First of all, with the fill light opposite the key, you run the risk of creating a second shadow—even the hint of one. There’s just one sun, so dueling shadows appear inherently abnormal to our eyes and brains.
Second, with too much fill from opposite the key, you can eliminate all of the beautiful shape and drama provided by the key. This is what shadows accomplish after all. And yes, you can overfill from any direction, but I find it easiest to step into this mistake when reflecting from the side.
Lastly, that sideways fill sometimes doesn’t brighten up shadows that are visible to the camera. It lights up from the side, and so what the camera sees may be slightly different than what that reflected light is able to reach. And this is a real problem with fill light.
So how do I eliminate this and other fill light issues? Simple: I add another light to my setup, placed as close to the camera as possible. This light, very near the lens, is known as “on-axis fill.”
Sometimes, my on-axis fill light is in front of my camera, with a grid spot or snoot meant to focus the light precisely on my subject and nowhere else. Sometimes, it’s a large softbox directly above or behind the camera or even a huge octabank placed behind me. Some photographers like to use a ring light, ensuring that the fill is literally surrounding the lens and making for the most frontal light. It works because the most important thing is that the light is coming from the axis of the camera lens—kind of like an on-camera flash. In fact, an on-camera flash also makes for perfect on-axis fill.
Because it comes from the lens’ position, on-axis fill, in practice, illuminates only the shadows seen by the lens. It’s a way to ensure that shadows are there, doing their job of providing dimension and shape and just the right amount of depth to an image. And because you’re using a separate light source for on-axis fill, you can dial it in very precisely in a way you can’t quite achieve with a reflector.
The biggest challenge with on-axis fill is ensuring you don’t overdo it. Too much fill and you can eliminate all the pretty shape and texture created by the directional key. Less is more, for sure, when it comes to on-axis fill.
By dialing the intensity of the fill light up or down, you can make the shadows show as much detail as you like, but not a drop more. My suggestion is to toggle the fill on and off, checking to see the difference between fill and no fill. Once it gets difficult to see a difference on your camera’s LCD, you’re in the right territory. That will subtly illuminate shadow detail, enhancing the feeling of depth and polish without eliminating the pretty key light.
The reason this technique adds production value is that the contrast can be so precisely dialed in. A novice gets “close enough” and calls it a day, but a real lighting master dials in every bit of contrast and detail in highlights and shadows. And the way those masters often do it is with on-axis fill.