I just came back from a wonderful trip to New Zealand that give me an opportunity to further test the setting model of “manual-plus-auto-ISO” that I’d been touting. The conditions were challenging because as most probably know, New Zealand tends to be cloudy or rainy, and in rain forests things are darker than even normal because of the extreme shade you find. What I found was that the method of setting Manual Mode with Auto ISO does indeed work well even under these conditions, but it’s critical to remember to “test-shoot” when you have a major change in lighting.
Imagine you are walking along a trail on a cloudy day, in dense cover. At some points you may be in a sunny/clear spot where exposure settings are fairly normal, and at others you may be in deep shade. Forest birds are always a challenge because they flit in and out, so they need a fairly fast shutter or you get a blur. So you set a 1/1000th shutter speed, and say you also set a fairly fast (f5.6) lens opening. This works in the clear areas, but in deep shade you’ll underexpose. I did that a couple of times, and the only remedy is to check exposure by half-pressing the shutter when you move into new lighting conditions, and resetting something (in this case, shutter speed) to get a workable exposure when needed.
Does this mean the M-plus-AutoISO method doesn’t work? No, it means that you can’t make light out of nothing. ISO, shutter speed, and lens opening are three independent variables in the exposure equation. There will be some conditions under which you can’t get any satisfactory solution. I photographed kiwis at night on a beach, for example. There’s no flash allowed, no bright lights. You are taking pictures of a bird on the sand by the light of a hand torch that isn’t even shining directly on the bird. Even at ISO 25,600 and 1/60th of a second (with an 80-400mm lens!) the shots were a half-stop underexposed. About a third of the pictures were trash because of exposure, subject movement, camera movement, or a combination of all of the three. But some worked, and with the M-plus-AutoISO method I was able to decide what to change to give me the best chance!