Focus is one of your most creative tools. Rather than simply aiming
to keep your subject in focus, use focus to tell the story, by
controlling what stays in focus and what becomes blurred.
In this lesson we’ll look at how, with an understanding of lens choice and aperture, you can keep your images in focus, right from the foreground to the background, or you can narrow that focus so there's a shallow depth of field and only part of the image is in focus.
Lens Choice and Focus
In general, short lenses show more focus from the foreground, all the way to the background. If you look at the shot below, the actor in the foreground, the tree in the mid-ground and the distant horizon are all fully in focus.
That's why we often use short lenses for moving shots; when the actor moves further away from the camera, you don’t have to refocus because everything stays in focus.
With a long lens, however, much less is in focus. Below you can see that the actor is in focus, but on the right the branch that's close to us, the rock just behind her and the far distant trees are all out of focus.
When you’re filming, it’s important to know how much of the image you want in focus. If you want it all in focus, a short lens might be better. If you only want a narrow range of focus, then a longer lens might be good.
The Problem With Long Lenses
Although the sort of cinematic effect people often aim for, with the actor being sharply in focus and everything in the foreground and background out of focus, looks great, it can be difficult to put into practice especially if the actor is moving towards or away from the camera. You have to focus very precisely on your subject.
In a clip in the video, you can see that I’m able to track focus as the actor walks for a while but then I start to drift off and occasionally lose focus. It’s very very difficult to hold focus when the depth of field is this shallow but that doesn’t mean you can’ try.
Everything is a Generalisation
Almost every “rule” you see about film making is a generalisation. The truth is that we can get a soft background that’s out of focus even with a short lens, and a deep depth of field with everything in focus with a long lens. You achieve both effects by changing the aperture.
In the shot below, I'm using my 10 mm lens and the mid-ground and the background are in focus. I’m using a narrow aperture like f/22 because of the bright light. The higher the f-number, the narrower the aperture.
To get a shallow depth of field, you go for a low f-number like f/5.6 or even f/4. Below I’m using an aperture of f/5.6 and it’s very soft in the background.
Of course, opening the aperture up brightens the image so you have to go for a low ISO. If the light's really bright you might need to put a neutral density filter on the front of the lens, like I did here, to limit the amount of light coming into the camera.
If I'm using a long lens, then the foreground and background are normally out of focus. If I want them to be sharper, I go for a higher f-number.
Here you can see that I’ve closed the aperture down to f/32 which is an extremely tight aperture. It’s making the image sharp right the way from the foreground to the background. I’ve had to turn the ISO up to brighten the image.
With a really long lens, say 200mm, it's going to be difficult to get absolutely everything in focus but you can get close. If you really want to get focus all the way through it would be better to use a slightly shorter lens and move closer. The actor will look the same size in the frame but you’ll have a sharper background
Wrapping Up
Whatever
lens you are using, and what aperture you choose have an enormous
influence on what's in focus in your shots. Don't just use your aperture
to set the brightness of the image. Remember that you can affect the
brightness by changing the shutter speed and the ISO, and use your
aperture to dictate how much of the image is in focus.
