Most of us who've been in photography for a while have at some point been exposed to the "Sunny 16" rule. This rule generally allows the use of ISO 100 at 1/100th and f16 on a sunny day, and of course, this rule has been existence since the days of film.
The standard settings for ISO on a bright sunny day in the outdoors is usually 100 or lower - depending entirely of course on how low your camera will actually go. For instance, on my old Nikon D70, the lowest rating is ISO 200, which was essentially the equivalent of ISO 100 on other brands (back then). Outdoors on an overcast day, you might bump your ISO to 200 or 250, but the general rule was to use the lowest ISO possible. This instruction came from my exposure to microstock photography, where there was almost no tolerance for noise in submitted images.
But in the world of portrait and wedding photography, small amounts of noise are not such a huge issue. For the most part, family portraits, head shots, wedding photos are going to be printed in standard sizes, and most likely not going to be made into huge posters, so small amounts of noise are less of an issue than for stock imaging and will not likely show in the final prints.
S = Shutter Priority « A = Aperture Priority
Shutter Priority: The photographer sets the shutter speed, and the camera selects the most appropriate f-stop.
Aperture Priority: The photographer sets the aperture f-stop), and the camera selects the most appropriate shutter speed.
When using either of these settings, the photographer only gets to select two of the three camera settings - provided your camera isn't set for auto ISO, you can select your ISO, as well as either the shutter speed (in s setting) OR the f-stop (in a setting). You will not be able to set both shutter speed and aperture while in this mode - you'll need to move back into Manual to select both.
There are both pros and cons to using these settings. If you aren't comfortable yet with full manual settings, then this might something you'd consider, although you will lose a certain amount of control over the resulting image and it's lighting.
What I wasn't familiar with, was the options for changing the ISO to make changes in these settings. Yes, I'm well aware of how ISO works with f-stop and shutter speed, but I work almost entirely in manual settings, and surprisingly enough, using these semi-automatic settings create a different balance between these.
Another photographer mentioned using an ISO of 1200 outside in the daylight and I have to admit, I nearly flipped. Because my experience is mostly with manual settings, I couldn't imagine needing an ISO of 1200 outdoors in the sun - outdoors my ISO is nearly always 100 or lower. That's normal. I don't know another photographer who would use an ISO of 1200 outdoors on a nice sunny day. Then the photographer mentioned using Aperture priority. And when I later thought about it - about how ISO, shutter speed and aperture work together, I understand why that would work.
The idea with photography is to get enough light to the sensor to create an image that is properly lit. If the camera is set for Aperture Priority and you've selected an f-stop of f11, and the camera selects a shutter speed of 1/40 based on the lighting, the only way to change the shutter speed without selecting fully manual settings is to change the ISO. If you use a higher ISO, you can get the camera to select a shutter speed of 1/100 or 1/160 so you can handhold the camera and avoid the blur you might get at 1/40 if you aren't too steady and don't have a tripod. This seems like a roundabout way of doing things - you might as well use manual. But, for photographers who have to make settings changes in a hurry, it's a workable solution (probably not one I'd use, but it does work). Below you'll see some example images - sorry I took these sitting at my desk, with no tripod. The long exposure times resulted in blur, but you can see from settings on each one the results. The camera was set for Aperture Priority, with a f-stop of f11 and the starting point for the ISO was 400. Before each photo, the only change made was to the ISO rating, if you check each photo's settings, you'll note the change in the shutter speed as selected by the camera. The last in this series was an outdoor photo (overcast).
When I shoot a wedding, I almost always have had 2 cameras - one often set for outdoors, and one for indoors, making it easy to transition from inside to outside without having to retest the lighting a reset the manual settings. For those starting out with only one camera, I suppose I can see the benefit of using the Aperture Priority setting (though it still makes me shudder) because the only transition needed would be to alter the ISO settings, making quick work of the camera adjustments. The drawback of course, is that you lose some of the artistic license you have with manual settings, but on the upside, you'll probably be able to move pretty quickly from one setting to another, even though you might not have a lot of experience, you'll have some confidence that the images you produce can be properly exposed.
There are other settings on a digital camera that can adjust the value of the light exposure without changing the shutter speed and aperture, and those are the exposure value settings. When using the flash, if you adjust the exposure value on the camera, you'll find you get different results using the same ISO, F-stop and Shutter speed settings.
If you've never made use of the EV settings on your camera, you might want to take some time to experiment with them and make note of the results. It can come in handy in a pinch if you need more light or less light in a hurry, without fiddling with your settings.
You'll note the results in these very quick snapshot samples below - no particular care was taken in the setup here. The photos were handled in a very "point and shoot" snapshot manner, but it pretty clearly shows the effect of the exposure value adjustments. The EV adjustments were the only settings changed after starting with a high ISO (1250) for shooting in low light.
The one thing you'll want to watch is the noise level - in general, you aren't going to be using an ISO of 1250, and if you need to, many cameras could generate a high level of noise. Make sure to check the noise in your sample photos.