I try to think in EV. It's not too hard to guess an EV for a particular ISO based on stops down from sunny/16.
Low light is hard, but I guess that's just a matter of experience. That I would approach by having a guess at it at EV100 (which is the same thing as LV, an absolute measure of amount of light) based on previous experiences and converting to EV at whatever ISO. That's a factor like EV400 = EV100 + 2).
The biggest thing for me was working out how to do the calculation from EV to actual exposure quickly. It's easy enough.
Start with 2, then add the number of aperture stops past f/2 you want(ie. f/16 is +6EV compared to f/2. You just have to memorise these, but there are only 6 maybe 7 values). Then whatever is left is the shutter speed in the closest power of two. (ie. 1sec = 1/2^0, so +0EV, 1/60 =~ 1/2^6, so +6EV). Powers of two are pretty easy.
So to get EV100 = 15, (sunny 16), you go 2 + 6 (for f/16) + 7 (1/125sec).
If you are shooting a scene at ISO1600 that is say -3EV from sunny 16 you say, sunny 16 is EV1600=19 (EV100+4). So I want 16. Which is 2+6 (f/16)+8(1/250s)
Easy. You can do the shutter speed and aperature around the other way if you want to say pick a particular speed for whatever reason. Say you want to open up as wide as you can using the last example and 1/1000 is the fastest shutter speed you have. 16 = 2+10(1/1000s)+4 (f/8).
The other thing that really helped getting my head around this is simply realising that an EV value is actually around the wrong way from what it sounds. It's called exposure value, and is referring to how much exposure you give the film, but it is actually less exposure the bigger the number. I try to think of it as 'degree of attenuation'. It's sort of seems to be measuring the amount of light, but this becomes nonsense when you start talking about EV100 vs EV400. It's not amount of light. It's amount of attenuation that light gets before it hits the film. An EV number always means exactly the same shutter speed/aperature irrespective of the ISO value it's for. So once you know what those are, or can get there quickly, the thought process gets much faster.
This is probably quite basic, but it's really helping me to get quicker at doing this without a meter. It doesn't take me long now from guessing the light to actually setting up the camera to take the shot. As for guessing the light correctly or making sure the photo was worth taking in the first place, still working on that
Last edited by octofish; 10-14-2012 at 11:02 PM. Click to view previous post history.