What is ISO? How to set it correctly on your camera

Camera Sensor ISO Setting

What is ISO on a camera?

ISO is a scale of numbers that refer to your camera’s sensitivy to light. They usually start at ISO100 and increase from there to ISO25600 and even higher. Following the megapixel wars of the Naughties, the leading camera manufacturers turned their attention to ISO. We’ve since seen some incredible advances in sensor technology with some cameras now able to record images in near total darkness.

The general rule of thumb for ISO is the higher the number, the more sensitive your camera is to light. Doubling the ISO doubles the sensitivity and increases the exposure by 1 stop but not without a cost. As the ISO increases so does the noise in the image. Set your ISO too high and you’ll either have to treat the noise in post production or bin the image.

How does setting the ISO actually work?

Contrary to popular belief and urban mythology, changing the ISO setting on your DSLR does not change sensor sensitivity! That’s important so I’ve pre-written a tweet that I’d love you to share:

Contrary to urban myth, changing the ISO on your #DSLR doesn't change the sensitivity of the sensor #photography Click To Tweet

If you look at what happens inside your camera, there’s an amplifier (just like the one in a hi-fi albeit much, much smaller) connected to the sensor output. Increasing the ISO increases the gain of amplifier thereby increasing the output signal from the sensor. (It turns up the volume if we use our hi-fi analogy.)

Camera Sensor ISO Setting
Camera Sensor ISO Setting

I’ve shown a very simple sketch of how camera ISO works above. Light (photons) enter the camera through the lens and fall onto the sensor. For simplicity I’ve only shown a single pixel here. The analogue output from the sensor is then amplified prior to being converted to a digital signal by the ADC (Analogue to Digital Converter). When you increase the ISO in reality you’re increasing the gain of the amplifier, creating a larger (louder in hi-fi terms) signal for the ADC converter.

The problem is the sensor output doesn’t just contain picture information. It also contains noise. Thermal noise, shot noise, crosstalk and many others prevalent in semiconductor devices (there’s a great article here for those of you interested in further reading).

It’s this noise that causes a problem as you increase the ISO and manifests as ‘digital grain’ in your photos.

5DmkII ISO25600 Pre Noise Reduction - 100% Crop
5DmkII ISO25600 Pre Noise Reduction – 100% Crop

Why is noise more noticeable in low light conditions?

Very simply, the signal from your camera’s sensor varies according to the amount of light in the scene whereas the noise level remains relatively constant.

When photographing a bright scene the (wanted) picture information from the sensor is much higher than the (unwanted) noise information. In electronics parlance we call this a high ‘signal to noise’ ratio. The noise element is much less noticeable as it’s completely swamped by the wanted signal.

In low light situations, the picture information is much lower but the noise level remains the same. This produces a much lower signal to noise ratio making it harder to remove the noise. Because the wanted signal is much lower, it needs to be amplified. The ADC needs a larger signal to sufficiently digitise all the image detail. Unfortunately boosting the wanted signal also boosts the noise giving rise to graininess as shown in the sample image above.

How to set your camera’s ISO correctly

The general rule when taking photos is to choose the lowest ISO setting you can that’s appropriate for the scene. I’ve summarised some guidelines in the table below:

Situation Typical ISO setting
Bright, sunny day 100-200
Cloudy, overcast day 200-400
Inside, bright room 400-800
Inside, dark room 800-3200
On camera flash and speedlites 100-400
Studio flash 100

Remember: these are just guidelines! I strongly encourage you to experiment with your own camera to see which settings produce acceptable results. My preference with my own kit is not to go above 1600 if I can help it. Dialling in 3200 on my DSLRs can produce acceptable results if treated carefully in post production but I use this as a last resort.

There are also occasions when I choose to ignore the guidelines I’ve added above. They’re not that common but they do occur. Typically when I need a faster shutter speed for a handheld or monopod shot. Good camera holding technique is very effective at reducing camera shake but if your subject is moving you’re still going to get a blurry image.

Live, indoor events often fall into this category. Despite being brighter than a typical room they’re not that bright. In this situation it’s a compromise and I very often trade off a higher ISO setting to get a faster shutter speed and freeze the action. Fashion shows, theatre and ice hockey matches are all examples where I’ve had to do this.

Special note about Auto ISO…

Whatever you do, DON’T USE IT!!!

I can’t think of a single use case where anyone would ever need to set this mode on their camera. (If you know of one, please enlighten me by leaving a comment below).

The whole point about learning how to use your camera better to improve your images is you need to take control. That means moving away from the automatic settings. You don’t need to shoot everything in full manual mode. The semi-automatic modes like Aperture Priority and Shutter Priority are very good – you still have control. Why on earth anyone would want to let the camera control the sensitivity is beyond me. Why? Because it can and will choose values that are too high, resulting in images that are too noisy and therefore useless.

In the days of film photography, ISO was ‘fixed’ — it was a characteristic of the film. You set it when you loaded the film and it remained and the only way to change it was to change the film.

Nowadays with digital photography, you can set it on a frame by frame basis and you should, if the situation calls for it. BUT it should be your decision, just like in the days of film, not the camera’s.


Hope that’s made sense — it can get very technical very quickly and I’ve tried to keep it as jargon-free as possible.

If you’ve learned anything new, please leave a comment and let me know. It’s always good to get feedback from my readers. Alternatively, please consider sharing it on your favourite social media network using the buttons on this page

As always, have fun,



2 thoughts on “What is ISO? How to set it correctly on your camera

  1. Thanks for answering my question. I didnt understood iso before but it all makes sense now.
    Take care
    Kat x

  2. Thanks Kat, glad you found it useful.

Leave a Reply

Your email address will not be published. Required fields are marked *