What strategy do you follow when setting ISO, shutter speed and aperture to achieve the correct exposure?
Do you have different strategies for different shooting conditions?
I believe that all photographers develop an informal strategy for choosing the right exposure settings.
I want to find out if there is an effective general purpose strategy that can be adapted to a wide range of conditions.
This question is inspired by the question about a photography cheat sheet
What strategy do you follow when setting ISO, shutter speed and aperture to achieve the correct exposure?
Obviously there are different strategies for different situations, I usually use an iterative process that goes a little like this:
If yes, set it, if not pick something optimal like f/5.6. Then I look at the shutter speed and ask similar questions:
If yes, set it, if not pick something "safe" e.g. 1/2*focal length. Then I look at the exposure and set ISO - there's no artistic consideration here (if I want noise I'll do it in Photoshop so I can get a nice fine grain) so the only question is:
If not, set the ISO to whatever is necessary to get the correct exposure, even if it seems quite high! It's important not to underexpose as this is much worse for noise than upping the ISO. If there isn't a high enough ISO, or I want to reduce noise by letting in more light, I will go back to the earlier questions and re-evaluate any arbitrary decisions. If I can open the aperture I will, likewise if I can slow the shutter I will.
If not it's time to make a compromise and weigh up how important the artistic and technical decisions were, until a sensible balance is achieved.
This sounds quite complicated by I usually run through this in my head a few times by guessing what the exposure will be before actually setting the camera. It's also fine to use auto mode to fill in the aperture/shutter as appropriate if you're not setting it for creative reasons (the OP seemed to be asking about full manual settings).
Throw in flash an you've got another variable, with another art/technical trade off. It gets a bit complicated to generalize here, in general I'm either using flash for artistic effect in portraiture in which case flash rules and all other settings bow to it, or I'm using it for extra light in event/wedding photography, where I set the aperture and shutter how I want them both artistically and technically and use the flash to pick up the slack, going back to the other settings if I need faster recycles or want more ambient in the background.
I can offer help from a landscape/nature perspective. I can't offer much help in the area of portrait or indoor photography. Matt Grum may be able to offer helpful advice in those areas.
I guess this boils down to two different approaches. The semi-automatic approach, and the fully manual approach. I would say that 85-90% of the time, I use a priority mode, usually aperture priority, and let the rest be automatic. This is probably most common when I am doing wildlife, bird, and macro photography, as being able to set the aperture that gives me the correct sharpness and bokeh is critical in those types of shots.
Alongside a priority (or program) mode, my most common exposure adjustment is via exposure compensation. Unless I am shooting in dimmer light, I keep my ISO fixed, usually at 100, sometimes 200. Compensating exposure with simple +/- 1-2 stops is real nice, and keeps things simple. It lets me put most of my focus on composition and filtration, where I think it should be.
When it comes to fully manual mode, I guess it really depends. There are so many different situations that call for different settings. After sunrise, before sunset, or during the day, I try to keep my ISO low. At sunrise or sunset, or at night, you may need higher ISO settings. I usually use ISO 100 or 200, which keeps noise almost non-existent for landscape shots. When it comes to wildlife and birds, I adjust it as needed to allow the appropriate aperture and shutter speed. For aperture and shutter, I guess I generally blend a semi-automatic mode with manual mode. I make pretty heavy use of the histogram on my camera to see if I am in the ball park or way out of it when setting aperture and shutter speed. I often start in aperture priority mode, and get a feel for where the shutter speed might end up in the lighting I have. Once I have the aperture set as I need it, I'll adjust ISO until my shutter speed is fast enough to freeze motion appreciably (I generally don't use flash...but then again, I don't really know how to use it most effectively either.) I try to keep my ISO below 1600, but often enough when photographing birds, there isn't any choice.
The story gets a lot more complex when you involve filtration. Outside of a high quality UV filter that does its job well, pretty much all filters block some light. The most interesting experiences I have had with exposure involved the use of ND and GND filters, as well as polarizers. When it comes to ND filtration, I've found it best to meter your shots as you normally would without any filtration in place. (The Lee system makes this really easy...it is a synch to clip on your whole filter set in the foundation mount, and take it off, without affecting the shot much.) Again, I usually work with aperture, meter to determine what the shutter should be. ISO is more complex here. Quite often, I use filtration to allow me to purposely use a long shutter speed, to smooth out flowing water, flatten and glassify lake surfaces, etc. If that is the case, I always use the slowest ISO I can, which on my current camera is 100. That, in combination with filtration, allows me to use a longer shutter.
If I am using graduated ND filtration to reduce the contrast (dynamic range) of a scene, it gets even more complex. Before I can calculate how long my shutter should be (which can be very difficult, and sometimes a trial and error process, if I want to smooth out clouds or water), I need to spot meter my scene. The best way to determine how much GND filtration is needed is to meter the scene in at least three spots without filters in place: The brightest part of the sky, the darkest part of the landscape, and an area that appears to be as close to 18% gray as possible. Metering a middle tone helps you determine if you can capture the scene without filtration. It is easy to see with a histogram if this is possible or not. If not, metering the brightest spot and the darkest spot in the scene, and taking the difference between those two, will tell you how many stops of filtration you need at a minimum. I usually put on an additional stop of filtration and overexpose to give myself some extra shadow range (ETTR). Once you have determined the total dynamic range and necessary filtration, recompose your scene, slap on the necessary filtration, and set aperture, shutter, and ISO. I find it is easier to calculate everything if you use ISO 100, but any ISO could be used.
Finally, I try to follow the ETTR rule: Expose to the Right. With landscapes, once shutter speed and ISO have been set, it is easy enough to adjust aperture a little. Once a scene has been metered and properly filtered, it usually only takes about 1/3 to 1/2 stop, and at most 1 stop, to bump exposure to the right as far as it can go. If it takes more, then you might try adjusting your other settings some more to cover greater dynamic range.
Exposure can be a very complex thing, and I'm not sure there is really a rulebook that can tell you what you need to do. In my experience, setting exposure is the key thing that a landscape photographer DOES. Taking the picture is a momentary experience after that.
It's going to be really hard to add to the previous two answers, but I'm game! The three major types of photography I'm generally interested in are nature, wildlife, and macro, so each has a different set of thought processes in my approach...
For landscapes I tend to concentrate on aperture to achieve a depth of field appropriate to the situation. If it's a big scene, say a lake or a mountain setting, I'll aim for a more narrow aperture to bring more if it into focus. If it is a plant, I may go wider in the aperture to "pop" the plant from the remainder of the scene by creating a more shallow depth of field. Net effect, the thing that matters to me is aperture the most, shutter speed only comes into play if the subject is effected by things such as wind, at which point I may drive the ISO higher to keep my aperture where I want it and get the shutter speed to a point that it freezes motion. Or I may cheat and use a plamp to hold my subject. :)
This is usually a shutter speed situation for me. In general, I'm trying to get the shutter as fast as I can, especially with faster moving animals, so that I can freeze their motion. Depth of field has some play here, but usually it's telephoto and so it's going to be minimal in general anyways. Net effect, I want my shutter as fast as I can push it and will open up the lens or adjust the ISO as I need. On the upside, for me at least, Pentax has a shutter/aperture priority mode that allows me to pick the shutter speed and aperture with the camera selecting the ISO and this is very helpful in wildlife shooting. I'm still surprised that Nikon and Canon haven't added this mode.
Both shutter and aperture come into play here. When shooting macro, depth of field is razor thin, so tightening the aperture can be important and so I generally look to move that up quite a bit, even as much as f/11 or more. Since most of what I shoot macro is in motion, such as insects or water drops, shutter speed may play a role as well, but they differ...
If the subject is an insect, I want a fast shutter speed because they move and, being such tiny subjects, any movement is amplified in macro. At this point, with a tight aperture and a fast shutter, you either need a lot of available light, a high ISO, or a heck of a lot of patience.
If the subject is something like water drops, then my shutter speed is usually quite slow! My current technique, since I don't use devices to do the work for me, is an off camera flash, a cable release, and a faucet-based rig with a rubber hose and cake decorating ends (at some point I have to post some pictures of the rig on my site). In any case, I use a very dark room, set the drops going, and then trigger the shutter and press the test button on the flash with a very low power setting. Basically, this results in the camera capturing the result of the very quick light burst which freezes the drops. In any event, my ISO is always set at the lowest and I tweak the aperture as I go.
There's no "grand unification theory" that will apply to exposure. It is going to differ depending on the nature of the subject and the goal of the photograph. As you develop interests in certain types of subjects, you'll start to develop a feel for how to tweak your exposure options accordingly. As a general piece of advice, manual settings on the camera and a little note taking on decisions will help you achieve this feel in the long run. While you're doing that, don't worry too much about the missed shot, you'll get much, much, faster in the process as you practice.