Exposure Triangle (Part 4) – ISO Sensitivity

Welcome again to my blog – this is the fourth post in the “Exposure Triangle” Series: you can read the other posts by clicking their title from the main page or clicking here: Part 1, Part 2 or Part 3. This fourth, and final, entry will focus on ISO (or film) sensitivity – in other words, how quick a film or sensor reacts to light coming into the camera. As covered in more detail in my earlier posts, the more sensitive a sensor or film is to light, the quicker it will go from black to white.

For clarity’s sake – keep in mind that every image starts off as a fully black picture until light starts coming in. Colours are registered on the film or sensor during the time the shutter is open. If an excessive amount of light goes into the camera, the part on the sensor or film that received excessive light will be “burnt out” and it registers as white. If all the sensor or film receives excessive light, than all the image simply becomes a totally white rectangle.

Now, it should be simpler to understand sensitivity – it essentially means how quickly your image will transition from a black-image state to one whereby colours are registered and possibly have areas completely burned to white. The higher the ISO, the quicker this process will take place.

That should have already been made clear in my earlier posts – what we didn’t discuss, however, was the trade-off we must keep in mind when using a higher ISO. This trade-off comes in the form of “digital noise” or “grain”. I’ll focus mostly on digital noise, given that most of you are likely using digital cameras anyway. I can go all technical to describe noise, or stick with simple methods – I’ll go for the latter…

So – what is noise? Essentially it can be described as imperfections in the image that are a result of a sensor (or film) that “misread” the real picture out there. The quicker a sensor tries to register light (and colour), the more likely it is that there are “errors” or misreadings in the process. As a result, the higher the ISO goes, the more the final image loses detail, colour accuracy or both. That is the side-effect that higher ISO leads to, and it goes without saying, it’s a bad thing. Check the uploaded image below (you can click on it to enlarge it). This is the same image taken at ISO 100 and ISO 3200. Can you spot the noise and imperfections? Can you spot the loss of detail in the image?

Noise Comparison

So – why would you want to use higher ISO? Well, there are situations where it is inevitable – if light levels are low and you cannot increase them, then more often than not an increase in ISO would be one of the options that you have to consider. How high should you go? That depends, and it depends on what other factors you can and you want to control. But that question, dear readers, will be the theme of the next post – that is, what to change, when, and why. In other words, it’s time to start seeing the exposure triangle in practice now!

Recap of this post:

Higher ISO = Faster transition from Black to White BUT more NOISE.
Lower ISO = Slower transition from Black to White BUT less NOISE.

Thanks and come back later for more!