Welcome to Exposure basics: Aperture, Shutter Speed & ISO.

Disclaimer: I am a Nikon user, so most of references to camera equipment, features & specifications mentioned below will be based on Nikon systems and will use Nikon’s terminology. This is not my way of saying “buy a Nikon camera” – all of the following material, including methods, tips on technique & my personal advice will also be relevant to any major DSLR camera brand.

What is exposure?

– it is the amount of light reaching your camera’s sensor.

If too much light gets in – your photo is going to come out too bright (or OVEREXPOSED), not enough light – and it will be too dark (or UNDEREXPOSED).

Let’s have a look at these three examples. All three photos have been taken using Nikon D700 with 70-200mm f2.8 VRII lens (click on images to enlarge):

Exposure basics: Aperture, Shutter Speed & ISO

1. Camera settings: f/5.6, 1/500, ISO 200 (above)

Exposure basics: Aperture, Shutter Speed & ISO

2. Camera settings: f/5.6, 1/1000, ISO 200 (above)

Exposure basics: Aperture, Shutter Speed & ISO

3. Camera settings: f/5.6, 1/250, ISO 200 (above)

As you can see, all three photographs have the same aperture setting (f5.6) and ISO (200), but different shutter speeds.

The first photo is correctly exposed. Considering that this is quite a high contrast scene anyway, i managed to retain most of the important details in both highlight and shadow areas (clouds & trees).

The second one has been taken with a faster shutter speed of 1/1000, and as a result we start to loose some of the details in the darker areas. The whole image looks kind of dim.

The third one has been taken with the slower shutter speed of 1/250, and the opposite has happened: we have completely lost details in those beautiful clouds – the whole sky looks washed out…

So the first thing you as a photographer have to learn is how to control your exposure.

To help you do this every DSLR has three controls: APERTURE, SHUTTER SPEED & ISO.

So, what are they?

APERTURE – is the size of the “hole” (or diaphragm) through which the light passes through;

SHUTTER SPEED – is the time for which the shutter curtain (in front of the sensor) stays open;

ISO – is the sensitivity of camera sensor (what used to be film speed in the old days).

By balancing these three components, the photographer controls how much light hits the camera’s sensor. If you increase one of the elements, you have to decrease one of the other two in order to maintain the correct exposure.

Let’s imagine that the correct exposure is a complete circle divided into segments of three different colours representing three elements of exposure: blue – aperture, red – shutter speed and green – ISO (Fig.1)

Exposure basics: Aperture, Shutter Speed & ISO

Fig.1

If we take out two green segments (decrease our sensor sensitivity), we have to fill the circle with alternative segments to maintain the correct number of segments within the circle. We can open up our aperture (add two blue segments) or we can slow down our shutter speed (add two red segments). Or, in fact, we can add one of each, but as long as the total amount of segments remains the same – we have our correct exposure! (Fig.2)

Exposure basics: Aperture, Shutter Speed & ISO

Fig.2

Exposure values are expressed in number of stops.

Decreasing exposure by one stop means you are letting in half of the amount of light, and increasing by one stop means you are letting in twice the amount of light.

So how does it translate into our three elements of exposure?

Lets’ look at the shutter speed first.

Let’s say you have a shutter speed of 250 of a second, or 1/250. Decreasing your shutter speed by one stop will make it 125 of a second, or 1/125. On the other hand, if you increase you initial shutter speed by one stop you will get 500 of a second, or 1/500.

Shutter speed of 1/250 will let in twice the amount of light of the shutter speed of 1/125, but only half as much light as the shutter speed of 1/500.

Here is the list of all common shutter speed values in 1-stop increments, starting with 30sec:

30s;  16s;  8s;  4s;  2s;  1s;  1/2s;  1/4s;  1/8s;  1/16s,  1/32s;  1/64s;  1/125s;  1/250s;  1/500s;  1/1000s;  1/2000s.

The same method lies in the basis of aperture values, with the only difference that the numbers are quite different. The aperture increments are expressed in f-stops. The smaller the f-stop – the larger the opening – the more light gets in. This could be quite confusing at first, how the smaller number represents the larger opening. It’s all to do with the formula: f/#, where “f” is the focal length and “#” is the diameter or the opening. Thus, f/2.8 will give you a larger figure then, let’s say, f/11. So the aperture of f/2.8 will let in more light then f/11.

Here is the list of all common aperture values in 1-stop increments, starting with f/1:

f/1;  f/1.4;  f/2;  f2.8;  f4;  f5.6;  f8;  f11;  f22;  f/32;  f/45;  f64.

This is how a diaphragm ring looks at varies aperture settings (Fig.3).  Notice: the larger the f-number – the smaller the aperture:

Exposure basics: Aperture, Shutter Speed & ISO

Fig.3

Now – the ISO (sensor sensitivity value).

The smaller the number – the less sensitive the sensor is to light, i.e. if you want more light registering on your sensor you have to increase you ISO.

Here is the list of all common ISO values in 1-stop increments, starting with ISO 50:

50;  100;  200;  400;  800;  1600;  32000;  64000.

Now that we are familiar with the proper numbering system for all three elements of exposure, let’s take away our colourful circle with wedges and try and read exposure values, as they are read by professional photographers.

Let’s say the correct exposure settings are:

F5.6; 1/125; ISO 200.

We now know what all those numbers mean:

You lens should be set at aperture of f/5.6,

Your shutter speed – to 125 of a second,

And your ISO should be 200.

We take a picture – and it’s perfectly exposed.

Now let’s change one of the numbers, and stop down our lens aperture by 1 stop – f/8.

We take another picture of the same subject – and it looks darker. Why? Because we reduced the overall value for our exposure by 1 stop. So now we have to bring it back up to the same total value as we had before.

We can do it by either raising our ISO by 1 stop, or slowing down our shutter speed, also by 1 stop.

Let’s take two more pictures with the following settings:

F8; 1/64; ISO 200

and

F8; 1/125; ISO 400.

Both photographs will come out perfectly exposed, the same as our very first photo. This is because we have compensated the loss of 1 stop by raising one of two other values: shutter speed and ISO.

These are the basics of controlling exposure by balancing three of its components: aperture, shutter speed & ISO.

So now you may ask: if those three controls all do the same thing – changing the amount of light which reaches your camera’s sensor – why not to have just one simple control: more light or less light?

Well, first – this is how the camera’s mechanism works.

But also, each one of the three controls – aperture, shutter speed & ISO – has it’ own “secondary” function! Each has its own affect on how the final image will look. And it is exactly those “secondary” functions which give you that wonderful creative control to take photos, just the way you want.

But this is a topic for the next sketch (which hopefully will be coming soon) – Aperture, Shutter Speed & ISO: Creative Control.