Many photographers - especially those using studio lights - use hand held incident light meters to measure the brightness of any light source. An incident reading measures the light as it falls upon the subject and not after it’s been reflected by the subject. Your camera’s internal meter reads reflected light.
So, now the big question. Where do you point your light meter? Some will say point it at the camera, others say point it at the light source and I’ve even heard some say half way in between. So what’s the correct technique?
Well, I’ll spare the suspense. The correct answer is at the light source. After all, you’re measuring the brightness of a light so the most accurate way to measure it is by pointing the light meter, from the subjects position, at the light source.
Now you know that nothing in photography is ever that easy. And yes, there are some caveats to consider. If you want to delve a little deeper then read on. If not, have a wonderful week and see you next Tuesday.
So, why the caveats?
In any photograph, your sensor is recording light that has been reflected off of a subject. But reflections aren’t so straight forward. Reflected light can bounce off a subject in three different ways - you can have a diffuse reflection, direct reflection and glare.
A direct reflection is like that off of a mirror. It is a mirror image of the light source that produces them. Most photographers have in their vocabulary the term “specular” to describe highlights from bright light sources. The sun reflecting off the waves or the bright highlights in a car’s windshield or chrome. Specular highlights, or more correctly, specular reflections, are a direct reflection of the light source. So, when I refer to a direct reflection, you can think specular - that’s ok.
Glare is a direct reflection but the light is polarized. (We’ll leave the discussion on glare to another blog post.)
A diffuse reflection is quite different. As it’s name implies, the reflection is diffused in all directions and the brightness is the same regardless of the angle from which they are viewed.
Now, obtaining an exposure value for either of these is pretty straight forward. But in reality, we seldom photograph subjects that are 100% diffuse or direct reflective. Most of the light reflecting off of normal subjects is a mixture of all three types of reflections. It’s this mixture that creates challenges for the photographer.
For years students have express frustration when instructors demonstrate how to accurately meter a subject and then, seemingly off the top of their heads, pull out some type of fudge factor. Think of all the discussions about the placement and brightness of hair lights and accent lights.
The blend of diffuse and direct reflections varies greatly in the hair from one person to the next. Some refer to lights working more efficiently with one persons hair than another. In reality you're actually seeing a different blend of diffuse and direct reflections.
As you move accent lights farther behind a subject, they often look too bright. Even if the light is set the same value as the main light. This is especially true for subjects with a high percentage of direct reflected light.
In our workshops we talk about needing to compensate when setting the value of an accent lights that are moved more than 90° degrees from the camera. The reason is that our subjects reflect not only diffuse light but also direct reflections. There are always those who want to know the “formula”. Exactly how much do you compensate for something at, say, 120° degrees? The real world answer is “I don’t know”. It all depends upon your subject and the blend of diffuse and direct reflected light.
In their book Light Science & Magic: An Introduction to Photographic Lighting, Fil Hunter and Paul Fuqua surmise that photographic lighting is “primarily an exercise in reflection management.” If you want to really understand how light behaves then I’d highly recommend this book.
The fact is that in the real world we seldom photograph subjects that have totally diffuse or direct refections. Just about any subject has a mixture of both. I suggest above that the correct technique for using a light meter is to point it directly at the light source. But there are many very accomplished photographers who may use a different technique. My suggestion is to not blindly follow anyone. Try and understand “why” they suggest a technique. Research it yourself and establish habits and techniques that compliment your style and choice in subjects.