When talking about the quality of the light, often what people mean is whether the light is hard or soft. Specifically this relates to what happens in the transition between areas that are lit and areas that are not lit by the light – how gradual a transition is there between a highlight from a light, and a shadow due to the lack of a light.
Soft light has a very gradual transition – it’s hard to say where the highlights end and the shadows begin. Hard light, on the other hand, has a very defined edge to the shadow it creates. The actual source is not what determines if light is hard or soft though – it’s how it appears relative to the subject.
Think of your own shadow on a bright sunny day – it’s well defined and has a clear edge to it. The source of light is the sun, and that’s hard light. But on a cloudy day it’s hard to find the edge of your shadow, because the cloudy sky is a soft light source. The source of light again is the sun, but this time there are clouds between it and you.
Equally, stick a flash on your camera, fire it directly at your subject and examine the the shadow it creates on their face and you’ll see that it’s a hard light source. Bounce that same flash off the ceiling, and the edge of those shadows gets harder to define, so now that flash is a soft light source.
Time for two more definitions:
A hard light source is one which the subject perceives as small, relative to the size of subject.
(The sun is enormous, but it’s very far away so when there are no clouds in the sky, that sun appears small to us on earth.)
A soft light source is one which the subject perceives as large, relative to the size of the subject.
(Your hot shoe flash is a small source of light, but use it to photograph a ladybird from 3 inches away and it appears big to the ladybird.)
When it comes to hard light and soft light, you’ll find people think soft light is prettier light, but one thing I’ve learned is that hard light can be wonderful, if used well. That’s good because it’s not always easy to create soft light, especially at a wedding, and especially outdoors. So if we can learn to work with hard light, things definitely get easier.