When you start talking about teaching the fundamentals of anything, people tune out. I'm hoping I got you here with the whole FUNdamentals thing. Seriously, this post is gonna be a lot of fun (no, not really).
Soft light vs. Hard light
We've all heard people use these terms and many of us probably know what these terms mean intuitively, but for the sake of comprehensiveness, let's nail this one down. Between light and shadow, there is an area that photographers call the “gray zone.” This is the gradient that transitions from the highlight to the shadow. Suppose you put a flashlight on the ground. You see the beam of highlight, the shadow everywhere else, and a ring around the light that transitions from the highlight to the shadow. That's the gray zone.
So what is soft light? It is a light which has a wide gray zone. It means that the light won't cause harsh shadows under people's noses, or under their chins, etc. Soft lighting does not show texture (such as pimples, wrinkles, blemishes, leprosy, etc) as well as hard light. This is one reason why soft light is generally more amenable to portrait photography, because we want the light to wrap around the texture on our skin so we don't show off our blemishes. Remember: the softer the light, the wider the gray zone. The harder the light, the narrower the gray zone.
What makes light soft?
One thing, and one thing alone makes lighting soft: size. It is not about how bright the light source is, nor how diffused the light source is, nor anything else. It's all about size. The larger the light source, the softer the light. This explains why the (horrible) photographers at the department-store photo studio your wife forces you to shoot their flash units into umbrellas. The flash unit itself is only a few inches wide, but when you shoot it into an umbrella, it becomes 43 inches (sorry, I don't do metric, but I'm sure you know what an umbrella looks like) in diameter! Making the light large makes it soft. The same is true with the relative size. If you're shooting a portrait of someone and the light is too hard, do you put the flash closer or further away from the person? It may surprise you that the answer is to put the flash closer to the subject. When the light is closer, it seems larger than if the light is further away. This relative size of the flash also affects softness.
What about natural light outdoors? Now you know why shooting portraits, or even wildlife, in mid-day lighting from the sun is so horribly ugly. The light, while giant in terms of actual size, is only a puny little dot in the sky. This small relative size of the sun makes for hard light. Portrait photographers always hope for a cloudy day when shooting outdoors. The clouds spread the size of the sun to make it relatively larger, which produces softer light.
How can I achieve soft light without fancy umbrellas or soft boxes?
Flash portrait photographers go to great lengths to increase the size of their lights. In addition to using light modifiers like umbrellas, we often bounce the light from the flash off walls to make the light appear larger (the light on the wall spreads to make it seem bigger). This not only gives the appearance of being directional light like off-camera flash, it softens the light too. (Read this article to learn how to prevent hard shadows on the wall when doing portrait photography.)
If you don't don't even have a flash unit for your DSLR but still want to use soft light for your outdoor portraits, head right over to Amazon.com and spend a whopping $20 on a reflector. This is probably the most important piece of equipment I own for shooting portraits. Heck, I'd rather have a reflector and no camera than a camera and no reflector (not really). Reflectors are large reflective discs which spread the light from the sun to be larger. 5-in-1 reflectors, like the one I just linked to, also include a diffuser, which is a piece of translucent material that can be held to lightly shade the subject and spread the light source. When I shoot portraits outdoors, a reflector is my secret weapon. I'll simply have my wife or someone around hold up a
reflector diffuser to enlarge the light source, and I get BEAUTIFULLY soft light without any fancy equipment. Also, it's a lot simpler than bringing all my studio flash gear.
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