Determining the ISO sweet spot in a camera

Seeing grain (noise) in a photo at some point becomes difficult, yet its there.  So is there a way to tell the ISO sweet spot in a camera from a scientific standpoint?  

ISO (in stops, not the numerical value) is directly proportional to image noise (as the ISO increases, so does the image noise), and the progression is nearly linear.

BTW, you can see it in the measurements published at DxOMark.com .

Some cameras let you set an ISO limit, and most, if not all, allow you to set manual or automatic ISO. In my studio I always use manual ISO and usually select 100 or 200 because I want full control of my camera and most everything that I'm photographing isn't moving or not moving quickly.

When out shooting somewhere, usually moving people, things, etc I'll use Auto ISO, though frequently I'll also be using the manual settings of the camera for these shots. I use the shutter speed when outdoors shooting to control the exposure of the shots.

I adjust the light levels on my flash units, when indoors, to control the exposure of the shots and usually leave the camera at the shutter speed of 1/200 of a second. Some cameras require a different sync speed, like 1/250 second, so read your manual to find this. Indoors in my studio Then I adjust the F-Stop settings high enough to block the constant room lighting of my studio from affecting my shots, and then set it higher or lower later when the depth-of-field needs correction for the shot. If a lower F-Stop setting is required for the shot, I turn off the room lights for these and use the lower F-Stop needed by the shot. I have wireless 3-way "room lights" switches on my camera stands, so that I don't have to move around while in the darkened room to turn the room lights off and back on. 

ISO is a sensitivity adjustment based on film sensitivity when using film in older cameras. The ISO setting was based on how sensitive the roll of film was and there was no ISO adjustment on the camera. You had to buy each roll of film based on how sensitive the film needed to be for the shots you were planning, Because you had to shoot the whole roll before you could change the ISO with a new roll of expensive film, we used 100 ISO sensitivity for as much as we could get away with. Yes, it required brighter flash sometimes. High ISO settings increase the camera sensitivity so it can take photos in lower light. When you increase the sensitivity, you are also making it more sensitive to optical and electronic noise. Film cameras could take great low light shots with higher ISO film too, but the light sensitive coating on the film became grainy with very high sensitivity. The Polaroid 3,000 speed rapid developing film produced photos without flash and just room lighting, that developed very quickly, but the photos taken this way were very grainy and low quality. Their 100 ISO film produced acceptable photos, if you had adequate light.

So learn about ISO and light sensitivity. Then respect it and use it when necessary, realizing that maximum ISO or nearly so is going to produce lower quality photos with noise in them. When the next generation of cameras come out, they will likely have even higher ISO adjustment capability. Stay away from the maximum ISO and you should get good shots with whatever digital or film camera that you use. Add more light and you won't need the maximum settings to get good shots.

Charley 

The sweet spot is the base ISO. That's why it's the base ISO.