What is Camera ISO? Camera ISO Explained

ISO is fundamentally the sensitivity at which your cameras sensor reacts to the light when you open the shutter

ISO is measured in a scale (On standard digital cameras) called ISO. ISO is the number indicating a digital camera sensors sensitivity to light, the higher the sensitivity, the less light is needed to make an exposure. It is very helpful to be able to control ISO when taking pictures, let’s use the following example:

I’m at a party, inside a house at night, not much available light apart from a few normal house lights / lamps etc, I want to take a picture of my friend sitting on the chair. So I point the camera at him, press the shutter half down and look at the camera readout – it tells me that that I can properly exposure the picture (NOT using a flash – assume it’s broken for now) with a shutter speed on AUTO which shows to be 1/15 (One 15th of a second) – I take a picture and check the display, hmn, all blurry. I Check the ISO on the camera… it’s ISO 100… .

Right, so we can see the problem here is that we need to be able to use a faster shutter speed to control some of that blur. But if we just set the shutter speed to be faster, we will be letting in less light – so the although the shot WILL be sharper and less blurry, it will be much darker, much more under exposed.

So we need to increase the shutter speed to say 1/125, 125ths of a second will make sure that the shot is sharp and not blurry (This is dependent on focal length too – but that’s for another day) but somehow keep good exposure… so in this instance we would want to INCREASE the ISO, from ISO 100 to say ISO200, so the sensor will now react twice as fast to the light – giving us an effective double amount of light.

So, take the picture again and have a look at the display, did you manage to get the shot? Was it exposed enough? If not, increase the ISO again to ISO 400 and take another shot.

So why not just slap the ISO up as high as you can to get the sharpest picture possible?

This is because (dependent on how good the camera is) the HIGHER the ISO, the more ‘grainy’ the picture looks, grain is best avoided. You’ve all seen them, images of your friends taken in low light conditions, but all ‘grainy’ or ‘noisy’ as it’s also called. Ideally you want to have the ISO set as low (low being the ISO100 end) as possible to get the shot you need – but sometimes it is just not possible to get the shot you want at such a low ISO. Most compact cameras will set the ISO automatically, but some do give you basic control – all digital SLRs have full control over ISO.

Here is a quick overview of each ISO level

ISO 100 and lower – Excellent quality for brightly lit places, such as in bright sunlight outside. Allows for lots of detail and great image quality.

ISO 200 – Should be generally be used on dull days such as overcast and cloudy, only a tiny bit of noise

ISO 400 Great for indoor photography for flash and no flash – best use for high speed photography – but noise can be seen.

ISO 800+ Only really used effectively on high quality digital SLR cameras, most shots taken like this with a small compact camera will look extremely noisy.

I will talk about how to control noise in another article, and also about the relationship between ISO, aperture and shutter speed.


So remember, the higher the ISO number the quicker the sensor reacts to light – thus giving you effectively MORE light to use – BUT the trade off is increased noise / grain.

If this has been a help to you please leave me a comment or hit the ‘like’ button, it’s always appreciated – Thank you.

What is camera ISO