Beginners Photography: ISO Explained

Understanding how ISO works, in addition to aperture and shutter speed, is fundamental to getting better exposures and becoming a more skilled photographer. The good news is that ISO is quite easy to grasp once you have learned a few important concepts. So let’s get started.

ISO is a measure of the sensitivity of your camera’s sensor to light.

The higher the ISO value, the more sensitive the sensor. For example, a camera set to an ISO value of 200 is twice as sensitive as an ISO value of 100. This is a very important point, so I’ll repeat it: if you double the ISO value, you double the sensitivity. Typical ISO values are 100, 200, 400, 800, 1600, 3200 etc.

ISO 100 example

Example image taken at ISO 100 (and ISO 12800 below)

Most cameras have a “base” ISO setting of 100 or 200 and an upper limit that is dependent on the make and model. Some cameras allow you to unlock higher ISO ranges, but I recommend you leave this setting alone until you feel comfortable in your understanding of the trade-offs involved.

So let’s slow down for a minute. Why would you want to change the sensitivity of your sensor? The answer is almost always to raise your shutter speed. To explain this better, imagine the following scenario:

You’re indoors at a party taking photographs of your friends, and you notice that your shots are turning out slightly blurry. Your settings that produce a correct exposure according to your camera are as follows:

  • Equivalent Focal Length: 80mm
  • Aperture: f/4
  • Shutter Speed: 1/30
  • ISO: 100

Now, there can be multiple causes for blurry photos, but in this example let’s assume that the reason is because your shutter speed of 1/30 is too slow. It is not fast enough to stop the motion of your subjects and the small movements of your hands affecting the quality of the image. So what can you do? By adjusting your ISO setting you can double the sensitivity of the sensor to halve the amount of time it takes to make a correct exposure. Your new settings are:

  • Equivalent Focal Length: 80mm
  • Aperture: f/4
  • Shutter Speed: 1/60
  • ISO: 200

You take a few more pictures, and you’re getting less blurry pictures but you still think you could do better. So let’s increase the ISO from 200 to 400, halving the amount of time it takes to make a correct exposure again:

  • Equivalent Focal Length: 80mm
  • Aperture: f/4
  • Shutter Speed: 1/120
  • ISO: 400

This is much better, and you’re now getting sharp shots most of the time because your shutter speed is sufficiently fast enough not to be affected by unwanted movement in the environment.

Now you might be thinking why not just shoot at ISO 400/800/1600 all the time? Well that’s a good question, and it’s not necessarily a bad idea as long as you understand the trade-off – which is that the higher the ISO value, the more susceptible your image is to digital noise. I’m not going to go into the specifics of exactly why this is the case, but I can show you an example of what it looks like:

ISO 100 close up

ISO 100 sample. Click for larger view.

Above is a 100% crop of an example of an image taken at ISO 100. The image is clean and you can make out the fine details of the subject.

ISO 12800 sample

ISO 12800 sample. Click for larger view.

The second image is an example the same picture taken at ISO 12800. Notice the grainy/fuzzy quality and the loss of fine detail. This is the trade-off of boosting the sensitivity of your camera’s sensor.

How much noise you can tolerate in your images is a subjective issue you must decide for yourself. I will, however, say that I would much rather have a sharp noisy picture than a blurry image shot at a lower ISO. Experiment yourself to find out your own tolerances and how well your camera performs at the higher ISO ranges, because performance varies. Generally speaking, newer cameras and larger sensors will yield cleaner images at higher ISOs than older models and smaller sensors.

I also encourage you to make some prints of photographs taken at high ISOs. You will find that, unless you’re printing at very large sizes, the appearance of noise is much less pronounced on paper than it is when inspected on large computer monitors. There are also various ways to reduce the appearance of noise during the post-processing stage, but that’s an article for another day.

I hope you found this post useful and have a better understanding of ISO for having read it. If you have any questions or feedback then please leave a comment below.



Filed under Beginners Resources

2 responses to “Beginners Photography: ISO Explained

  1. The Jagged Man

    This is absolutely the best post/article/lesson I have ever read on ISO and how it relates to picture quality! I shoot with a Canon S3is and have shot within the aperture or shutter priority modes. I never have had satisfactory results. With this info I believe my approach has been off due to my lack of practical knowledge which you have now provided. I do shoot more than not in the programable setting and adjust exposure and ISO setting with satisfactory (to me anyway) results.
    This Page by the way is getting bookmarked and read often! Thanks for sharing.

  2. Wow, thanks for the kind feedback. I’m glad you found it helpful! There are many great articles out there that go into lots of detail about ISO, but I think for beginners it can be information overload sometimes and practical advice is more appropriate to get people out there and shooting!

    As a rule of thumb, without taking into account image stabilization/focal length, I like to shoot people shots (portraits, groups) at absolute minimum 1/60 and ideally 1/120 or faster. At that sort of speed I’m confident that any natural movement of people isn’t going to show up as blur in the image.

    Sometimes, though, that’s just not possible indoors with available light. That’s when it’s best to just pop the flash, which pretty much freezes everything (even if those tiny flashes look pretty bad).

    I’ll be posting more articles over the coming weeks and months – aperture, shutter speed, exposure, when/when not to use flash, image processing, RAW vs JPEG etc. so keep checking back! I’ll be aggregating them on the “Learning” page so hopefully people like yourself have a nice little bookmark to come back to and share with others.

    Thanks again for the nice comment, it makes it “worthwhile” to know the time I invest in writing and taking the pictures helps someone.


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