Exposure Value and balancing light - Prepare your brain!

I have just uploaded a new video about Exposure Value and balancing the light in your image.

When we use the exposure meter on the camera it gives us an average balance of light to work towards to match the light in the room for an accurate picture. In a lot of situations, we may want this light to be a little brighter or vice versa but it's always handy to be able to find an average exposure level to begin with.

The exposure meter changes depending on what settings we are using, so when we make an adjustment to our Shutter Speed, Aperture or ISO, the light in the image changes and the meter will show us if those settings are causing the image to be over exposed (too much light) or under exposed (too little light).

The measurement of exposure is often referred to as a 'stop'. This is not referring specifically to just shutter speed, just aperture or just ISO, it is a combination of all of them. However when we are changing our exposure, the change we make is measured on a scale that uses the 'stop' system.

Stops for Shutter Speed are seen as this:

1/8000, 1/4000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1" etc

For Aperture:

f22 (f/22),16 (f/16), f/11, f/8.0, f/5.6, f/4.0, f/2.8, f/2.0, f/1.8 etc

And for ISO:

100, 200, 400, 800, 1600, 3200

So each of these numbers are a stop measurement.



If I had a shutter speed of 1/60 and I was told to increase my shutter speed by two stops, I would then change it to 1/250 as this is two numbers up the scale.

Sometimes we need to use a specific setting. As an example, if I wanted to take a slow exposure of water, but I had already balanced my exposure to 1/250 and f2.7 and ISO 400 then I would need to adjust my shutter speed - this would then put my exposure out of balance. But if we use the stop measurements as a guide we can easily get back to a balanced exposure without guessing at our settings.


So we have 1/250 SS, F2.7 A and 400 ISO but we want a shutter speed of 1/4000. So we are moving 3 stops up the shutter speed scale:

1/8000, 1/4000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1"
                           +3          +2        +1

So we have moved 3 stops and our image will be too dark as we have sped up our shutter and not compensated for the lack of light.

We can then balance this out by moving either out Aperture or ISO an equal amount of stops. In this situation, we started off with an Aperture of f2.7 and as this is the lowest we can go on the SX40 we won't be able to move this setting to let in more light. This leaves only our ISO to change, so we move our ISO 3 stops up the ISO scale in order to compensate for the Shutter Speed and to let in more light:


100, 200, 400, 800, 1600, 3200
                        +1      +2      +3

So we now have a new setting for our image of 1/4000 Shutter Speed, f2.7 Aperture and 3200 ISO which allows us to take that picture with a faster shutter, and to maintain a good level of light in our image.

It seems a little complex but it's actually quite straightforward.

One last thing to mention is that on the SX40 you can also do fractions of a stop so instead of our Shutter Speed jumping from 1/30 to 1/60, the SX40 allows us to use 1/40 and 1/50 too. These smaller fractions are not part of the scale to measure light changes. Only the parts of the scale that have a number above them are full stops. You'll see what I mean in the new video.

I hope that is fairly easy to follow as it isn't so easy to explain!!