ISO: The Essential Guide To ISO in Photography

Man changing ISO settings on a camera
ISO is one of the three critical aspects of photography that allow you to control your exposure. In this guide we will teach you What ISO is and guide you to getting the perfect ISO setting for each photo.
Updated: 11th June, 2020

This article contains affiliate links and Square Pixel Photography may earn a small commission if you make a purchase. This comes at no additional cost to you. Read our full disclosure here

    What is ISO?

    In very simple terms: ISO is a camera setting that allows you to control the brightness of your photos.

    It is very closely tied to the camera's shutter speed and aperture settings. The combination of all three is what gives your photo it's final brightness value.

    Confused? Stay with me!

    When you press the shutter button on your camera, your chosen aperture and shutter speed allows a certain amount of light to enter your camera and hit the sensor. The sensor then turns that light into a digital signal (your photo).

    If the amount of light hitting the sensor is too low, the signal will be weak, and the resulting photo will be dark. This is where ISO comes in.

    Choosing a higher ISO will 'turn up' the signal, much like turning up the volume on your radio (do people still use radios?). The amplified signal will then create a brighter photo.

    You can see the result of 'turning up' the ISO in the series of photos below. The shutter speed and aperture are the same in all of these shots. Only the ISO setting has been changed.

    How does ISO affect exposure?

    Technically, ISO doesn't affect the exposure at all, only the aperture and shutter speed settings do that, as they control the physical amount of light that enters the camera.

    From a practical viewpoint though, doubling the ISO value will double the brightness of the resulting photo. This has the same effect as increasing the exposure by 1 stop.

    Before we move on to figuring out which ISO we should use at any given time, first we need to discuss a very important topic.

    High ISO Image Quality

    Unfortunately, choosing higher ISO Values comes at a cost to image quality. There are three main tradeoffs that come with increasing the ISO of a photo.

    Digital Noise (grain)

    All photos have noise.

    The process of turning light into a digital image is not a perfect conversion. Any imperfections in the process of converting light into a digital image manifest as noise, or grain in an image. When the light is strong, and the ISO is low, the noise can't be seen unless you're looking very closely.

    Going back to the 'turning up the radio' analogy I used earlier:

    If you turn up the radio loud enough, you will hear distortion and the sound quality will go down. A similar thing happens when you 'turn up' the ISO. Not only does the photo signal get brighter, so does the noise, making it more visible.

    Dark areas and areas with smooth tones (sky, skin, etc) will show the noise most noticeably, but as the ISO increases it can also degrade the lighter areas and fine details in an image as well.

    The example below shows the effects of high ISO noise on the sky and trees in a photo.

    Noise can be somewhat reduced in post processing. We use and recommend Topaz DeNoise as our go-to solution. You can download a free trial at the Topaz Labs Website.

    Read our Noise Reduction articles here

    Topaz Denoise AI
    15% Off with code:

    Lower Dynamic Range

    Dynamic range is a little out of scope for this beginner article, but it is important to mention.

    As ISO values go up, the dynamic range available to you goes down. This means you need to be more careful about getting your exposure correct as you will have less recovery options in post-processing.

    Higher dynamic range affects photos that have very bright and very dark tones in the same image.

    Lower colour fidelity and saturation

    At high ISO settings, colours may become less saturated and may not be captured as faithfully as at low ISO. Modern cameras generally control this very well and it will only affect the highest of ISOs.

    This is probably not something you will notice in many photos, but is something you should be aware of if you're shooting at very high ISO settings.

    What ISO should you choose?

    Now that you know about the issues affecting higher ISO photography, we can talk about which ISO you should use.

    First the bad news:

    I can't give you a number.

    And now the good news:

    The answer is: Set the ISO as low as possible that will give you the shutter speed and aperture needed for the shot.

    I can give you some general guides to help you make the correct choice. The information below is very general, and you should experiment to find what works for you and the scene in front of you.

    Click on a title below to expand the explanation

    ISO For Action Photography

    Action photography means anything where the subject is moving. Sports, birds, wildlife, street, pets, etc.

    A moving subjevt will need a shutter speed fast enough to freeze that movement. How fast you need to set your shutter speed will depend on how fast your subject is moving. For fairly slow subjects it could be as low as 1/100th. For birds in flight it might need to be as high as 1/4000th.

    • Set your camera to Shutter Priority
    • Dial in the shutter speed you need to stop the motion
    • Set the ISO to the camera's base ISO as a starting point
    • Take the phoot

    If your photo is:

    • too dark: Increase your ISO
    • too bright: Increase your shutter speed
    • blurry: Increase your shutter speed
    • well exposed, too small depth of field: Increase your ISO (allowing your camera to choose a smaller aperture)

    ISO For Landscape Photography

    Most landscape photography should be done on a tripod and at the lowest ISO value your camera has. You generally don't need to worry about movement and will want an aperture that makes the entire scene sharp from back to front.

    If you don't have a tripod or there is movement in your scene:

    • Set the camera to Aperture Priority mode
    • Dial in the aperture you need for front-to-back sharpness
    • Set the ISO to the camera's base ISO as a starting point
    • Take the photo

    If your photo is:

    • too dark: Increase your ISO
    • too bright: choose a smaller aperture (larger f-number)
    • blurry: Increase your ISO

    ISO For Portrait Photography

    In a studio you will probably be in full control of your lighting conditions. In that case you will want your ISO at the lowest setting and control the brightness by adjusting the strength of the lighting.

    For non-studio portraits you will probably want to set your lens to a relatively wide aperture to get a smooth background. 

    • Set the camera to Aperture Priority mode
    • Dial in the aperture you need for a smooth background (low f-number)
    • Set the ISO to the camera's base ISO as a starting point
    • Take the photo

    If your photo is:

    • too dark: Increase your ISO
    • too bright: choose a smaller aperture (larger f-number)
    • blurry: Increase your ISO

    ISO For Flash Photography

    Flash photography is generally about balancing the foreground brightness (affected by the flash) and the background lighting (not so affected by the flash). You will probably want to set your aperture to a 'middle-ground' setting to make sure your subjects are sharp, but your lens still lets in enough light from the background. Try starting at f5.6

    • Set the camera to Aperture Priority mode
    • Dial in the aperture you need for the scene
    • Set the ISO to the camera's base ISO as a starting point
    • Take the photo

    If your photo has:

    • a well exposed foreground, background too dark: Increase the ISO
    • a well exposed background foreground too bright: Turn down your flash
    • motion blur: Increase the ISO


    ISO For Macro and Closeup Photography

    Macro and Closeup photography can be extremely challenging, with depth of field at a premium and light hard to come by. In practice you will need to experiment with all ISO, Shutter and Aperture settings to get the balance right.

    • Set the camera to Manual mode
    • Dial in the aperture that will give enough depth of field for the subject (a high f-number)
    • Dial in a shutter speed fast enough to stop any movement of the subject AND the camera
    • Set the ISO to the camera's base ISO as a starting point
    • Take the photo

    If your photo is:

    • too dark: Increase your ISO
    • blurry: Increase your shutter speed
    • not enough front-to-back sharpness: Increase your f-number


    How To Change ISO

    It depends on the camera you use.

    Because ISO is such an important function in photography it is usually done via a button press and a dial movement.

    Consult your camera's manual for exact instructions, or type 'How to change ISO on <insert your camera make and model>' into Google. Alternatively, you can ask us in the comment section at the end of this article if you're still having issues.


    Auto-ISO menu on a Nikon DSLR

    Most modern cameras include a feature called Auto-ISO.

    When turned on, Auto-ISO will adjust the ISO setting to automatically set the correct brightness of your photo. You generally set up the highest ISO that you are comfortable with and the camera will do the rest, not going above that value.

    With auto-ISO enabled:

    • In shutter priority you pick the shutter speed and the camera will choose the aperture and ISO automatically. The camera will usually only increase the ISO once the lens reaches it's maximum aperture.
    • In aperture priority you pick the aperture and the camera will choose the shutter speed and ISO automatically. ISO will be increased once the camera determines that the shutter speed is getting too low for you to get sharp photos with your current focal length.
    • In manual mode you pick the aperture and shutter speed while the camera chooses the ISO.

    Turn auto-ISO off for landscape photography on a tripod, or any other situation where you want to keep the camera at the lowest possible setting.

    For all other situations, auto-ISO usually does a pretty good job. Just make sure that you set the maximum ISO value to something that you are comfortable with the noise levels generated.

    Remember: A noisy, sharp photo is almost always better than a blurry, clean one!

    Frequently Asked Questions

    Click on a question below to reveal the answer.

    Does Increasing ISO make the sensor more sensitive to light?

    No. Most image sensors have a single base sensitivity to light. Increasing the ISO will amplify the signal from the sensor more, but will not make the sensor more sensitive to light. Some modern cameras have two 'base' ISO settings that it will switch between based on your ISO, but this is relatively uncommon.

    Can I just brighten the image on a PC?

    Generally speaking, the camera will be able to do a better job of increasing brightness than your photo editor. Images will have more noise if brightened on a PC vs increasing the ISO.

    Some cameras are very close to being 'ISO invariant' - which means the results are very close between increasing ISO and doing it during post production - but this is still fairly uncommon.

    It is currently always safer to do it on the camera than during post production for best results.

    Leave a Reply

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