Guide to the ISO Setting on your Camera

How Does the ISO Setting Affect Photographic Exposure?

We’ve already looked at aperture and shutter speed in the previous tutorials. In this one, we are going to look at the third element of the exposure triangle: ISO. We will learn how the ISO setting works and how it affects your photograph. ISO is a tricky one to explain without getting into some very technical jargon. In this chapter, I will do my best to avoid this jargon.

Let’s get technical…… not.

I’ve written tutorials online about ISO before and received long, detailed and even angry messages from certain photographers who took serious issue at my explanation of ISO! Some people really need to get out more. One such person told me in no uncertain terms that I was an idiot and was completely wrong about ISO and that this was in fact the correct definition:

“ISO works by having amplifiers in the image sensor’s circuitry increase the gain before sending the analogue voltage read from the photon well to the A/D converter to be digitised. E.g.: If at ISO 100 the signal is 100 mV, we can get ISO 200 by using an amplifier to boost it to 200 mV. For ISO 1600, we can five-double it to 1600 mV, and so on.”

So….. eh, what he said. I mean he’s not wrong but talk about taking the fun out of photography! I’m going to try to explain ISO in much simpler terms. Also, knowing the above does not necessarily make you a better photographer. It’s possible to get too caught up in the technical aspect of things to the detriment of the creative side of photography.

So…. what is ISO?

Increasing the ISO creates the impression that your camera’s sensor is more sensitive to light. It is in fact a little more complex than this. Most cameras have an ISO range that starts at a base of 50 or 100. This can then be increased into the thousands on most cameras.

What is ISO in photography?
I won’t sing…… I promise.

Increasing the ISO value on your camera is bit like using an amplifier and microphone to make your voice sound louder. The loudness at which you actually speak doesn’t change but the amplifier takes the sound of your voice and makes it sound louder or “amplifies” it.

ISO works in much the same way. When you increase the ISO value, the amount of light entering your camera remains the same. However, your camera now amplifies this light in the same way the amplifier amplifies sound. This means that less light is now required in order to make the correct exposure for the scene you are photographing.

Higher ISO = Faster Shutter Speeds

This allows us to use much faster shutter speeds than if we were just using the base/lowest ISO of 50 or 100. The higher the ISO, the faster the possible shutter speed is. This can be useful when we are shooting in low light and don’t have a tripod.  This could be at a location where tripods are not allowed for example. In these cases we have no choice but to shoot handheld.

We saw in the previous tutorial on shutter speed that low light can mean that the shutter speed becomes too slow to shoot handheld without risking camera shake. Increasing the ISO is a way around this. As we also learnt in the last tutorial, using a wider aperture is another way of allowing us to use a faster shutter speed. If it’s too wide however we risk running into issues with not having enough depth of field to keep everything sharp enough.

When can increasing the ISO be useful?

  • If you are shooting in a location that doesn’t allow tripods and the light is too low to shoot handheld.
  • If you simply don’t have a tripod with you in a low light situation.
  • If you need to increase your shutter speed significantly in order to freeze motion in a low light situation.
  • When using a wider aperture to increase shutter speed is not an option due to depth of field issues.

So why not just increase your ISO all the time to ensure fast shutter speeds?

In theory this sounds great. I can finally ditch the heavy tripod! The thing is that while increasing the ISO amplifies the light entering the camera and allows for faster shutter speeds, there is a trade off in terms of image quality. Let’s go back to the sound amplifier analogy from earlier. The amplifier will certainly make my voice sound louder but if I increase the volume settings too much, the sound quality will begin to degrade. The sound will be less clear. This is not so much of an issue for me as my singing voice sounds like a goose farting in the fog anyway.

What is Digital Noise?

The same principal applies to ISO (not flatulent geese). As you increase the ISO value, the image will become “noisier” or grainier. Random specks of colour will begin to appear and degrade the image quality and detail.

Take a look at the following photos of a wine bottle photographed at different ISO settings:

A low 100 ISO setting results in smooth images with no very little digital noise.

The first one above was taken with the lowest possible ISO setting of 100. The shutter speed in this case was 1/40 of a second. This would actually be too slow to handhold without risking camera shake. I used a tripod so this was not an issue.

A high 3200 ISO settings results in a very noisy image.

In this second photo, I’ve increased the ISO setting to 3,200. Now my shutter speed is a much faster 1/1250 of a second. I would have no problems hand-holding the camera at this speed. However, if you look very closely at the photo, you will see that the image quality has degraded somewhat. There is a lot of digital noise visible especially in the background. There is also a noticeable loss of clear detail.

Increasing the ISO is a balancing act. Sometimes, I’d rather have a reasonably sharp but slightly noisy image than a shot that is completely unusable due to camera shake. Digital noise can also be reduced to some extent in post-processing. Lack of sharpness due to camera shake however cannot be fixed afterwards.

After taking the above photos I drank the wine that was left in the bottle. This had the effect of degrading the image quality from my own eyes quite significantly.

ISO and Digital Noise Infographic

The chart above shows the effect increasing the ISO setting has on the digital noise levels in a photograph.

Does this mean I should avoid using very high ISO settings?

Absolutely not; a lot of modern cameras actually handle high ISOs very well and produce high quality images with minimal digital noise. This is especially the case with full frame cameras.

Take a look at the following photographs taken with a high ISO setting:

Example of a photograph taken with a high 1600 ISO - Belfast City Hall
Staircase at Belfast City Hall

Belfast City Hall has a stunningly beautiful interior. Tripods are forbidden however. By increasing the ISO to 1,600, I was able to achieve a fast enough shutter speed of 1/100 second to be able to take this photo handheld. The full frame camera handled the high ISO very well with very little digital noise visible.

Example of a photograph taken with a high 3200 ISO - Merchant's Arch, Temple Bar, Dublin
Merchant’s Arch – Temple Bar, Dublin

Above is a photo I took while walking through the Temple Bar area of Dublin. I saw a musician playing under Merchant’s Arch and though he would make for a good subject. The area he was standing in was quite dark. It was one of those times when setting up a tripod may have meant missing the moment completely, so I just increased the ISO to 3,200 and took the photograph handheld.  This enabled me to set a shutter speed of 1/400 second and avoid camera shake. As with the previous photograph, the full frame camera meant that the image quality suffered very little.

Now that we have covered the exposure triangle of aperture, shutter speed and ISO, we’ll put it all into practice in the next tutorial. We will be looking at how to use the aperture priority, shutter priority and full manual settings on your camera.

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I am a travel photographer from Dublin, Ireland. I specialise in urban, street, architecture and landscape photography. One of my favourite activities is wandering around a new and unfamiliar location with my camera at the ready. In the blog section, I share photography tutorials and tips as well as some of the stories behind my photographs and travels.

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