They say it is all in the eyes. And in portraits the eyes come alive with the presence of a catch light. But before we talk specifically about catch lights, a refresher on reflections. After all, we don’t photograph people, places, or things. We photograph the light that reflects off of those people, places, and things. There are two basic types of reflections. Diffuse reflections define the subject and its texture. We base our exposure on the diffuse reflections, whether that be skin, a tree, a flower, etc. Specular reflections are direct reflections of the light source, like a mirror. We also have shadows, which are areas that don’t get lit.
As you change the distance between your light and your subject the diffuse reflections get brighter or darker. Move them closer together and you have to lower your exposure to maintain the correct exposure. Move them farther apart and you need to provide more exposure. Pretty simple and intuitive. The diffuse reflections loosely follow the Inverse Square Law of physics. I am going to skip the math and science part here, but basically it tells us that changing the distance between the light and the subject will have a bigger effect on the exposure than you might expect. Light falls off rapidly as you move the light farther away.
Then we have the mirror-like specular reflections. Specular reflections are the same brightness as the light source, no matter the distance. You can easily see this by pointing a flashlight (torch) at a mirror and observing the brightness as you move the light closer or farther away. At this point you might start to ask why these highlights don’t follow Inverse Square. The answer is that they do! But in a different way. As you move the light source farther away from your subject the specular reflections get smaller, as you move the light closer the specular reflections get bigger.
Once you have an understanding of this you can start to consciously control the relationship between the various highlights. And that is where we come to discussing the catchlights in the eyes of our subjects. This stems from a question I recently received, “Can I make the catchlights less noticeable by moving the light farther away from my subject?” And the answer is, No. Moving the light farther away will make the catchlights MORE noticeable. They will be smaller, yes, but they will also be brighter in comparison to the diffuse reflections. The followup question was if dimming the light would reduce the brightness of the catchlight. Again, the answer is no. Dimming the light lower both the diffuse and specular reflections and you will need to increase the exposure, leading to the same relationship between them.
Let’s look at what is happening. Start with a light in a softbox at about 36-inches from our subject. We meter the light and find that the proper exposure is at f/8. Now move the light back to 48-inches from the subject. To maintain the same f/8 exposure we have to raise the power of the flash up about 1 stop. Now the face is still at the same exposure level (moving the light back and raising the power level compensated for each other). But the light is 1 stop (2X) brighter and because the catchlight is a mirror reflection, it is now twice as bright than it was before, making it much brighter than the diffuse reflection.
We can also look at this going in the opposite direction, bringing the light in closer to our subject. If we bring our light in 3 times closer, say it was originally at 9 feet away and now we move it in to 3 feet away, what happens? We have to stop down (change aperture or lower the flash power) by 3 stops to maintain the proper exposure. That makes the specular highlights 9 times darker than they were. Let’s see that that looks like in pictures.