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We all have to start somewhere with our photography, and part of the journey between beginner and seasoned amateur involves several eureka! moments.

Over the next few weeks I’ll detail some of the things I’ve learned that have been fundamentally important in improving my photography.

2. The relationship between aperture, shutter speed and ISO

One of the most complex things for most newcomers to digital photography with a DSLR is understanding the 3 things used to control exposure: aperture, shutter speed and ISO. I’ll quickly explain what these things are:

1. Aperture – The aperture of a lens works in a similar way to your eye’s iris. The smaller you make it, the less light gets in over a given space of time, and vice versa, the larger you make it, the more light gets in over a given space of time.Aperture on a camera is expressed as an f/ number: f/4 or f/11 for example. Confusingly, the smaller the f/ number, the larger the aperture, so f/4 is a bigger or wider aperture than f/11.
2. Shutter Speed – The shutter of the camera is essentially a curtain. When you press the shutter release the shutter opens and closes at a predefined speed, allowing light coming through the lens aperture to reach the sensor. The predefined speed that the shutter opens and closes at is shutter speed.Shutter speeds on a camera are expressed like 1/200, meaning a 200th of 1 second. The higher the number, the faster the shutter speed. 1/400 is twice as fast as 1/200.
3. ISO – The term “ISO” comes from the days of film and was a standard measurement (ISO stands for “International Standards Organisation”) of the light sensitivity of the film. In the era of the digital camera it means the same thing, but the film has been swapped for a sensor that allows you to change its sensitivity from shot-to-shot, unlike film, where you were stuck with that ISO until you’d used up the roll.ISO is expressed in numbers like 100, 200, 400, 800, 1600, 3200, 6400 and 12800. The higher the ISO number, the more sensitive the sensor becomes, and the more sensitive the sensor, the faster the light is recorded.

Stops and exposure

If you frequent any photography groups, read tutorials or books or listen to photographers you may well have heard them saying something like “overexposed by one stop”.

“Stops” are simple to understand, they’re simply a way of expressing relative measurements of light. Sounds complicated? It isn’t. Think about it like this:

You have 1 lightbulb to light your room + You add another lightbulb of the same strength… You have now increased your light by one stop.

Doubling your current amount of light is adding a stop, if you take away a stop you are halving the amount of light (removing a lightbulb.)

How do you know what to dial in to the camera to double or halve the light?

If you know how to count to three you’ll find this pretty simple.

Remember when I said aperture was measured in f/ stops? For example: f/1.4 – f/1.6 – f/1.8 – f/2.0. Each of these increments are equal to 1/3 of a stop. When you turn the aperture adjustment dial one click on your camera you are adjusting the amount of light by 1/3 of a stop, so if you turn the aperture dial 3 clicks you have adjusted the amount of light by one full stop. Here are some examples:

1. You are at f/8, you rotate the dial 3 times to f/11. This means you have halved the amount of light (1 stop) – remember the higher the f/ number, the smaller the aperture, and the smaller the aperture the less light getting to the sensor.
2. You are at f/11, you rotate the dial 3 times in the opposite direction. This takes you back to f/8 and you have doubled the amount of light. You made the aperture wider, which lets more light in to the sensor.

Note that for this to work with three clicks your camera should be set to adjust at 1/3 stops. If your camera is set to 1/2 stop increments then 2 clicks are equal to a full stop.

The same rules apply for shutter speed, 3 clicks is equal to one full stop of light adjustment. For example: 1/125 – 1/160 – 1/200 – 1/250 are all 1/3 of a stop increments.

If you were at 1/125 second and you clicked the shutter dial 3 times to get you to 1/250 second you will have halved the amount of light entering the camera (1 stop). If you clicked 3 times in the opposite direction, taking you back to 1/125 you will have doubled the amount of light entering the camera… remember, the longer the shutter speed, the more light entering the camera.

ISO stop increments vary from camera to camera. I have mine set at 1/3 increments, but if your camera’s ISO options are “100, 200, 400, 800, 1600…” then going from 100 to 200 will be a 1 stop change, doubling the amount of light, and going from 200 back down to 100 will half the amount of light – a reduction of one stop.

I guess the simplest way to explain it is: 3 clicks for a one stop (half the light or double the light) adjustment in exposure.

Simple eh?

Reciprocity (easier to understand than it is to pronounce)

Stops are just a way for us to measure light, and because they are a measurement scale we can use them to compare the 3 things we use to control the overall exposure: aperture, shutter speed and ISO. Being able to compare these three settings also allows you to shuffle these three settings around, while keeping the original metered exposure the same.

Example 1

For example, you’re shooting your friend standing in front of his house at a shutter speed of 1/125 of a second, your aperture is set at f/4 and you’re using 200 ISO. The house in the background is in focus,  you decide to make the background blurry by opening up your aperture to  f/2.8.

That’s a change of 1 stop, and would make your current shot slightly overexposed. Why? Simple,  you’ve doubled the amount of light entering the camera. To make it right again you need to use either shutter speed or ISO to compensate for that 1 stop increase in light, so your options are:

1. Increase your shutter speed by 3 clicks to 1/250 of a second
2. Decrease your ISO to 100, making the sensor less sensitive and cutting out 1 stop of light

Example 2

Another example would be taking the same picture of your friend, but this time your camera’s shutter speed is set to 1/60th of a second, your aperture is set to f/4 and the ISO is 200. You notice that camera shake is resulting in slightly blurry photos, so you increase your shutter speed to 1/125 of a second.

Again, this is a change of 1 stop. You have halved the amount of light getting into your camera, so you need to compensate by giving back that stop, using either a wider aperture, or a higher ISO:

1. Open the aperture up by 3 clicks to f/2.8
2. Increase the ISO to 400

Pros & Cons

There are pros and cons to most compensation adjustments, and if you’re careful they are not really problems, more things to be aware of:

• Using higher ISO numbers introduces more noise to your images, and this is particularly noticeable and ugly in the shadowy areas of a photo.
• Opening apertures to numbers like f/2.8 significantly reduces depth-of-field, meaning that you have to be very careful about where you focus on the subject. If you are shooting at a very wide aperture like f/1.6 and you have a person turned slightly sideways and you focus on the nearest eye, it’s likely that the furthest away eye is going to be in the out of focus area of the photo.
• Reducing the shutter speed certainly allows you to take a brighter image in low light conditions, but it also introduces the possibility of camera shake at shutter speeds of less than 1/60 of a second. It also stops you from being able to “freeze” movement, like you would be able to do at faster shutter speeds for moving subjects. Use a tripod :)

That’s all for now.