Lighting Q and A, 09-26-08
"Is it possible for a specular highlight to be anything other than blown out? If so, how do you meter and control its intensity?"
Yes it is, Christine.
Keep reading for how to control specular intensity -- and for your second question, which I liked even better...
Christine's question went into more detail, and was asking about the process of using one light as both a key light and as a background element via the light's specular reflection.
The short answer is yes, the specular can be just about any tone you choose. It depends on two things: The reflective efficiency of the object creating the reflection and the intensity of the light source, on a per-square-inch basis.
The more efficiently your object reflects, the brighter your specular will be. But you might not have much control over the object, so you can also control the specular intensity by increasing the size of the light source.
Let's say you have two lights, each set to illuminate your subject to, say, f/5.6. One is a hard light, and the other is a big, soft shoot-through umbrella.
They will both correctly expose the subject at f/5.6. But in addition to creating very different qualities of light on the subject, they also will create very different specular highlights whatever is reflecting the specular -- your subject and/or your background.
The specular of the hard light will be tiny and very bright -- and almost certainly blown out. This is because the light is very small -- and very intense over those few square inches of light size.
The specular of the big, soft light will be large and manageable. The size of the light source is a determining factor in the intensity of the specular. And that highlight could be a variety of tones based on how close the light is to is to the subject and background.
This is because, while both light sources are putting out the same amount of light, they look very different when you measure them on an intensity-per-square-inch basis. And since a specular is basically a reflection of the light source, the intensity of each of those square inches is the main factor in the brightness of the specular.
As for metering those highlights, you can use a flashmeter in reflective mode to check how bright a specular is (vs. your shooting aperture.) Or simply zoom into only the specular highlight and shoot a photo. Then look at your image and histogram on the back. The spike will tell you the tone where the specular is, as compared to medium grey.
Now, Christine is Thinking:
She goes on to describe a second setup:
"Possible Scenario 2: The specular is caused by a light placed for the sole purpose of adding a specular--i.e. a rim light on the edge of someone's face or a background light on a shiny wall--and changing the power and position of that light will not affect the overall exposure."
Absolutely, but I would do it in a slightly different way. Why not position the light so that it is very easy to line up the specular highlight? And yes, you could squeeze some double-duty out of it, too.
And that is exactly what we did in the photo at left. This was one of a recent series of photos for a software company. My assistant Patrick Smith and I were working very quickly in a hotel conference room to do eight full-page portraits in just two hours.
My main light is coming from camera left, a few feet out of the frame and pretty close to my subject. It is warmed up with a 1/4 CTO and is lighting his face with soft light via a shoot-through Westcott double-fold.
The second flash is also firing through an umbrella, but that flash is directly behind me and a tad high.
That is basically a "buy one, get one free" flash, as it not only provides the perfect angle to create a soft specular on the dark wood background, but also serves as soft, on-axis fill for my subject.
As mentioned in the SB-III post, on-axis fill is something I am using more and more, and in several different ways. More to come on that soon.
:: Read More: Using Specular Highlights ::
New to Strobist? Start here | Or jump right to Lighting 101
Connect w/Strobist readers via: Words | Photos
Got a question? Hit me on Twitter: @Strobist
Save Money: Browse MPEX Weekly Strobist Deals