One of the fundamental things you need to understand (need – alright, you don’t need to know – there is alway “Auto”) if you want to be a better photographer is exposure, or as it is often referred to, the exposure triangle.
ISO, Aperture, Speed
ISO measures the sensitivity of film or nowadays, your sensor to light. The lower the ISO, the less sensitive – alternately, the higher the ISO the more light sensitive. Why, then, don’t you always shoot at the highest ISO – because the higher the ISO the more “noise” in your shot. Why then, you ask, don’t you always shoot at the lowest ISO – because in some low light situations, you just can’t get the shot without cranking up your ISO.
The aperture, represented as ƒ/999 (where 999 represents a number between 1.2 and 32 (generally) – these numbers will vary depending on the lens you use) and also known as your “f Stop”. The smaller the number (1.2) the LARGER the aperture – conversely the larger the number (32) the SMALLER the aperture. The larger aperture (1.2) will let more light onto your sensor (or film) and the smaller aperture (32) will let less light onto your sensor.
Source: WikipediaBy Mehmetaergun [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
This is how long your aperture will stay open. The speed settings will depend on your camera – the EOS 60D allows me to shoot from 1/8000 sec to 30 seconds. If I choose BULB mode then I can keep the aperture open for as long as I like.
These three things work together to form an image – hence the term “Exposure Triangle”.
Try this little exercise – set your camera to manual and then look through the viewfinder and 1/2 press the shutter so that the display lights up. Set you aperture to the lowest ƒ number you can – with my 70-200 ƒ/2.8L lens, the lowest setting is 2.8 at any focal length (from 70 mm – 200 mm) – then slowly adjust the shutter speed from the longest (30 sec) to the fastest (1/8000 sec) – you will notice the light meter (the series of dashes from -3 – 0 – +3) will probably have an arrow pointing to the right (+3) telling you that you are way overexposed – as you move the shutter speed to a lower number, eventually the light meter will indicate that you are getting closer to the correct exposure (0) and as you keep increasing the shutter speed it will start moving towards the left (-3) – indicating that you are starting to underexpose.
This time set your ISO to the lowest possible number for your camera (the EOS 60D lowest is ISO 100), set your aperture and shutter speed so that your light meter is set at 0 and then slowly start increasing ISO without changing anything else. You will see the light meter start to slowly move towards the overexposed side (+3).
What I am trying to explain here is the relationship between aperture, shutter speed and ISO – the 3 things work together to create the “correct” exposure.
Of course, with all things photographically/artistically, the end result is always subjective – what to one is perfectly exposed to someone else is underexposed or overexposed – only you can decide what is right for you.