Why do specular highlights often look sharp even when I'm focused on a nearby object?
Asked 7/11/2017
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I'm trying to understand why specular highlights don't always appear obviously out of focus when shooting close subjects. Since a specular highlight is a reflection of the light source, I would expect it to behave like focusing on a reflection in a mirror: if I'm focused on the object's surface, the reflected light source should often be at a different apparent distance and therefore blur.
But in real photos, highlights on shiny objects like fruit, glass, or water often still look quite sharp, and their appearance seems to depend more on subject distance and depth of field than on the distance to the light source itself. Why is that? Can a specular highlight ever be more in focus than the surface it's reflecting from?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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We see and photograph objects that are mostly illuminated by reflected light. Now most surfaces are irregular, thus the light they reflect is highly diffused. However, frequently, objects have shiny surfaces that are polished. Such surfaces reflect away a high percentage of the incident (about to hit) light. When by chance, a lustrous surface is situated so that the angle of incidence and the angle of reflection are closely equal, an image of the illuminant (light source) can be seen and photographed. This is the specular highlight you are asking about. A light source as seen in a mirror is revealed as a flawless duplicate, however, most surfaces in nature are not perfect, and we see a diffused reflection of the light source.
When we attempt to scale the distance to an object as seen in a mirror, we are frequently misled. We stand before a mirror 1 meter/yard distance and view our refection. The image we see gives us the illusion that we are situated 1 meter/yard behind the mirror. Do we focus on the mirror or on the image in the mirror? The specular highlight seen on the apple is an image of a light source that is far more distant than the apple. When focused on the apple, the reflected image of the light source will go out of focus. On the other hand, if the highlight is tiny, i.e. a tiny point, the point is focused correctly along with the rest of the apple's surface.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
9y ago
0
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A specular highlight is the image of the light source formed by a shiny surface, but on real objects it usually isn’t a perfect mirror image. Most natural surfaces are only partly smooth, so the reflected source is spread into a small, bright patch rather than a detailed scene.
That bright patch is recorded at the subject surface, so its sharpness in the photo is governed mainly by whether that part of the surface is within the depth of field. In other words, the highlight “lives” on the object in the image, even though it comes from reflected light.
Also, many highlights are just blown-out or nearly featureless bright areas. A small bright patch can still look crisp because there’s little detail in it to reveal defocus. What you mainly notice is its edge and position on the subject.
If the reflective surface is very smooth and mirror-like, and the reflected scene has detail, then focus on the apparent reflected distance can matter more—similar to what you noticed with your eye and reflections in glass. But for typical specular highlights on apples, bottles, or rippled water, surface roughness and limited detail in the reflected source make them appear sharper than you might expect.
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