Pulling focus to surface reflections — like a mirror — is not as uncommon of a shot as you might think.
How many times have you seen an over-the-shoulder of somebody at a bathroom sink? Or a character looking longingly through a window pane?
Like any shot, these moments require sharp focus to be effective, but when you pull your tape measure out, what do you measure for focus marks?
It’s not as simple as you may think.
To pull focus to a reflection, you have to take two measurements:
- Measure from the film/sensor plane to the reflecting surface (i.e. a mirror)
- Measure from the reflecting surface to the subject
- Add the two distances together to get the correct focus mark
In this scenario, the shot will hold the same principles of depth-of-field. So, if the subject is further away from the reflecting surface, objects closer to it will be more out of focus.
Follow the Optical Path
Basically, you need to follow light through its entire optical path up until it enters the lens. Even though an image appears to be at the mirror, it is in fact away from the mirror.
The light from a reflection is actually traveling to the mirror, then from the mirror to the lens — which is why you must measure those same distances.
If you measure only to the mirror, you will focus on the surface, but not necessarily the subject reflected within it.
(I don’t want to get into dirty details about lens optics, but you can check out an introduction to the topic over at Digital Photography School)
For a better visualization of this phenomenon we can look to Toy Story 3.
In the scene where Woody escapes from Sunnyside daycare, there is a great rack focus that illustrates the differences between focusing at the mirror and through the mirror.
Click to enlarge
In this case, Pixar took advantage of this effect for a small shock moment and subsequent reveal.
As you can see in the side-by-side, there is a noticeable difference.
In one image, Woody is sharp while the janitor and smudge (both objects near the mirror) are soft. In the other image, the opposite is true.
Double the Marks
Checking marks before you roll camera is always important, but is twice as important in this instance.
When pulling focus to reflections, you actually have two different marks to account for — from the camera to the reflective surface and then from the surface to the subject.
If you are in a position where the subject and the camera are both dynamically moving, you will have to be extremely accurate with both measurements to keep the shot in focus.
Either that or beg the director of photography (DP) to give you a decent amount of depth-of-field!
Pulling focus to reflections is a unique, albeit common scenario, but it’s no different than any other shot — the same things that make you a good focus puller still apply.
P.S.
For those using laser measuring devices: I tested my Hilti PD-40 on a mirror and it measured the total distance. However, I also tested it on a number of semi-reflective surfaces (like glass, windows, etc.) and it only measured to the surface — not the full optical path. Make sure you test your laser measuring device on different surfaces to know what it’s capable of.