This is confusing.
Let’s think of a photo of a car.
1. Stationary vehicle: If you set ISO at the base figure, meter manually to find the setting required, then you get the least noise.
2. Okay, that seems reasonable, but what if you need the same aperture for DOF reasons, but need to raise the shutter speed to cope with subject motion if that same car now starts to move? (Ignore panning shots.)
You can’t open the lens up without losing the required depth, and neither can you avoid raising shutter speed if you want to keep the car sharp. If you don’t raise the ISO setting you can’t change the shutter speed because then you underexpose, creating noise. If you do raise the shutter speed and raise the ISO setting you still cause noise. Either way, doing nothing to the ISO, or raising the ISO, you get noise.
How can you avoid this and not get noise?
Was it confusing during the film days? What I would have done is put the ASA 100 back in my bag and pulled out ASA 400 and I'd get the results with more grain.
The point is this: ISO has no direct effect on noise. It has a direct effect on an exposure recommendation that does affect noise. One doesn't have to take that recommendation from an automatic exposure system that uses ISO. But yes, there are absolutely times when you need more (resulting) brightness from the ISO attribute, and you'll get more noise because the capture will be underexposed. If you had enough light to get the job done as you wish, you would expose optimally (at any ISO). I outlined several circumstances when you
must raise the ISO (and get the resulting underexposure + noise). Just as one did with film resulting in more grain.
But again, high ISO (or low) isn't the attribute that affects noise; it's all about exposure.
Anyone you can test this thenmselives quite easily.
Set up your camera on a tripod, outside on a sunny day. We will not even use your camera meter, just use the good old “
Sunny 16 rule.”
Set ISO to 125 and camera to full manual. Set exposure (the only two exposure attributes) to 125th of a second at F16.
1. Take a capture using the above exposure
and ISO.
2. Set ISO to 800; change nothing else. The exposure is still 125th at F16.
3. Set the ISO back to 100. Change the shutter to a 500th of a second, nothing else! This is what the meter
would have recommended from capture #2. This 3rd image will, of course, be 2 stops underexposed. And it will need to be brightened in the raw converter.
#1 and #2 have
identical exposure; the only change is ISO. Both have identical noise!
#1 and #3 have identical ISO, but #3 will be far more noisy because it is underexposed. As #2 would be
if the photographer followed the automatic exposure recommendation.
And now we see that an increase in noise has little if anything to do with ISO and everything to do with exposure.
Yeah, sometimes the tradeoff of the capture you need is less exposure and more noise.
Well summed up by the creator of RawDigger here:
https://www.fastrawviewer.com/blog/mystic-exposure-triangle
