When I started photography 2 years ago with a cropped sensor dslr, ISO was one of the terms that would make me feel nervous about my own shots. Going through various blogs, there was a general perception about higher ISO values being always bad. I made a real solid picture in my mind that anything above 100 ISO would make pictures look really ugly and noisy even when the format was RAW.
Switching slowly from landscapes to event and indoor photography after one year, the struggle to get properly exposed shots started to become fairly evident. Cultural events, music concerts and indoor sports taught me how important is the value of ISO. The need for faster shutter speed in a low light environment is one of the most common scenarios every photographer has to face at some point in his photography journey.
Increase shutter speed -> Less light
Wider aperture-> More light.
Environment lighting -> Already very less.
As seen above, an already poorly lit environment makes it harder to do dslr photography even when a lens with wide aperture is available. ISO or external flash can be the only life saver here. I recently shot my first indoor dance event with a prime lens and a telephoto zoom lens. Interestingly even my 50 mm 1.8 mm struggled to get properly exposed shots because I was still in my old mindset of not bumping up the dslr ISO. The event was in progress and I was losing shots, there is no worse feeling than this. Had to swallow my low ISO pride and changed the value to 800. Voila! The shots looked much much better from an exposure point of view. I came home and copied the photos to laptop for post processing followed by 2-3 hours of post processing. Here is what I observed:
A poorly exposed low ISO shot when post processed is inferior in quality as compared to a properly exposed high ISO shot.
Pro tip: Use of external wireless flash units can help with properly exposed shots without bumping up the ISO, but most of the events don't allow this.
Astro and star trail photography is another huge branch of photography that involves usage of higher values. What would you think is a flexible range of usable ISO value for a cropped sensor dslr?