Understanding Exposure Part 2
Yesterday I gave you an overview of what I call the ‘Bermuda Triangle of Photography’. This is the relationship between ISO, Aperture and Shutter speed. I am going to help you forget about ‘auto mode’ and make creative decisions for yourself… today I am going to get you to forget about photography mumbo jumbo and we’re going to talk about buckets, bucket lids, water and hoses… stick with me, it will all make sense to you soon! If you haven’t read part 1, I suggest you do before moving on… you can find part 1 HERE
Imagine you have a bucket that will hold 10 litres of water, you place the lid on the bucket and set it aside. You grab your general 25mm garden hose to fill the bucket with water. You turn on the hose and aim it at the lid of the bucket, you then remove the lid from the bucket, you will need to aim the hose at the bucket for a while before it fills with 10 litres of water, when it fills to the brim place the lid back on to prevent any more water from entering… you now have exactly 10 litres of water in your bucket!
Let do the same thing, but this time use a hose with double the diameter, so instead of 25mm it will be 50mm, for the purpose of this example we will assume this doubles the flow of water. Will it take a longer time or a shorter time to fill the 10 litre bucket?…. If you said shorter time you are correct, theoretically, you will need to hold the lid off the bucket for half the time to fill the bucket. What if we doubled the diameter of the hose again, so now it is 100mm, assuming this doubles our water flow again, this would halve the time it took you to fill the bucket with the 50mm hose.
Each time we increase the diameter of the hose, we need to take the lid off the bucket for a shorter time to achieve the same result. Hopefully I haven’t lost you yet!
How does this relate to photography? The water represents ‘light’, the hose is your ‘aperture’ and the bucket lid is your ‘shutter’.
Imagine you left the lid off the bucket for too long, what will happen? That’s right, it will overflow… and if you put the lid back on too early, not enough water would be in the bucket. Overflowing is the same as overexposing an image, putting the lid back on too early will lead to an under-filled bucket, in photography we would refer to it as an under-exposed image.
To get a perfectly exposed image we have to be able to get exactly 10 litres in the bucket every time, if we have a high flow of water we need to shorten the time the lid is off, if it is a slow flow of water we have to increase the time we leave the lid off to allow the bucket to fill. The same is true for photography, the sensor in your camera needs the same amount of light each time to correctly expose an image, if it is really bright then the shutter needs to be open for a short time, if it is dark the the shutter needs to be open longer to capture the same amount of light. We also control how much of the light we let in by adjusting the aperture. So even if it is bright we can reduce the diameter of the lens opening to restrict the amount of light getting in allowing us a longer amount of time to leave the shutter open.
Now I am sure you’re wondering why, if you always need the same amount of light why do you need to be able to make adjustments to shutter speed and aperture? I will explain this later…
In addition to these two adjustments you also have the option of changing the ‘size’ of your bucket, imagine we used a 5 litre bucket instead of the 10 litre bucket, it would fill in half the time. In photography this is known as ISO, the sensitivity of your sensor. the downside of increasing your ISO (using a smaller bucket) is that it creates unwanted side effects to your images… we will explore this later too!
For now, I hope this has given you a better understanding of exposure control… in part 3 we will start to look at how you can change the aperture, shutter speed and ISO to take creative control and use your camera in more difficult lighting scenarios.
The Daily Pic – Beach Stroll
I enjoy walking along the beach watching the sun rise… I took this at Palm beach, I used an ND filter to help me create a long exposure, this is how I have made the water look so dreamy… I kept the shutter open for 20 seconds, to do that I had to restrict the light entering the lens by using a ND filter, it is basically a dark tinted cover that screws onto the lens… enjoy