Sensitivity and ISO – Technical Fundamentals Course

Sensitivity and ISO will be a bit trickier to explain than aperture and shutter speed, as there’s no convenient mechanical way I can show what it is and what it does. Bear with me!

Once light has passed through the aperture, and the shutter, it hits some sort of light sensitive medium. In the olden days, this was a roll or sheet of film, but nowadays of course, it’s a digital sensor. “Sensitivity” refers simply to how sensitive to light this medium is, and it’s measured in ISO. Since ISO is much quicker to say and write, I’ll be sticking to that.

Sensors from inside 2 DSLRs
The image sensors from inside two DSLRs. Do not attempt to take yours out of your camera, you’ll REALLY struggle to get it back in again!

As with the other two, the ISO scale is numeric, measured in stops, and helpfully this one is a very nice, logical scale.

Less sensitive <<<<<——>>>>>More Sensitive

50 – 100 – 200 – 400 – 800 – 1600 – 3200 – 6400

You can probably guess by now that the gap between each number is one stop, and that if you go from ISO 50 to ISO 100 you change by one stop, and it’s twice as sensitive, and if you go from ISO 1600 to ISO 800 you also move one stop, and become half as sensitive.

ISO, ASA and DIN

You’re unlikely to encounter these terms, but back in the days of film there were a couple of standard ways of measuring sensitivity. ASA (American Standards Association) was one, and was identical to what we now call ISO (Which stands for International Standards Organisation, worth knowing for the pub quiz). There was also DIN, which stood for Deutsche Institut fur Normung, and was a German scale that used a different set of numbers. Up until the turn of the millennium you would see film quoted as something like Speed: 100/21 but nowadays the latter number has been dropped and we just use ISO. Life is much simpler that way!

If you use film, each roll was a set sensitivity, and you were pretty much stuck with it. The way the film was made more or less sensitive was by increasing or decreasing the size of the light sensitive grains of emulsion that the film was made up of. Bigger grains were more sensitive, and vice versa.

ISO 400 slide film
Film – in this case ISO 400 slide film from Fuji

In the digital age, it doesn’t quite work like that. You probably know that the light sensitive bits in your camera are called pixels (stands for Picture Elements – someone added an X.) You’ll have seen that your camera has 10 million pixels (10MP) or 24MP or any number. To increase or decrease sensitivity in a digital camera, the pixels don’t get bigger or smaller. This would obviously cause a few engineering issues for camera designers!

Instead, the best way to describe how the ISO is changed is to think of a volume control. If you change the ISO in a digital camera, the processor inside the camera simply makes the pixels more or less sensitive, turning the “volume” up or down.

What effect does sensitivity have on the image?

Thankfully, the effect on the image is pretty easy to describe. Where you mostly see the effect of ISO is in what could broadly be called image quality. Think back to the “volume” analogy and you’ll quickly see what I mean. If you record someone talking with a cheap microphone, lots of wind noise, and a mechanical digger in the background, when you play it back at a normal volume it won’t sound great, but it’ll be OK. If you turn the volume up in an attempt to hear the voice better, you’ll make the voice louder, but you’ll also make everything else much louder too – all the distortions from the cheap mic, the digger reversing, and all the wind. You get what’s officially called “noise”.

In the same way, increasing the ISO makes the sensor much more sensitive, but it does so right across the board. You’ll amplify any areas where the sensor hasn’t done a great job of recording the image well, exaggerating any flaws. The “volume” analogy carries on right through to the definition for this problem, as the distortion in an image caused by high ISO’s is called, wait for it, noise.

100% crops of different ISO speeds
100% enlargements of 3 different ISO speeds. Notice how the difference between 100 and 400 is very subtle, but by 6400 the noise is very noticeable.

So, straight off the bat you’re thinking, “well, I’ll just keep the ISO nice and low, and get great image quality.” You are very wise to think this, and I’m chuffed you’ve been paying attention. Keeping the ISO as low as you can is a very good rule to live by – it’ll definitely help the overall quality of your images.

We all know there’s a “but” coming don’t we? Life just isn’t that simple! Whilst it’s a very good idea to keep ISO as low as you can, don’t let it rule your life. You see, some of the noise you get when you use a high ISO can be removed in software like Lightroom or Photoshop. In some cases you’ll be hard pressed to tell the difference between something shot at 100 ISO, and something shot at 800 ISO that’s then had the noise removed in software.

But why would you want to use a higher ISO? Ten bonus points to those who’ve been paying attention so far, and have remembered that aperture, shutter speed and ISO are all connected, and if you make a change in one area, you must make a corresponding change in another in order to keep the same exposure. Let’s say you want to freeze some fast movement, and therefore want a fast shutter speed. The amount of light you’ve got in the situation at the moment means that the fastest shutter speed you can get at ISO 100, with your lens at its widest aperture of f2.8, is 1/60s and you know this won’t be fast enough. Turn the “volume” up to ISO 800 and you can now shoot at 1/500s and still get the same exposure.

You see, you can remove the noise in the ISO 800 shot in software, but if you’d shot at 1/60s and got a blurred shot, you’re out of luck. There is, as of 2019, no way of making a blurry, out of focus image look properly sharp. So don’t be afraid to dial the speed up, and then take it out afterwards.

One last thing, before we tie all three exposure elements together properly. You may not realise it, but if you’re shooting digitally you’ve got at least one huge advantage over the olden days of shooting film (OK, there are several, but stick with me). You might remember I mentioned that a roll of film was all one speed, so if you wanted to shoot at a higher ISO you either needed to unload that roll, or have a second camera to hand that was loaded with different stock. In this amazing, 21st century, futuristic dream world we find ourselves in, that’s no longer the case. With a digital camera, if you want to change the ISO between shots, that’s exactly what you do. You can then change it right back for the next shot, or carry on as you were. Speaking as someone who’s been shooting a VERY long time, this is a rather handy thing. Just hope you appreciate it!

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