on the moon if you want. The closer you are to your subject, the smaller the DoF.
Finally, camera sensors come in different sizes which also has a bearing on DoF. Bigger sensors allow for a greater range. It's harder to achieve a shallow DoF using a compact camera because their sensors are much smaller than those in a DSLR.
To summarise, the 4 properties which will determine your final DoF are:
- Aperture - larger aperture, shallower DoF
- Focal length - longer focal length, shallower DoF
- Distance to subject - closer to subject, shallower DoF
- Sensor size - nothing you can do about that other than changing cameras
So you can now conclude that using the largest aperture and longest focal length, while standing as close as possible to your subject, will give you the shallowest DoF.
And now for the magic, the online Depth Of Field Calculator, courtesy of DOFMaster which is quite a handy tool. Simply enter the four variables on the left side, click calculate and a bunch of figures are displayed on the right.
The subject distance just displays what you entered but then it gets interesting. Under the Depth of field heading are three values: the nearest point to your camera that's in focus, the furthest point and then the total DoF which is simply the difference.
In this example, anything that starts 96cm away from the camera, extending all the way to infinity, which for practical purposes means the horizon, will be in focus.
The next two figures specify how much of the DoF is in front of and behind your focal point. This is helpful if there are several foreground elements at different distances which should all be in focus. Some cameras have a depth of field preview button which allows you to see the range. If the subjects can be moved you can reorganise them to be within the DoF. If not, change your aperture or focal length. Or move your feet!
The last thing to mention is the fancy sounding term Hyperfocal Distance. This is the exact point to focus on in order to maximise the depth of field. It's quite an important value in professional photography, especially for landscapes where you typically want everything to be in sharp focus going all the way back to the horizon.
But such precision isn't necessary to begin with. It's enough to understand the concept of depth of field and how it can be used and controlled. Once you're familiar with that, you'll more than likely be hitting the hyperfocal distance without even realising it. In the meantime, a good rule of thumb is to focus one third into the scene.
