Light follows rules. Whether you're using the sun, a flash, a lamp, or a reflector, light is light, and it all behaves the same way.
One important rule of light to understand is that the size of your light source relative to your subject determines whether or not you will have soft light or hard light. The bigger the light source in relation to your subject, the softer the light will be on that subject.
The easiest way to compare soft light and hard light is to look at shadows.
Hard light creates a shadow that has a distinct line and shape. Soft light creates a shadow that is not as defined.
Hard light creates a transition from light to shadow that is abrupt and instant. Soft light creates a transition from light to shadow that is gradual.
Many portrait photographers (and clients!) prefer soft light. To get soft light, you need a big light source in relation to your subject. Take a look below. The photos are straight-out-of-camera (SOOC), no adjustments made at all. One photo is lit with a bare speedlight and one photo is lit with a big softbox. Do you know which is which?
If you said that the first photo was the speedlight and the second photo was the softbox, then you're correct. Look near my model's nose. You see a distinct line from the shadow in the first photo because the bare speedlight is a much smaller light source in relation to my subject than the big softbox. The light on a speedlight is about the size of a credit card, and the softbox is four-feet by four-feet.
Here are the behind-the-scenes photos for both the speedlight and the softbox, respectively. Notice that both are the same distance from my subject. The difference is in the size!
Don't be fooled, though! A bare speedlight can produce soft light. You just need to be photographing a subject that is much smaller than the bare speedlight. For example, if you photograph a toy car with a bare speedlight, then you'll have softer light. Why? Because the car is about the same size as the speedlight, so the light source is big in relation to the toy.
A common misconception is that you can achieve softer light by putting the light source farther away. The thinking behind this is that the light will have an opportunity to spread and get bigger. Not true! First, light travels in a straight line, so it won't really "spread." Second, by putting your light farther away, it becomes smaller in relation to your subject, which creates harder light.
Check out the comparison photos below to see what I mean. Again, all photos are SOOC. The first photo is a still-life lit by a close light source.
The second photo is the same still life and the same light source, only the light source is now farther away (I raised the light stand.)
I'll put the photos side-by-side so that the comparison is clear:
Look at the shadows, especially the shadow under the fork. The softer, less-defined shadow is from the image with the closer light source. You can more clearly see the prongs in the shadow with the farther light source.
Want softer light? Don't put distance between your subject and the light source! Move your subject and light source closer together. By getting closer, the light source becomes bigger by comparison; therefore, the light becomes softer.
When it comes to creating soft or hard light, I'll repeat the principle from above: the size of your light source relative to your subject determines whether or not you will have soft light or hard light. The bigger the light source in relation to your subject, the softer the light.
Yes, in the end, at least when it comes to light, Size Matters.