I nearly always shoot in manual mode, unless I can't keep up with the pace of fluctuating light levels. If the lighting is constant then manual mode can save so much grief. I would use spot metering and pick my own target for metering, then set my exposure from the spot meter. Sample exposures would be....
- metering from the palm of my hand at +1 1/3 stops;
- metering from the brightest part of the wedding dress at +2 2/3 stops (maybe +2 1/3 for safety);
- metering from grass at between -1 and 0 stops (chimp the wedding dress, and perhaps the sky, for blown highlights and fine tune);
- If it's a bright, clear, sunny day and you are outside then select a manual exposure based on the "Sunny 16 Rule". e.g. f/4, 100 ISO, 1/1600 or f/5.6, 100 ISO, f/5.6, 1/800.
However, if you do choose to shoot using autoexposure I would be tempted to use evaluative metering and let the camera earn its living. Make sure you enable "Safety Shift" to dig you out of the worst of the deep holes that autoexposure might create for you. As to the specific choice of autoexposure mode....
- Av gives you control of DOF, for a more creative image, but can result in some surprisingly unwelcome shutter speeds if you're not paying attention. It can also wreak havoc with your peace of mind if you use flash and automatically assume you'll get shake free shutter speeds. If you do shoot in Av mode then you must be aware of the shutter speed you end up with and be prepared to juggle the ISO to keep it within acceptable limits. If you have a preference for extreme aperture values then you need to think not only about shutter speeds but also whether you have sufficient DOF. Too much DOF is far less of a problem than too little.
- Tv gives you control of movement/motion and you can certainly choose speeds that keep you above a shakey camera threshold, but you need to watch that you DOF doesn't become unacceptably shallow, or that you hit massive diffraction softening limits by pushing the lens to f/22 or f/32. Again you'll need to keep an eye on what the camera is giving you in case you need to change ISO to get a more acceptable mix.
- P should choose the most middle of the road combinations it can, avoiding excessive extremes of aperture or shutter speed. It will also ensure your shutter speed does not drop below 1/60 if you turn on the flash. It is the least creative mode, with the least control, but probably provides the largest safety net for getting a useable, if relatively unexciting, image.
As far as the actual exposure levels are concerned, any of the three AE modes will give you the same brightness levels so you need to pick whichever metering mode and AE mode best suits your shooting style and your willingness to get involved and override the camera when the need arises.
I'm not familiar with the Rebel series so I don't know how quick/easy it is to make exposure adjustments on the fly. With my 30D/40D/50D/1D3 it is a cinch to adjust any of aperture, shutter, ISO or exposure compensation and flash exposure compensation very quickly, and from the 40D onwards I have ISO displayed in the viewfinder as well, which is also a plus.
I don't know what your flash solution is but that does open another can of worms. Usually it's best to set a manual exposure for the ambient/background light and then you only have to worry about tweaking the FEC for your subject. On autoexposure you'll potentially have to juggle EC and FEC to get the right lighting balance. That's more than I want to have to deal with from one shot to the next.
You'll also need to ensure your shutter speed remains within no more than you flash sync speed, which might be a challenge outdoors in bright sunshine. For example, on a Rebel your max sync speed is 1/200. If you don't have a flash with High Speed Sync built in then you will need to shoot at f/11 and 100 ISO to get your 1/200 shutter speed without overexposing the background. f/11 will severely limit the power of your flash, rendering the popup flash useful as a fill only up to about 1.5m away. A 580EX, aimed directly, should still be good for up to 7m with an f/11 aperture and 100 ISO. Of course, a 580EX also has High Speed Sync, so you would not need to stop down to f/11 to limit the shutter speed.
perhaps it's because you didn't say anything like 'get to know your kit first'.
