The “Correct Exposure” – Part 2

If you’ve read Part 1, you should now be more familiar with the exposure meter, and how to read the information it displays. In a nutshell, you should know when your current settings will result in a correct exposure (zero-reading), an underexposed image (negative reading) or an overexposed image (positive reading). What we still haven’t covered is how to react to this information, and that is what this Part will address.

This is where the three factors of Shutter Speed, Aperture and ISO come in again. We will be assuming full control over these three factors for the purpose of this blog – this will make understanding the changes being made easier. Our second assumption is that we’re starting off with the following settings:

ISO: 100  / Aperture: f/8.0 / Shutter Speed: 1/100th of a second.

Now – suppose your current settings are showing an exposure meter reading of “-2”. What this is telling you is that if you shoot your image with these settings it will be underexposed, by two (2) stops of light. You might remember from my earlier posts that one “stop” of light less than required means that you need to double the amount of light coming in. Two stops of light less than required means that you need to double the amount of light coming in, twice – that is, two multiplied by two, which is equal to four. If you’re already dreading all the mathematics, fear not. The exposure meter will rescue you. In case you can do the math, you will probably know right away what you need to do, but if you don’t, let’s quickly recap on how to get rid of underexposure and make the image brighter:

  1. Increase your ISO
  2. Slow down your shutter speed
  3. Make your aperture wider.

The next question arises – by how much should I increase the ISO, slow down the shutter or widen the aperture? The answer is in the “-2” signal. Yes, that’s right – we need to let two more stops of light in. You can go about this as follows:

Keeping both shutter and aperture constant, your first option is to increase the ISO; by doubling the ISO from 100 to 200 you would get one more stop of light in, and another stop more would double it again from 200 to 400. By simply raising the ISO from 100 to 400, your reading (from the same scene) should show that you now have the correct exposure. But – suppose you don’t want to increase the ISO, because you want to avoid having more noise in the image (click here for a refresher), what else can you do? In that case, you have to turn to the other options.

A second option is to slow down your shutter speed by two stops – that means having a speed that takes four times more time to close the shutter. Our starting point was 1/100th of a second. If we want twice as much light to come in, the shutter speed must be half that time – so we go down to 1/50th of a second. But that would still be one stop less light than we need – so we need to go down another stop, to 1/25th of a second. With that shutter speed, and the ISO and aperture retained at 100 and f/8.0 respectively, we should get an exposure reading of zero, correct exposure once again! But – do you remember what you risk when doing this? No? You might need to revisit this for more detail. In brief, what a slower shutter speed can lead to is camera shake, resulting in blurry images as a result of movement of the subject, your hands, or both. In our example, the shutter speed required on f/8.0 and ISO 100 would be 1/25th of a second – a relatively slow shutter speed if you’re hand-holding your camera, as it’s difficult to stay perfectly steady enough for that “long”.

This brings us to our third option, which is to open your aperture wider, and let more light inside your camera. Same principle applies here – two (2) more stops of light are needed so the aperture needs to widen from f/8.0 to f/5.6, and that would let in twice as much light. We need more than that though – we need to double the amount of light once again to increase another stop. This means you have to go from f/5.6 to f/4.0. By doing this, we would have let in four times more light and the exposure reading with f/4.0, ISO 100 and 1/100th shutter speed should be zero – correct exposure. What does this do your DoF, however? As discussed in more detail in this post, it makes your depth of field much more “narrow”, meaning you have much less in the image that appears sharp and in focus. Whilst this is not always a bad thing, it might ruin a shot for you if you want front-to-back focusing in your image, for instance when you’re photographing a large group and there’s some distance between the front and the back row of people.

So, the million dollar question arises – what is the right thing to do? The answer is simple really – it depends. It depends on your creative intent, it depends on your equipment limitations, as well as on the type of subject you’re photographing. In the example above, you can have a fourth option, which is to mix and match as needed, for instance, increasing one stop by using a higher ISO and increasing the other stop by widening the aperture. The settings would read ISO 200, f/5.6 and 1/100th of a second. Those settings would still give you a correct exposure, and potentially lead you to a better picture. The trick is in knowing what each change will give and take from your image, and then select accordingly. There’s nothing like practice to learn these things, so it’s time for you to grab your gear and go shoot something. Switch to Manual exposure mode, and enjoy experimenting!!

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One Response to The “Correct Exposure” – Part 2

  1. Pingback: The “Correct Exposure” – Part 3 | ALISTAIR FARRUGIA | photography

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