In addition, we’ve added a 22 inch LiteDisc reflector to bounce some of the stray light onto the dark side of our subject. 6, we see a typical setup where the shadow of the subject and the incident reflection are avoided. As long as the light is off-axis from the background, the hot spot will not appear in the photo. Shooting directly into an on-axis background is easily accomplished by using an off-camera light source positioned to the left or right of the camera. Some situations with an on-axis background include photographing flat art, showing a person in front of a large window, or showcasing a feature of an architectural interior. However, there may be times when you may want the wall on axis behind your subject. (figure 4)įigure 5 shows the improved result when we change an on-axis flash to an off-axis flash.
In the diagram of figure 4 we see that the camera is now off-axis from the wall, and the reflection from the camera flash is no longer bouncing back into the lens. Taking a few steps to one side or another causes the (camera mounted) flash to hit the background surface at an oblique angle, so the light misses the camera when it bounces back. The hotspot of light from a surface behind our subject perfectly illustrates that a flash aimed directly at a wall or window (on-axis) will bounce directly back at us. ‘On-axis’ flash, creating the ‘bounce back flash’ that frequently ruins photos, is shown in fig. If the on-camera flash is fired when the camera is on-axis, a direct incident reflection results, as illustrated. When the camera sensor is parallel to a surface in the photo, we call it ‘on-axis’ positioning. This occurs when the light from a flash hits a surface that’s parallel to the camera sensor and bounces right back at the camera. One of the most common problems in flash photos is the ‘flashback’ reflection behind the subject. That’s the most basic illustration of the concept, but as we’ll see, there’s a lot more to consider. Finally, we’ll explore the useful and fun aspects of the angle of incidence.In this simple diagram we see that a beam of light enters from the left, strikes a surface, and exits to the right at the same angle. Then we’ll provide solutions to problems that arise from unintended reflections. This often mentioned law of lighting physics is easily demonstrated, as in the photo of me holding a laser pointer, but how can it be used for creative purposes? In this lesson we’ll show you how the angle of incidence works in scientific terms. Understanding additive and subtractive lighting.Using reflections to achieve interesting effects.Avoiding common problems from unwanted reflections.Learning about this aspect of lighting all begins with the phrase: The angle of incidence equals the angle of reflection. We don’t think about the physics of light and reflections very often, but a basic understanding of light theory can be helpful. The surfaces might be skin, water, leaves, flowers, clouds, buildings, even the sky, but they’re all reflections. Unless your camera is pointed directly at a light source, like the sun or a light bulb, you’re photographing light reflecting off of a surface. So the length of the path ACB is the shortest possible between A and B which touches the mirror.Almost all photographs are simply a record of reflections. The line segment AB' is the shortest of all possible paths between A and B'.ĪB' intersects the mirror at C. How can you draw the the light ray so it reaches B in the shortest time? It must travel along the shortest path.ĭraw the mirror image of B: it is B'. You want the light reach from A to B in the way that it is reflected from the horizontal mirror. In other words, a ray of light prefers the path such that there are other paths, arbitrarily nearby on either side, along which the ray would take almost exactly the same time to traverse.
However, this version of the principle is not general a more modern statement of the principle is that rays of light traverse the path of stationary optical length with respect to variations of the path. This principle is sometimes taken as the definition of a ray of light. In optics, Fermat's principle or the principle of least time is the principle that the path taken between two points by a ray of light is the path that can be traversed in the least time.
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