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Why You Should Know What ISO Does

ISO is one of three factors which determine the exposure of a photo, along with aperture and shutter speed. To really get the most out of your photos, you need to know what all 3 do and how you can use them. Read this post to gain a more in depth knowledge of how to use your camera properly and start taking expert photos.

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Section 1 – What exactly is ISO?

The ISO (International Standards Organization) determines the sensitivity of the sensor in your camera which, in turn, affects the exposure of your photos. The ISO scale typically starts at 100 and continues to double from this point to the boundary of your camera’s capabilities: 100, 200, 400, 800, 1600, 3200, 6400, etc. with 1/3 stops in between.

The International Standards Organization are those responsible for setting this widely used standard and all you need to know is included in the information below:

Section 2 – How does ISO affect Exposure?

ISO is one of three determining factors of the exposure of a photo, along with aperture and shutter speed. These two affect the lens and exposure time respectively, with the ISO affecting the sensor (or film). To be more specific, the ISO determines how well exposed a photo will be by changing the sensitivity.

The ISO scale is similar to shutter speed in the sense that, when doubled, the exposure is also doubled; they are proportional to one another e.g. a low ISO number would give a low exposure and a high ISO would give a high exposure – much simpler then aperture.

This is much easier to demonstrate using actual photos as you can see in the slideshow below…

The photos are displayed in the following order: 100, 200, 400, 800, 1600, 3200. The aperture and shutter speed remain constant throughout these photos, with only the ISO changing, so that you can clearly see it’s effect on a photo.

The ideal exposure is shown in the fourth photo which was taken at ISO 800.

100
200
400
800
1600
3200

As you may have noticed by now, nothing affects the exposure in one single way; there are consequences to using different ISO’s.

Section 3 – How does ISO affect the quality of photos?

As a general rule, the lower the number, the better the quality of the photo. By doubling the ISO, you’re effectively doubling the exposure taken by the camera and, in turn, doubling the digital noise. This noise reduces the detail of a photo by making the image appear grainy and uneven.

Lower number = Lower sensitivity = Finer quality photos

To best demonstrate how ISO affects the quality of the image, I’ve taken another series photos and displayed them below. For the purpose of this experiment, I have changed the shutter speed and aperture of each photo rather than simply changing the ISO. This is to give even exposure making it easier to compare the difference in quality.

The photos are displayed in the following order: 100, 200, 400, 800, 1600, 3200.

iso-vs-noise-1-600
iso-vs-noise-2-600
iso-vs-noise-3-600
iso-vs-noise-4-600
iso-vs-noise-5-600
iso-vs-noise-6-600

As you can see, the higher the number, the stronger the unsightly noise becomes. There is noise reduction software that can help to correct this but you’ll find that this only really “smooths out” the noise, which can result in an airbrushed effect on faces, reducing the detail in a photo as shown below (cropped to 1% of actual image).

Clearly it has it’s uses but should be used in moderation and with consideration as to what the photo is being used for.

Cameras with larger sensors handle noise better as they allow more light into the photo but, as technology improves, the difference between sensor size and noise is reducing – what used to be a big problem is much less of a one now in high quality cameras.

All cameras are different so I suggest taking yours into low light conditions in order to discover the maximum ISO that you’d be comfortable using.

Enhancing the exposure in post production has the same effect as increasing your ISO, so make sure you get the exposure right in the camera first time round in order to avoid this.

Now that you know what the ISO does, let’s take a look at situations where it might be used.

Section 4 – Which ISO and when?

ISO 100-200: Your photos will have the most detail and the best quality; great for shooting in daylight as there is no need to boost the ISO any higher. Shooting at 1600 in bright conditions would be a waste as this will result in the presence of easily avoidable grain.

ISO 200-400: For slightly darker conditions, such as in the shade or indoors where it is brightly lit.

ISO 400-800: I like to use this range when shooting with a flash indoors as it helps to produce a more even exposure with a detailed background.

ISO 800-1600: Event photographers frequently have no choice but to use this range as live events often happen in low light conditions where flash is not allowed.

ISO 1600-3200: Again, event photographers will use this range for live gigs, but it’s also used in extreme low light conditions where using a tripod is not an option. ISO 3200 is the highest I tend to push my camera to as I’m not a fan of digital noise (grain).

ISO 3200+: This range is reserved for extra low light conditions and artistic effect as, with most cameras, it’s impossible to avoid a grainy result in this range. 

Your Free Quick-Start Photography Cheatsheet

In order to simplify the process of learning photography, I’ve created a free download called The Quick Start Photography Cheatsheet and you can download it below.

Here’s what you’ll get:

  • A downloadable cheatsheet to carry with you as you shoot
  • Detailed summaries of each section of this post
  • External links to relevant articles and blog posts
  • At-A-Glace Images that will explain how each exposure works
  • And much, much more…

Your Free Quick-Start

Photography Cheatsheet

This downloadable cheatsheet gives you detailed summaries of every section of this post, as well as links to relevant articles, and at-a-glace images that will explain how exposure works.

Understand ISO in 4 Simple Steps

A note from Josh, ExpertPhotography's Photographer-In-Chief:

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Josh

Hey I'm Josh, I'm Photographer in Chief here at ExpertPhotography, and I'm in charge of making sure that we provide you with the best content from the most knowledgeable photographers in the world. Enjoy the site :)

  • I had no idea about ISO. Thanks for sharing!!

  • Great overview of ISO – you did a great job of explaining it using the photographs.

  • Ben

    This is definitely good to help people learn. I found that with newer cameras, i.e. D7000 and 7D, it is possible to get usable photos using ISO 6400, just takes a but of “Lightroom” touching up, if ya know what I mean 😀

  • CMR

    Thank you for explaining this so clearly.

    It’s not the first time I read stuff concerning ISO but shortly I forgot and when I get my camera I’m still confused about what to use.

    Now I have a much better idea !

  • Brian

    Thanks so much… easy to understand and very educational!

  • Charlene

    Thanks so much….this helped alot!!Easy to understand with your pictures.

  • I often tell people to learn ISO first…before Aperture and Shutter speed…It is so often overlooked and setting between 200 and 800 can make or break a picture

  • Heloisio

    It was very good your exposition.

  • pixelkobold

    Great. Thank you for that. Never thought so intense about the topic ISO.

  • I found this article to be well written. The only additional information I would give would be considered for the “expert only”:

    Depending on lighting situations you will be able to get better Dynamic Range (More information in the very dark AND very light areas of the photo) using specific ISO’s. LowerISO’s are not as sensitive to allow for this in certain lighting conditions, and the inversely for the higher ISO’s.

    The other small tidbit of information I could offer is to not assume that Film ISO and digital ISO will react the same. Even between manufacturers of digital cameras the ISO’s are not a match up. If you are upgrading be prepared for a learning curve.

    The “rules of thumb” above are probably the easiest to keep in mind until you better know how your specific camera responds in any given situation.

  • Elisa Donahue

    Thank you for making it easier to understand the ISO concept. I’m still experimenting with all three, ISO, F-stop and the shutter speed.

  • Finir punktar.

  • A really good article. I used a Canon 5D and could only push it to ISO 800 for decent quality. 100 is my favorite for clarity. I just got the Canon 5D Mark ll and will experiment with the ISOs.

  • David

    One question – you mention that for indoor shots you like ISO 400 – 800 as it gives a more even exposure with detailed backround. When else is it a good idea to bump up the ISO even if you don’t need it.

    Night shot – you’ve got a tripod so shutter speed is not a problem (taking a phto of a still landscape). Would you set it on anything but ISO 100?

    Cloudy day – ISO 100 is possible but would you increase it?

    I’ve got into the habit of having ISO set on the minimum possible with still acceptable shutter speed and aperture for the situation – I realise now this is probably wrong. Auto ISO is an option on my camera. Would you recommend it?

    Love the website. Would love to buy you a pint one day!!!!

    • I’d never use auto ISO myself, it’s a bit too unpredictable for me. Always use as low as you can, but there are reasons where you might want to increase it. At night, I might want a higher ISO, even if i’m using a tripod as it would allow me to capture the image as a faster shutter speed, which I may want if i’m looking to avoid movement. – Josh

      • I tried the opposite of that. Having the ISO at 100, I had the camera on a tripod with the shutter speed at 10 seconds. I also used a remote to start the shot and my picture ended up awesome with absolute no grain. You only need the higher sensitivity if you can’t use a slow shutter speed. Since you were using a tripod, you didn’t have to worry.

        • JoshDunlop

          My argument was that there may still be movement that I wanted to freeze/capture. Babbling brooks, moving cars, grains blowing in the wind, etc.

  • Lori

    Thank you so much Josh. I love your web site. Thanks for sharing all the info. It has been a great help.

  • Regina

    Thanks for the great info. I have always used ISO 100 when I use my flash. I am going to experiment with ISO with my flash. Thanks!

  • rick

    grea tutorial on iso. ow i nderan t better

  • Donald Robitaille

    Great website, very informative. My question involves the ISO settings on the flower pictures. It appears to be daylight, why are the flowers so dark at ISO 100, 200 & 400. This has me a little perplexed. Am I missing something ??

    • “The aperture and shutter speed remains constant throughout these photos with only the ISO changing so that you can clearly see its effect on a photo. The ideal exposure is shown in the fourth photo, which was taken at ISO 800.”

  • Dave

    ISO has nothing to do with exposure in digital cameras. It adds analog gain to the image after exposure similar to the way raising brightness in Photoshop works. Brightness and exposure are not the same thing.

  • iNtervengo

    Josh, ISO doesn’t mean “International Standards Organisation” (with an ‘s’ in English btw) at all. If you go on their website they’ll even explain that in details that the etymology derives from the Greek word ‘ISOS’ which means ‘equal to’ and it’s plain wrong to pronounce it I.S.O. (as an acronym). It must be pronounced in one go as a word. (Actually the correct pronunciation should be ESO with an ‘E’ sound and not an ‘I’ sound). All the rest, good job.