Of the three pillars of exposure, ISO is probably the least understood. Many new photographers turn on auto-ISO settings and let the camera decide. While that isn’t ALWAYS a bad idea – it can certainly lead to lower than desired image quality. This article explains what ISO is and how you should use it in your photography.

What Is ISO?

In very basic terms, ISO is a camera setting that controls how sensitive your camera’s sensor is to light. A higher value means your camera is more sensitive to light; lower values mean it is less sensitive. 

The series of photos below simulates the effect changing the ISO value and leaving all other exposure settings the same has on a photograph. Every time you double the ISO value, the exposure becomes 1 stop brighter (twice as bright).

What ISO does to exposure

Some of the more experienced photographers out there will have noticed that in terms of the exposures ‘brightness’, doubling the ISO value does the same as doubling the shutter speed (i.e. going from ½ second to 1 second) or opening the lens aperture by 1 stop (i.e. from f5.6 to f4).

What ISO Should I Use?

To put it simply: You should use the lowest ISO you possibly can that still allows you to get the exposure and/or shutter speed you need. The lowest ISO available to you depends on your camera model, but it will generally be somewhere between ISO 50 and ISO 200.

Why Keep ISO Low?

As with most things in life, using a higher ISO has tradeoffs. There are several aspects of image quality that suffer as the ISO levels increase.

ISO Noise (Digital Grain)

Digital camera image sensors have come a very long way in recent years, but there is no getting around the limitations imposed by physics. Using a higher ISO setting increases the amount of noise (sometimes called ‘digital grain’) in your images. 

The image comparison below shows the difference between ISO 100 and ISO 3200 on a Nikon D750

What ISO does to colour

The amount of noise produced at each ISO varies between camera models and is dependant on the scene you are photographing (dark scenes show more noise).

How much digital grain is too much depends largely on both how the photo is to be used and your tolerance level for noise (see the second point under ‘Is High ISO Bad?’ below).

Colours & Dynamic Range

You can also see in the example above that the colours on the ISO 3200 sample photo are not as good as in the ISO 100 shot (most notably in the colour of the sky). As you increase the ISO levels, the range of colours that the camera is able to capture gets more limited and less saturated. Post-processing can fix this to some extent, but the fixes generally introduce even more noise.

At higher ISO levels the camera’s sensor is also able to capture less dynamic range. The darkest and lightest areas in the photo will have less detail in them and turn into pure black/white blocks sooner than at lower ISO values. 

Image Sharpness

Images at higher ISO values also won’t appear as sharp as those taken at lower ISOs. There are two reasons for this:

The additional noise present in the photo makes solid edges appear to be slightly fuzzy. This is due to the noise obscuring the solid edges.

You won’t be able to apply as much sharpening to the photo in post-processing as increasing sharpening also increases the (already high) levels of noise in the photo.

What ISO does to sharpness

When Should I Increase The ISO?

As you may have guessed from the information above, you should only increase your ISO when you need to. There are two simple questions you can ask yourself to work out if you need to:

Can I use a slower shutter speed without introducing unwanted motion blur?

Can I open my aperture and still get the depth of field that I need?

If the answer to both of those questions is ‘no’ – then you will need to raise your ISO setting to get the exposure you need. Only increase the ISO value until the answer to question 1 is ‘yes’.

Is High ISO Bad?

The examples given above of what increasing your ISO does to image quality would seem to imply that anything above the lower ISO setting on your camera is going to ruin your photos – however, this is not the case:

For a start – it is usually better to have a noisy photo with slightly less saturated colours than a blurry one! Using higher ISO values mean you can take photos that would have otherwise been unobtainable.

Secondly: How much your image is affected by using higher ISO values depends entirely on how you are going to use the photo. If you are printing your images below A3 size, you probably won’t notice any noise at all until you go above ISO 1600, and even then the amount may be acceptable to you. If you’re sharing them on the internet, then your photos will more than likely be downsized and you won’t see objectionable noise even with very high values. The same applies to colour saturation and dynamic range. If you’re heavily cropping or printing huge, then you might see the effects of noise a little sooner.

Finally: Having some visible noise in your photo (however you are presenting it!) isn’t necessarily a bad thing. There seems to be an obsession lately where all images MUST be noise free – however, a well composed, emotive photo that includes noise is always better than a noise-free photo that is poorly taken. As long as the noise doesn’t overwhelm the subject, most people probably won’t even notice it is there! Also, unless you are at extreme ISO levels the drop in colour saturation would also probably not raise any questions.

Using the auto-ISO setting

Most modern cameras have an auto-ISO setting. Turning this on lets the camera choose the ISO level based on your shutter speed, aperture and the focal length of your current lens (the longer the lens, the higher your shutter speed needs to be to avoid camera shake). Using auto-ISO certainly has its uses.

During high-speed photography when conditions and lighting are continually changing, and you may not have time to change the ISO levels manually.

Just ensure that you tweak the settings so that the maximum ISO values that the camera can only choose values that produce acceptable levels of image degradation (as outlined above).

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