Photography Basics 1: What is Aperture or f-stops/f number & Depth of Field
Introduction – Aperture Basics
This is one of the fundemental parts of photography when you advance past a point and shoot, the biggest controls you have how your picture appears is Aperture and Shutter speed. Shutter speed is more obvious in that a slower shutter gives a longer exposure and more movement or motion blur, where as a faster shutter speed freezes action.
We will discuss creative ways to use shutter speed later.
For now we want to discuss Aperture, many people have a vague idea what it means, but don’t know which aperture or f-stop to select for different situations, or why you would chose that aperture.
To start an Aperture is basically a hole in which light is admitted, in terms of cameras the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane.
You can read more about Aperture in depth here.
F-number or F-stop
The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked “f-stops” that the f-number can be set to.
(f-number is also sometimes called focal ratio, f-ratio, or relative aperture)
The very basics you need to remember is a big number is a small hole, so f/22 is a very small aperture, small hole, less light so longer exposure. Something like f/1.8 is a very large aperture, more light and faster shutter speed (shorter exposure).
Here you can see a very common 50mm lens with it’s Aperture set at the smallest number f/1.8, remember that’s the largest opening in figure one.
Figure two shows the smallest aperture, or highest number which is f/22 for most lenses including this one.
Most cameras with some manual controls at least have something called Aperture Priority mode, which refers to a shooting mode used in semi-automatic cameras. It allows the photographer to choose an aperture setting and allow the camera to decide the correct shutter speed. This is sometimes referred to as Aperture Priority Auto Exposure, A mode, Av mode, or semi-auto mode.
This is the mode I most frequently shoot in as Aperture has the greatest effect on the picture you are taking.
The common f stops are as follows:
When you are buying lenses generally the bigger the maximum aperture the better, most professional zoom lenses have a constant f/2.8 aperture which makes them very expensive and heavy!
You can read more on F-number and the science behind it here.
Lenses
Cheaper lenses usually have a variable aperture between f/3.5 and f/5.6. When a lens has 2 numbers for f-stops it means it varies as you zoom it, so if a lens is Nikon 18-70mm f/3.5-4.5G AF-S for example, it means at 18mm it’s f/3.5 and at 70mm it’s f/4.5.
Aperture greatly effects light gathering capabilties, so for low light or night work the lowest aperture possible is preferred, some lenses go as low as f/1.2 or f/1.4!
Prime lenses tend to have bigger maximum apertures as it’s cheaper to build with a fixed optic, prime lenses don’t zoom, the most common and cheapest is the 50mm f/1.8 which we’ll discuss more later.
Depth of Field
The most important thing to understand about Aperture is how it controls depth of field, this will directly effect your pictures and the artistic capability in which you can create the images you want.
Here is an example I took yesterday, Macro photography has a very small Depth of Field as the subject is very large in relation to the sensor in the camera.
Depth of Field is the amount of the picture that is in focus, the basic rule is the smaller the aperture, the more is in focus, so at f/1.8 the Depth of Field would be very very small and at f/22 everything would be in focus.
For this picture the Aperture was f/4.2, the largest the lens can manage during this macro focusing range.
Watch at f/4.2
As you can see not a lot is in focus, the focus point for all 3 is the bottom of the question mark, you can see here only just that is in focus the rest is not. The Depth of Field is small.
The shutter speed was 0.7 of a second.
The next was taken at a medium Aperture f/11 which is normally used for landscapes or large scenes.
Watch at f/11
As you can see more is in focus here, the picture is more aesthetically pleasing. The top of the watch is still out of focus and the background is nicely blurred but the subject is fully in focus and part of the watch.
The shutter speed was 4.2 seconds.
The next is at f/22 which is used for long shutter speeds and macro photography.
Watch at f/22
As you can see now virtually everything is in focus and the shutter speed was very long at 18 seconds.
So you can see as you increase the f-number the aperture hole gets smaller, more of the pictures is in focus and the shutter speed gets longer.
You can use this to control how the picture looks and small apertures are especially good for seperating the subject from the background.
Another example are flowers, these are two shots taken at f/5 and f/22, you should be able to work out which is which now.
We’ll discuss more about DoF as it’s called later and more advanced ways to use it, plus the terms such as Bokeh which are commonly used now.
You can read more about Depth of Field here.
Summary
Aperture is the most powerful tool in taking the pictures you want, in creating artistic effects and interesting eye popping captures of animals, people and flowers.
Just don’t get carried away and use an aperture that is too large (small number), especially on portraits as you can have one eye in focus and one out, or can create confusing pictures that look completely out of focus.
Choose the correct aperture to give the background a nice blur and make the subject stand out.
It will take some practise and you will have to learn each lens, also note most lenses are not super sharp wide open, wide open means the maximum aperture (the smallest number) so stop it down 1-2 f-stops.
So for a f/1.8 lens it should be sharp around f/2.2 and above.
As a rule of thumb for portraits you can use the maximum aperture stopped down 1-2 stops, for group shots try around f/5.6 and for landscapes try f/11-16.
If you need and clarifications please leave a comment below, next to come is What is ISO or ASA – Camera/Film Sensitivity AKA Filmspeed!
