In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain. Higher ISO settings are generally used in darker situations to get faster shutter speeds.
Something that I was learnt early on with my DSLR was you had to keep the ISO as low as possible, the higher the ISO the grainier the image would be. This just isn’t true, especially with the current range of DSLR and Mirrorless cameras. If you are shooting portraits and want to keep a constant F/stop ie F/2.8 and you can’t drop below a certain shutter speed(I used to be 1/100 second), I was also taught to put my camera on a tripod. Yes, you can do this, but you are losing versatility and the freestyle for composition. The ISO was more important in rating with film cameras than it is in camera sensor and relates differently. Many of these cameras, grain doesn’t even become an issue until about IS3200 or higher. I have read (not sure HOW accurate it is) ISO works better in increments of 160, so ISO160, ISO320, ISO640. The grain is more evenly dispersed. I have happily shot at ISO320 and ISO640 and have no issue with grain (On my Sony a7ii), I think grain was actually more of an issue with the Nikon. Nikons are supposed to be very good in dark lighting, but the D7100 is grainy with bad light leaks.
My shoots have changed especially in the last six months, mostly the last three months. I used to shoot on a tripod ISO100 and never shot above ISO800. I have been doing some experimenting and shooting at a much higher ISO even up to 2000. The grain is minimal and I have even got to a point, where I am adding grain to photos, to smooth and even out the image, particularly with portraits.
There is something artistic and almost old school and ‘real’ about grain in portraits.
This shot is gorgeous and handheld, look at the detail, but there is grain (very subtle) added to the image, the whites are white and bright but not glaring and the skin tone is nice and soft and even.
This is also shot at a fairly high ISO, with added grain, it is quite dark, but the face is well lit. I love the softness, but the pop of colour.
So there are probably 1000s of tutorials online, so you can check it out, but I do recommend you try it shoot from ISO100 – to ISO2000 and you will see that the grain really is not as bad as you think. Then try going up to ISO3200, obviously, if you are in a really bright light, this isn’t always possible.
So next time you are shooting and it is really bad light, don’t be scared of a higher ISO, a little grain is not a bad thing. And a higher ISO will not ruin a photo, and can often add some artistic flair. You can always soften grain, but nothing can fix a photo that is blurry or out of focus because you are scared of high ISO.