Why bother changing base ISO?

Started Jul 18, 2007 | Discussions thread
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Jim Kaye Senior Member • Posts: 2,791
Why bother changing base ISO?

OK, maybe I'm getting too lazy. But I generally shoot raw, and say I'm using my 80-400 f4.5-5.6 VR handheld outdoors on moving subjects, I want to keep the aperture at f/8 or smaller and the shutter speed at 1/500 or faster (even with VR on), and the light is changing (as clouds roll by, for example). Why not just leave the ISO at 100 and bump the "exposure" in post by various amounts depending on the light in each shot?

If I'm shooting in bright sunlight (LV 15) I won't have to make any "exposure" adjustment at all, and if there is one or two stops less light (LV 13 or 14), I'll have to increase the "exposure" setting by one or two stops in post. At LV 12 and below, where I'd need ISO 800 (or 3 stops of increase in the "exposure" setting in post) or higher, I have to decide if I want as much noise as I know I'll get (and have to deal with that with extra post processing) or whether I'd rather take a chance on a wider aperture or a slower shutter speed.

The only downside I see is that the image in the LCD looks dark and I may not be able to judge whether I've framed the shot as I wanted to or whether I hit focus accurately (although I'm usually loathe to delete shots based on focus assessments made on the LCD). The important thing is that I don't see any difference in the noise level depending on whether I bump ISO in the camera or change the "exposure" setting in post. Julia Borg and others have talked about this in other posts -- changing ISO being equivalent to making "exposure" adjustments in post -- but I've started shooting this way more frequently, and I just can't see any difference, so I'm becoming more convinced it makes sense. I'm in the habit of turning off any noise reduction in the NEF conversion step because I prefer the level of adjustability and control interface I get in Noise Ninja over what comes in Capture, ACR, or Bibble, so whatever NR Capture would do on my higer ISO NEFs automatically doesn't come into play.

I know some people use auto ISO to keep their aperture and shutter speed where they want them when light conditions are changing moment to moment, but the minimum shutter speed for auto ISO only goes to 1/250 and that's not quite fast enough for the kind of shooting I'm talking about. Also, using auto ISO leaves the exposure decision more to the camera, and although adjustments can be made with exposure compensation on the fly or in post processing later, I find it's too easy to blow out small highlights if lighter areas make up a small part of the scene (for example, a deer with a white tip on its tail). The biggest advantage I see of just shooting at the base ISO with a fixed aperture and shutter speed is that it becomes virtually impossible to blow out anything but purely "reflective" highlights (which I never care about anyway).

Yes, there's more post processing involved in this approach than trying to adjust the ISO on the fly. But in a way it's more foolproof with regard to retaining detail in the highlights. (As much as I try not to, I occasionally change the ISO to 500 or 640 when a cloud covers the sun, then forget to change it back to 100 when the sun comes out again.) And from what I've read, there should be more noise when an image is "underexposed" and the "exposure" is bumped up in post processing compared to "exposing" it correctly to begin with. But I'm talking about a situation where using a wider aperture or a slower shutter speed are too much of a compromise, so the choice is between increasing ISO in the camera or increasing "exposure" in post processing.

Anyone else ever shoot this way?
--
Jim Kaye

'I believe that the electronic image will be the next major advance. Such systems will have their own inherent and inescapable structural characteristics, and the artist and functional practitioner will again strive to comprehend and control them.' -- Ansel Adams, 1981

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