Evenness and Color
If you thought dealing with contrast and exposure in an image was challenging, then just the mention of the inverse square law may send you reeling. But fear not, it's here to help you! In very basic terms, the inverse square law dictates that the farther away you move a light source from the subject, the less bright the subject becomes, which is pretty obvious. But what the light also does is fall off in a more extreme way the closer you get to the light source. So, for example, if you're lighting a group of four people or more, say for instance a group photograph at a wedding and the group is in a line with a single flash that is right next to the end person, that person will be very bright, but the luckless soul at the other end will not be so fortunate. However, if you move the flash farther away, say another 12 feet, the relative difference in brightness diminishes. The person closest to the flash will still appear brighter, but not a lot brighter than the last person. In practice, this means that if you're lighting a group, or want even coverage on a big subject then you have to move the light farther away.
However, this not only affects the brightness, but also the quality of the light. The brightness of the subject will be reduced, and the light will become harder. You can counter this by making it bigger, perhaps with a softbox, (not always appropriate at a wedding, times is quite often against you in these situations) or simply by increasing the power.
Congratulations, you now understand the basics of light! Soon it'll all become second nature, particularly once you master the final property of light—its color. Color temperature is measured in Kelvin, which you can accurately measure by using a color temperature meter, human eyes naturally compensate for this effect, however, so anyone inside the church at a wedding wouldn't perceive the light being particularly warm in hue, If you shoot Raw format then you can adjust the color balance in post-processing. You may think you can do the same if you shoot JPEGs, but you can't—not with any great accuracy or quality.
Ambient light changes color at different times of the day and under different weather conditions. Bright midday light is most often thought of as "normal,' and is usually marginally warmer than your camera's flash white balance setting. Low evening sunshine, like sunset, is very orange, and light in open shade or on a cloudy day is much cooler and "blue." For artificial light, tungsten bulbs—like many typical household bulbs—are very orangey and warm, whereas fluorescent bulbs are often quite green, although they can be mixed in with some magenta.
The only time it matters to get it spot-on is if you are mixing light sources—such as using a flash on a subject in a room that's lit by household bulbs, for example. If you want to match your flash to the ambient light, you'll have to change its color by putting a specially colored gel in front of it.
The best option for you is to shoot in RAW, I generally get my exposures spot on however gettting that white balance spot on can be quite a challege so even though I would personaly prefer to shoot my weddings in Jpeg I do shoot manly in RAW just to ensure I can make any minor adjustments to the color balance.
This wonderful Photograph of the beautiful Jess was taken at her parents home and bouncing the flash towards the natural ambient window light added just a touch more light for me, this image was taken as a jpeg as I could see that the color of the light was perfect enhancing her beautiful skin tones.