OK, I've been a photographer for a long time (as in, I've spent more years with analog than digital cameras). After looking at some really long threads about ISO elsewhere, I'm starting to understand how ISO works in digital cameras. It's not ASA. I'm convinced: you lower noise by giving the sensor as much light as possible. For cameras that are not ISO invariant, for any given exposure (shutter/aperture), a higher ISO setting may produce less noise than a lower one. For an ISO invariant camera, the ISO setting won't matter. The light meter is always going to factor in the ISO value. So what's the best strategy for setting exposure on ISO variant and ISO invariant cameras? Let's pick a specific scenario: I'm going to photograph birds with a hand-held 400mm zoom. I need at least a shutter speed of 1/400, but even faster would be better. I pick a shutter speed, not less than 1/400. For the aperture, I'll live with whatever I can get. If the camera is "ISO variant", I'll set the mode to shutter priority and ISO to auto. The hope is that the camera will raise ISO only after the aperture is maxed out. If the aperture is also important, I'll go to manual and also set the largest aperture I can live with. ISO is still auto. Some bird shots may turn out to have a lot of noise, but this will always be the best possible result given the camera and the lighting conditions. If the camera is ISO invariant, I could use the same strategy or I could leave the ISO at 100, so that the camera maximizes the amount of light received by the sensor (given the limitations of the shutter and possibly aperture settings). The camera might flash warnings that the scene does not have enough light; that's OK, as long as it allows the photo to be taken. An alternate scenario: I'm going to photograph landscapes and I can put the camera on a tripod. Nothing is moving, so I can use a long exposure. The aperture might be the only thing I care to set. What's the best strategy now? I'm less sure here. For an ISO variant camera, I want the longest exposure that doesn't blow out the highlights, so I might set the ISO to 100 and adjust the shutter speed by going to manual mode. Or I could rely on the camera's "evaluative metering", but check the histogram. I could then raise the ISO, if needed, until the highlights are no longer clipped. If the dynamic range is extreme, I could take multiple photos to combine as an HDR. I'm not sure that an ISO invariant camera would be set any different, although I see articles which seem to suggest that these cameras have an advantage in this scenario. It's not clear; for every camera, you want to maximize the amount of light received by the sensor. The brightest areas of a scene may limit how much light you can allow in before those parts are clipped. This limit affects all parts of the scene, so it's not clear how ISO invariance helps. Any tips on how to set exposure in various scenarios or corrections to my guesses above would be welcome. Thanks!