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, 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… The starting and ending points of this range and how well the camera handles the ISO depends solely on the camera that you’re using. Most modern cameras these days have many more points at which you can set the ISO in between those that i’ve mentioned.
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 factors which determine 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 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.
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 = Lover sensitivity = Finer quality photos
To best demonstrate how the 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 to produce an even exposure so that it is easier to compare the difference in quality. The photos are displayed 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 it only really smooths out the noise, which can result in an airbrushed effect on faces, reducing the detail in a photo, like the one below (cropped to 1% of actual image). Clearly, it has it’s uses but should be used in moderation and with consideration to what the photo is being used for.
Cameras with larger sensors handle noise better as they allow more light into a 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 the 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 easily avoidable grainy results would be achieved.
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 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. 
