Part one of this series, on exposure, is here. The first section of part two, on aperture and f-numbers, is here.
Welcome back. Let’s start with a reminder of the f-number ‘scale’, from widest aperture to narrowest:
wide — 1.4 2 2.8 4 5.6 8 11 16 22 — narrow
It’s fairly intuitive that the wider the aperture of the lens, the more light can get in, and vice versa for the other end of the scale. Your eyes do this as well - if you look at a bright light, the ‘aperture’ of your eye gets smaller so that you can see more than just a whiteout. In a dark room, the opposite happens.
Version two of the scale:
wide and bright — 1.4 2 2.8 4 5.6 8 11 16 22 — narrow and dark
The picture below was taken with a 35mm lens at f/1.2, which is very wide and very bright. It was a dark room, but using such a wide aperture allowed me to take a nicely exposed image anyway.
Something else you may notice about that picture is that only the snowflake decoration is in focus, or sharp. This leads us to the term ‘depth of field’ which just means how much of your image is acceptably sharp, and how much is blurry or soft. The above image has a very small depth of field: the branches and decorations even a short distance further away or closer than the sharp snowflake are very soft.
Depth Of Field, commonly abbreviated to DOF, is a bit of a confusing term. What field? Are we talking sunflowers here? To make it a little easier, it can help to think of DOF as meaning ‘Depth Of Focus’ instead.
It’s important to realise that DOF is concerned only with how far away things are from your lens. Look at this picture:
David’s face is sharp, but his sister is very soft, because she is a fair bit further away from the camera then he is. You can also see that David’s hand and front leg are a little out of focus, because they are closer to the camera than his face. DOF can be very very small indeed, depending on your camera and lens.
A different example. In the picture below, Madeleine and Lindsay are both in focus, because they are exactly the same distance away. If they were twenty feet apart from each other, but still the same distance from me when I took the shot, they would still both be in focus.
Finally, an example of a large depth of field/focus. Here I focused on the background, but made the DOF so long from front to back that everything was acceptably sharp:
Everything from the nearby leaves to the distant hills is acceptably sharp - literally miles of depth of field/focus. Compare this with the portrait shots above, where the DOF is only a few inches!
Okay, hopefully you have a decent hold on what depth of field is at this point. The creative options are pretty much endless, so the last important thing to cover is how to change it. There are two ways we’ll look at here, the first being your choice of aperture. Here’s the expanded scale:
small DOF — f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 — large DOF
The bigger that f-number gets, the larger your depth of field gets. Finally, something that’s intuitive to remember! We won’t delve into the physics here - just google ‘circle of confusion’ and you’ll start diving down the rabbit hole. The only extra point I want to make is that the f-number actually refers to aperture size only, and the effects on light entering the camera and depth of field are secondary effects of your aperture size.
The other thing that changes your depth of field is relative distance. The closer you are to your subject, the smaller DOF becomes. Similarly, the further away your background is from your subject, the more out of focus it will be.
That’s it for now! Enjoy your cup of tea.