When photographing an object with a smooth surface, the reflection of the light source that can be seen in the object is known as the specular highlight. The size and characteristics of this specular highlight change with the distance to the light source and the size of the light source. As the size of the light source gets larger or is moved closer, the highlight appears larger and more spread out. Since it is spread out over a larger area on the surface of the object, it appears less intense. In other words, the highlight is less bright as it gets larger. The inverse is true as well, if the light source is moved away or made smaller, the specular highlight will appear smaller and brighter.
This becomes useful in predicting the size and intensity of light on your subject. Catchlights in the eyes of a portrait subject is a good example of these principles at work. Photographers who prefer a smaller, brighter catchlight simply need to use a smaller light source or keep the light source at a longer distance from the subject. Photographers who use large softboxes and keep them close to the subject will see large reflections in the subject’s eyes which are much less intense.
The specular highlight is not the only thing that changes when the light source size or distance to subject changes. Be aware of the other results that will affect the mood of your photo such as brightness and softness of shadow edges, each time you change your light source.
Another noteworthy thing to consider is the shape of your light source. Photographers can take advantage of the many available light modifiers to change the shape of the reflection. You may like a round highlight more than a rectangular one for a particular shoot. To achieve this, simply use an umbrella rather than a softbox. Another common example would be using a narrow softbox to “draw” a line along the side of a reflective object to draw attention to its shape. Using the principles of shape control and specular control together will help you achieve the look you want.