The 3 Factors of Exposure – Part 1 – ISO

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Let me just say before beginning this guide, that it’s my strong belief that you should only ever be shooting in Manual or Aperture Priority modes… oh OK you can shoot in Shutter Priority mode too, I’ll let you off for that.

This isn’t some sort of photography snobbery, this is about control and results. Until you stop letting the camera make decisions for you, you’ll never learn why an image was captured the way it was, and you’ll never get consistency in your shots.

Exposure is how much light is captured to generate your image. There are 3 factors that can be adjusted together to control this amount of light exposure. I like to refer to them as:

  • Sensitivity
  • Duration
  • Amount

These 3 factors apply no matter what type of photography you’re doing, film and digital alike.

What is ISO?

Sensitivity relates to how sensitive the media is that you’re exposing to the light. We call this the ISO with both film and digital. When I was a kid, it used to be called ASA, but the principal was the same: the higher the number, the more sensitive to light, but the more sensitive to light, the more noise/grain you will get in the image.

One of the major advantages that the digital age has brought about, is variable ISO i.e. the sensitivity can be changed for each image. With film, if you put in a 200 ISO film, you’re stuck shooting with that sensitivity until you replace the film. ISO in the digital world is simply how sensitive the little chip (sensor) that the light hits will be, and within that chip’s overall operating range, you can set its ISO to whatever you want before each shot.

To demonstrate ISO easily, you’ll need a camera with full Manual mode, i.e. we will need to be able to adjust the ISO, Aperture (F-Stop) and Shutter Speed manually and not have the camera automatically adjust these for us.

I’m going to demonstrate on one of my Canon DSLRs, but even some high-end compact cameras have this functionality.

Let’s set a base level of all 3 settings so that we can easily see what impact they are having when we change them. I’m indoors, on a fairly overcast day, so I’m going to set my camera to ISO 500, F/3.5 and a shutter speed of 80. Experience tells me that these settings are likely to give me a reasonably good exposure, maybe a little dark and moody. I’ll take a picture:


Not a bad exposure at all. So what impact on an image will changing the ISO have?

A quick change to double the ISO to 1000, and suddenly we can see that the next image is brighter:


And similarly, if we half it down to 250, we end up with a darker image, one which is too dark.


So, the question you might be asking is “Does ISO simply change the brightness of the image then?” and the answer is both yes and no. Changing the ISO certainly does impact the brightness of the image, by making the sensor chip less or more sensitive to light, but it’s only one of the 3 main controls of the brightness of the image. Changing the ISO allows you to make the image brighter or darker, without having to change the other exposure controls.

But then, why have the other 2 controls? This will become clearer as we get through this topic, but the basic concept is that each 3 settings will adjust the amount of light used to make up the image, but all 3 have additional “side effects” and don’t just simply change the brightness of the image. It then becomes a balancing act of deciding which combination of settings to use, to a) achieve the exposure you desire, but also b) which additional impacts on the image you’re willing to have.

So what are the additional side effects you’ll get when you adjust the ISO? The most noticeable is noise.

What is Photographic Noise?

Photographic noise is simply grain on your image, which can impact colours, sharpness and detail. The higher the ISO, the stronger the grain becomes. In digital photography the performance of the camera’s sensor determines just how much noise will be present at which ISO, for example my Canon EOS 6D has a fantastic high ISO performance, with grain only becoming noticeable when the ISO is in the thousands, whereas my very old Canon EOS 400D becomes too noisy in ISOs over 600.

Here’s an example to demonstrate this. With the aperture set to F/3.5, I’ll adjust the ISO and Shutter speed to achieve pretty much the same exposure, while raising the ISO (I’ll come onto why adjusting the shutter speed allows us to do this, later)

Here’s what we get with the ISO set to 500:


And if we zoom into some detail, you’ll see it’s nice and clear.


If we raise the ISO to 1000 and take the photo again, you’ll not notice much difference at first glance:


But if we now zoom in again, you’ll see the noise (you might need to zoom in on your display even more depending on how you’re viewing this page):


As you can see, some of the detail and sharpness is starting to be lost and there’s a grain appearing across the image.

Let’s now take this to something very high, and set the ISO to 8000. On this camera, 8000 ISO is still quite usable in many circumstances:


Even viewing the whole image, you can see the impact of the noise, and if we zoom in, you can really see the high levels of noise present:


It’s important to mention, that not all noise is bad, there are many photographers out there, including myself, that like a little bit of grain to the image; it gives it character, and sometimes a film-like quality.

While on the subject of film, the same thing happens if you use a high-ISO film. The ISO and quality of a film determines how much grain you’ll get on your developed photographs.

So that’s ISO covered for now, let’s move onto Shutter Speed in the next section and see how that affects the image, and how it can be combined with our understanding of ISO.

Next, Part 2 – Shutter Speed ->