Part 2 – Measurements and Adjustments
Aperture is measured using ‘f-stops’.
On your camera, you’ll see ‘f/’ followed by a number.
The number denotes how wide the aperture is. This affects the exposure and depth of field (also explained briefly below); the lower the f/ number, the wider the aperture. Yes, I know, strange, but keep reading...
This may seem confusing; why is a low number is used for a higher aperture?
The answer is actually logical, but first you need to know the f-stop scale.
The scale is as follows: f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/90, f/128.
The most important thing to know when moving across this scale is that, each number (stop) will decrease the aperture by half its size, allowing 50% less light through the lens and onto the sensor.
This is because the numbers come from the mathematics used to work out the size of the aperture from the focal length.
You’ll notice, on modern day cameras, that there are apertures in-between those listed above.
These are ½ or ⅓ stops, so between f/2.8 and f/4 for example, you’ll also get f/3.2, f/3.3 and f/3.5. These are just here to increase your control over the image.
Now things begin to get a little harder. If you get confused, skip to part 3; the most important part has been covered.
If you have a 50mm lens with an aperture of f2. To find the width of the aperture, you divide the 50 by the 2, giving you a diameter of 25mm.
You then have to take the radius (half the diameter), multiply it by itself (giving the radius squared) and multiply that by π (Pi). The whole equation looks something like this: Area=πr^2.
A few examples for you:
A 50mm lens, with the aperture of f/2: 50mm/2 = a lens opening 25mm wide. Half of this is 12.5mm and using the equation above (π*12.5mm²) we get an area of 490mm².
A 50mm lens, with the aperture of f/2.8: 50mm/2.8 = a lens opening 17.9mm wide. Half of this is 8.95mm and using the equation above (π*8.95mm²) we get an area of 251.6mm².
Now, it doesn’t take a genius to work out that half of 490 is less than 251 – this is because the numbers used are rounded to the nearest decimal point. The area of f/2.8 will still be exactly half of f/2.
Part 3 – Aperture and Exposure
Any change to aperture will directly affect the exposure: the larger the aperture, the lighter exposed the photo will be. The best way of showing this is by taking a series of photos, and keeping everything constant with the exception of the aperture.
All the images below were taken at ISO 200, 1/250 of a second and without a flash. Only the aperture changed.
This set of photos was taken in the following order: f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22.
A good way to see the changing size of the aperture is to look at the size of the out of focus area in the image (in the Bokeh).
Part 4 – Aperture and Depth of Field (DoF)
Now, depth of field can be a big topic. For the sake of this tutorial we will say that it is the depth of the image that stays in focus in front of and behind the main point of focus, the acceptable focus.
All you really need to know in terms of how depth of field is affected by aperture: the wider the aperture (f/1.0), the shallower the depth of field, and the narrower the aperture (f/32), the deeper the depth of field.
Before you look at a selection of photos taken at different apertures, take a look at the diagram below which helps explain this.
If you don’t understand exactly how this works, it doesn’t matter too much.
For now, it’s important for you to know the effects.
Here’s a selection of photos all taken on aperture priority mode so that the exposure remains constant and the only changing variable is the aperture.
The photos below are in this order: f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16.
Notice how the depth of field increases every time the aperture is decreased.
The first thing to note is that there are no rules when it comes to choosing an aperture. It depends greatly on whether you are going for artistic effect or to accurately reproduce a scene in a photo.
To best make these decisions, it helps to have a good knowledge of traditional uses for the apertures listed below.