On-Axis Fill And Flash Fall Off
I recently presented at a flash workshop for the IPF to an audience of about 50 photographers (blog to follow) and one subject that came up was that of fill flash. So I thought it would be useful to discuss fill flash and in particular the use of on-axis fill.
But let’s begin at the beginning with a key light, or main light. This light is the light that we use to light or expose the subject. It is what gives our subject it’s shape in that we use it to create both light and shadow. Let’s start with a simple, single light setup, using an SB800 fired into a softbox, triggered by a pocket wizard. The light is elevated above the model, so that shadows fall downwards (another good tip). In this case, I have kept the light back about 12 feet from the model/mannequin.
If we look at the mannequin, the shadows are not very deep at all. This is because the light is so far away from the model. People often confuse this with soft light, but it’s not If you look closer at the shadows, particularly at the one on her nose, just look how quick the shadow goes from light to dark. It is an extremely quick transition. This is because the light is relatively harsh due to the fact it is “apparently small” (because it is far away). The reason that the shadows are not deep (dark) is because the distance from the front of the models face to the light is almost the same as the distance from the back of her face to the light. So there’s very little change in the amount of light hitting both parts (called fall off) hence very little shadow.
Let’s move the light closer
By moving the light closer (even slightly) we see two notable effects. The first is the shadows become somewhat deeper/darker as does the background. Again this is due to the effect of fall off. In the case of the background, the ratio between model to light and model to background has changed. Previously it it would have been 12 feet from model to background and 12 feet from model to light. A 1:1 ratio. But as I have brought the light closer to the model, this ratio is now 12:10 (smaller than 1:1). This means that proportionately, more light is falling on the model than on the background, hence the background turning darker. To compensate for the extra light on the model, I have compensated by stopping down the aperture on the camera.
Also note the shadow transition. It is marginally (notably?) longer than previous, because the light is a touch softer, due to it being larger/closer.
Moving the light closer again, all of these effects become more pronounced. The background becomes darker again, the shadows on the model become deeper and the light becomes softer.
As we continue to move the light closer again, the effects become more pronounced again. In fact the closer we get to our subject with the light, the more pronounced the effect becomes. So much so, that even a slight movement in model towards or away from the light source, can have a dramatic effect on the exposure. In each case, as I move the light towards the model, I meter the light and adjust the aperture accordingly.
We can really see this dramatic escalation by moving the light again. The step movement is still only about 2 feet (give or take) but the difference between the two consecutive shots is quite dramatic. The background is really starting to fade away to black, the shadows are much more pronounced, but the transition is much more gradual.
Getting In Close
Getting the light in REALLY close (approximately 2 feet), we now have some pretty dramatic lighting from a single source. The light is relatively soft given it’s apparent size/distance to the subject. You can really see this by looking at the shadow on the neck. Just look at how long it takes to go from light to dark; it’s a very smooth transition. But the shadows are deep This is due to the fall off, because there is a big difference in distance (ratio wise) between the front of the face to the light and the back of the head to the light. This would be my “typical” light setup for most shots.
Key Light Summary
So we can see that the key light/main light is what both lights our subject and gives it it’s three dimensional feel, by also creating shadows. The term for this is “modeling”. By changing the position, size and direction of this light we have control over how soft it is, how deep our shadows are and where they fall.
For some, when the shadows are this deep, they may want to “lift” the shadows by introducing a fill light. This is typically a second light source, that is usually less powerful than the main light. Unfortunately most people “incorrectly” place the fill light on the opposite side of the subject to the main light. But this is far from ideal. We have gone to the trouble of creating and placing shadow, of making our subject three dimensional using the key light. What we don’t want to do now is “fight” these shadows. A fill light from the opposite side would do just that. Not only that, but it would also create it’s own new shadows! The best fill is on-axis fill, where the light comes from the direction of the lens. This is a flat light source creating no new shadows, but has the effect of just lifting the depth of the shadows created by the key light, but still keeping the same modeling/3D effect.
With two lights, I need a way of triggering them both. One option would be to optically slave one of the lights, but in this case I opted to use a second pocket wizard. The fill light uses the same soft box as the main light and also uses an SB800. It is placed behind me and slightly above.
With the on-axis fill flash power dialed down to a pretty low output, I start to gradually introduce fill into the image. The shadows are ever so slightly lifted (not as deep), but the other qualities still remain in that the light is still soft and we still maintain our modeling on the subject.
Increasing the fill light by one stop, I am still keeping the ratio between the fill flight to main light pretty small, but I lift the shadows even more and still without reducing our modeling. So the shadow transition is still lovely and soft but the shadows are less deep.
Mannequin lit by single soft box: Approximately 2 feet away & On-Axis fill (increased by one more stop)
Increasing the power of the fill light again, we change the ratio between main to fill, reducing the depth shadows even more.
There is no right or wrong ratio to use between main to fill. Some people suggest a 2:1 ratio in that the main light is twice as powerful as fill. But this is a subjective thing. The fact is that the fill light controls the contrast in the shot and only the photographer knows what contrast they want. So it’s done to taste. The trick is though, to keep it on-axis