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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.


Section 1 – What Exactly Is ISO?

The ISO (International Standards Organization) determines the sensitivity of the sensor in your camera. This, 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. A low ISO number will give a low exposure and a high ISO will give a high exposure. It’s 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. Only the ISO is 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.


As you may have noticed by now, nothing affects the exposure in one single way. There are consequences to using different ISOs.

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 of 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 have ISO in the following order: 100, 200, 400, 800, 1600, 3200.


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.

This can result in an airbrushed effect on faces. It reduces the detail in a photo as shown below (cropped to 1% of actual image).

Clearly it has its uses but in moderation and with consideration as to what you’re using the photo for.

Cameras with larger sensors handle noise better as they allow more light into the photo. As technology improves, however, 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. 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 you might use it.

Section 4 – Which ISO and When?

ISO 100-200: Your photos will have the most detail and the best quality. This is great for shooting in daylight because 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 since 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 you’re not allowed to use flash.

ISO 1600-3200: Again, event photographers will use this range for live gigs, but you can also use it 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 because 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.

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

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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 :)