Along with shutter speed and aperture, ISO is one of the three fundamental exposure parameters, and as such, mastering it is crucial to taking full control of your images. If you are new to photography, check out this fantastic video tutorial that will show you the ins and outs of how ISO works and how to use it.
Coming to you from David Bergman with Adorama TV, this excellent video tutorial will show you everything you need to know about ISO. By far, the most common mistake I see beginners make with ISO is leaving it too low in an attempt to preserve image quality, resulting in shutter speeds that are too slow to prevent blur due to subject motion or camera shake. The important thing to remember is that you can always work on the noise in a photo in post (in fact, modern noise reduction software is impressively advanced), but once a shot is blurry due to one of the aforementioned factors, you generally can't do much to save it. Check out the video above for the full rundown from Bergman.
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The exposure triangle is a myth.
Exposure is light filtered by time and aperture. ISO is electronic amplification.
I can see why newbies are taught this. But it is distressing to see people that should know better not understanding it.
Great points BUT! Even in manual there is something you need to look at while adjusting ISO. I am using Sony camera but should be the same across all makes. Whither in M or other modes at the bottom of the LCD screen there is the M.M. (metering mode) and as you adjust any and all setting it changes from - value or + value, this is the cameras light meter and you will want it to be +/- 0.0. With different makers of camera there is another part of ISO and it is dual ISO where at 640 and above is a point the camera gets cleaner (less) noise electrically then again around 12800. Again newer cameras now have ISO Invariance, basically if your ISO settings are low and increasing exposure in post noise level will be the same all the way up, it is like you can capture either at a low ISO or a normal ISO looking at the M.M. but noise in shadows will not increase when increasing exposure. Example out capturing white Egrets on a sunny day with zebras on and they cover most of the screen adjusting the +/- EV dial to a negative number the zebras disappear but the captured image is very dark even using auto ISO, in post increasing the exposure brings the image as seen. The difference is the darker image saving feather detail vs a brighter image no feather detail. This also comes into effect when doing astro Milky Way captures when bright lights are in the image again with zebras on you will see them and turning EV to neg value the highlights are reduced giving details of maybe a building and the ability to see maybe someone at a window lit on the inside but in post just increasing the exposure the highlights with detail are saved as well as bringing out the detail of the sky. You pay for the Auto ISO function and also in camera noise reduction (NR) along with ISO Invariance and the math involved may cause longer time to capture what you want. Knowing and using the tools built in will speed your capture. As far as astro work even out the darkest of areas there is light on the horizon and zebras will show up and again adjusting yes a darker image but knowing in post you can brighten the image some with details to boot is a modern miracle. Lastly when out at night the M.M. at zero will capture a very bright image but may lose some detail in highlights of lit building on the horizon (yes at 300% zoom) with detail of lights in hotel rooms with people/TV's on small details BUT you can capture it using all camera options.