Dear Robert and others:
The question has arisen about “hot spots” on foreheads, noses etc., etc. and how to control them. This is an excellent question because people rarely attribute this problem to what actually causes it and sadly, most people who teach about professional lighting techniques, rarely even mention it.
The “hot spots” of which we speak are actually reflections called SPECULAR HIGHLIGHTS. The word specular literally means “mirror like” so we are talking about mirror like reflections of our light sources themselves as seen in the subject’s skin. To help understand precisely what a specular reflection is, it helps to understand what a diffused highlight is as well as the other components of light.
Let’s say we have a blue beach ball and we place a light source on one side to light it. Now as we look at it, there are four distinct components of the lighting that we can control and they are all linked to each other in varying degrees.
They are:
DIFFUSED HIGHLIGHT
SPECULAR HIGHLIGHT
SHADOW (also correctly called the “umbra”)
SHADOW EDGE (also correctly called the “penumbra”
The SHADOW of course is the area not lit by the main light source. In portrait as well as other types of photography, we control just how dark the shadow areas of the subject appear by using a separate light source called a fill light. In some cases, a non powered reflective surface is used instead to redirect a portion of the main light back into the shadow areas of the subject to control them.
The SHADOW EDGE or penumbra is that separation line between the diffused highlight and the shadow. It is neither highlight or shadow but rather a transition between the two. When astronauts observe the Earth from space and see the fuzzy line that separates night and day, it is call the terminator. It is precisely the same as the shadow edge created when lighting a portrait but simply on a much more grand scale.
Just how abrupt the change is for this dividing line or SHADOW EDGE is determined by the size of the light source. The larger the light source, the more gradual the shadow edge will be. Smaller light sources create a more abrupt transition between shadow and diffused highlight. Photographers sometimes use the term “harder” to describe the look created by smaller light sources because the shadow edges have a harder look whereas larger light sources produce a softer look to the shadow edges. It is precisely this characteristic that allows photographers to control how a portrait looks by changing the size of the main light source.
The true tonality of the object is called the DIFFUSED HIGHLIGHT and in our beach ball, this is the brightest part of our subject that is lit by our main light source and appears blue. The light that strikes the surface is bounced away in random directions. Just how random is controlled by the SURFACE EFFICIENCY of the object. Surface efficiency refers to how smooth or rough the surface is. A smooth surface such as that of a billiard ball, is said to have a high surface efficiency while a pillow would have a low surface efficiency. The higher the surface efficiency, the more you will be able to see a reflection of the light source itself.
This SPECULAR HIGHLIGHT is controllable by the photographer as to how bright it appears in a photographic image. The key thing to remember is that the brightness level of the specular reflection is always RELATIVE to the brightness level of the diffused highlight. When we affect a change in one, we affect a change in the other.
Now, what are the mechanisms used to control the brightness level of the SPECULAR HIGHLIGHT relative to the DIFFUSED HIGHLIGHT? The answer lies in the nature of what a specular reflection is. A mirrored reflection of the light source itself. The best way to illustrate this is to conduct a test. Let’s pretend that your portrait subject is standing before you, holding a mirror in front of their chest. A mirror large enough for us to easily see a reflection of the light source that is lighting them. Let’s say that we are using a 2 foot square soft box as our light source. We will assume that the surface of the soft box is relatively uniform in it’s brightness levels from corner to corner and edge to edge. This is a feature that soft boxes generally do well at.
Ok, let’s place our light source at a distance of 4 feet from our subject holding the mirror. Now, let’s use a spot meter than enables us to measure the brightness of a specific area of an object. Remember, we are not measuring the light striking our subject (as in an incident reading) but rather the light coming from it. This is no different than being outdoors, and taking a reflective spot meter reading off of the side of a building. We are reading the light reflecting off of that object. In the case of the soft box, the light is emanating from within but the principle is precisely the same.
Ok, let’s take a spot meter reading of our soft box as seen through the mirror. We just point the meter so that the bull’s eye is centered on the soft box that we see in the mirror. We take a reading and make note of it. Now as the subject remains in place, we move the soft box from 4 out to 11 feet. We can still see it reflecting in the mirror being held by the subject but it looks a bit smaller now. Let’s take a new spot meter reading of our soft box as seen through the reflection in the mirror. What do we notice about this reading as compared to the first? They are precisely the same. Why is this? Because the soft box never changed in it’s brightness level. We simply moved it away from the subject. Taking a spot meter reading of several square inches of it’s surface area will yield the same brightness level even though we have adjusted it’s distance.
So now we know that the brightness level of a specular reflection of the light source never changes when it is moved. It just gets larger or smaller. So why then do we have to take a new INCIDENT meter reading to determine our taking aperture anytime we move our light source nearer or further from our subject? This can easily be demonstrated by sitting on the posing stool and observing the size of the light source as someone pulls it away or moves it in closer to you. From the point of view of the subject, if we move the soft box out to 11 feet, it looks considerably smaller than if it is moved in to only 4 feet. At 4 feet, the fact that the box is filling more real estate in front of the subject, is why the subject, now receives more light and we have to adjust our aperture accordingly. Think about opening the curtains for one window in your living room about half way to let in light. Now open the curtains up all the way. More light. Same principle.
Ok, now that we have that figured out, let’s discuss human skin and see how these so called “hot spots” are created. If we looked at our skin under a microscope, we would observe that it is covered with microscopic droplets of oil and moisture. Any liquid has a high surface efficiency like that billiard ball we spoke about earlier. The more oily our skin is, the more surface efficiency and the more we will see specular reflections of light sources on our skin. All those tiny droplets of oil and moisture add up to a reflective surface. More or less depending on just how dry our skin is. This is why women use powder. They are adding material that both absorbs moisture, and also adds to the roughness of the skin thereby reducing the skin’s surface efficiency. The result is more diffused highlight area showing the natural tone and color of the skin and less specular highlights or “hot spots”.
For generations, photographers have asked why African Americans or any darker skin person tends to look more oily than a person with light skin. The answer is quite simple. The specular highlights on the darker skin have a darker background to be seen against. In reality, it is only an illusion of greater contrast. Nothing more than that. Sadly, many photographers believing that it was the right thing to do, moved their main lights further away when photographing people with dark skin. They thought that it would reduce the oily look to the dark skin. Unfortunately it accomplished precisely the reverse effect.
With this knowledge of light, how then can we control effectively just how oily a portrait subject looks with those dreaded “hot spots”? The answer is decidedly simple. Let’s pretend that we have our soft box located at 11 feet from the subject. We can see it’s effect on our subject’s face and we can also see it reflecting in the mirror that the subject is holding. That mirror is nothing more than a larger version of all the tiny mirrors that live on the skin in the form of oil and moisture droplets.
If we take an incident reading of our main light from the position of the subject with the dome of our meter facing the main light, we will have a recommended aperture to set our camera on. Let’s pretend that it is f 4. Let’s set our camera’s lens to f 4 and make an exposure.
Ok, now let’s move our soft box in to a distance of only 4 feet from the subject. We take a new meter reading just as before from the position of the subject with the dome pointing at the soft box. Now what do we read as the recommended aperture? The new reading is f 11. A smaller aperture because we now have three f stops more light on the subject than a moment earlier. We adjust our lens to f 11 and make an exposure. Let’s print both of these images so that the subject’s flesh tones are identical. If both exposures were correctly metered and the aperture’s set correctly on the lens, we should have two identical looking images but with one difference. If you remember, the brightness level of the soft box surface measured the same regardless of its distance to the subject. What this means is that when we stopped down by three f stops, to maintain a proper exposure for the diffused highlights, our side benefit is that we now underexposed all of the specular highlights by precisely three f stops. The soft box that is seen as bright white in the first image, now looks 3 f stops darker in the second image. In like manner, the thousands of tiny specular reflections on the surface of the skin creating those “hot spots” that we don’t want have also been reduced by precisely 3 f stops. Those bright oily looking “hot spots” that are so troublesome on the skin, have been all but eliminated by reducing their brightness level RELATIVE to the diffused highlight by 3 f stops. The brightness of the specular reflections never really changed but because we had to stop down, we reduced the brightness level of the specular reflections.
To sum all this up, larger light sources used closer to the subject will reduce specular highlights relative to the diffused highlights. Smaller light sources on the other hand used further away increase specular highlights relative to the diffused making your subject look more oily.
Other benefits to working with light sources closer to the subject, include far greater control over the amount of main light reaching the background. Simply put, moving your light source in closer to the subject, will actually create less light on the background relative to the subject rather than more which is what many people mistakenly believe.
It all comes down to a few simple scientific principles. If you understand these, it becomes relatively easy to manipulate and control the various elements of light. With this knowledge, you can achieve your photographic goals, and spend your valuable time creating new images, rather than making unnecessary corrections with Photoshop.
I hope this has been helpful.
