There is nothing difficult about taking a photograph of a scene reflected in a mirror. What the eye sees when it looks at the mirror from the camera position will come out in the photograph exactly as though the objects reflected were being photographed directly, but there are one or two points that must be remembered.
The reflection in the mirror must be regarded as being as far behind the mirror as the object is in front of it, so the lens must be focused on that distance plus the distance from the camera to the mirror. It must not be focused on the surface of the mirror. For example, suppose the photographer is taking his own portrait and he is 3 feet from the mirror with his camera 1 foot in front of him. His reflection will be 3 feet behind the mirror and, as the camera is 2 feet in front, then the total distance from the camera to the reflection will be 5 feet. Accordingly the camera lens must be focused on that distance.
This simple relation only holds good so long as the camera is looking straight into the mirror. If it is pointed at an angle, to photograph objects lying to the right or left or above or below, then the focused distance must be equal to the distance from the camera to the object via the mirror measured along an imaginary line joining the two. i.e: along the path of a ray of light which proceeds from the object to the mirror and is reflected from the surface into the camera lens. This distance can be found by measuring from the object up to the surface of the mirror in the direction of the reflection of the camera, and from that point on the surface of the mirror to the camera itself.
With rangefinder or screen focusing, so long as the reflection and not the surface of the mirror is focused on, the image will be sharp, no matter what the angle of the line of sight or the distances of subject and camera from the mirror.
Effect of Glass
The ordinary domestic "looking-glass" is silvered on the back of the glass, and there is always a dim secondary reflection from the surface of the glass in addition to the bright reflection from the silvered surface. At a distance of several feet, this double reflection is not usually noticeable, but it may be objectionable in close-up pictures. The answer then is to use a polarizing filter in front of the lens, turning it until the surface image is no longer visible in the focusing screen, or when viewed directly through the filter.
The image reflected by the mirror is reversed from left to right. This does not matter with some subjects but it is undesirable in portraits and pictures where there is any printed matter. If the photograph is intended to look what it is, a mirror reflection, then there is no need to do anything further about it, otherwise the negative must be printed with the emulsion side away from the paper to cancel out one reversal with another.
Photographs may be taken of subjects reflected in the surface of distorting mirrors of the type set up in amusement arcades and fairgrounds. Here the same rule holds good as for plane mirrors, but it is not possible to fix the position of the reflection accurately. With rangefinder or screen focusing the lens should be focused on the important part of the reflection, e.g: the face-and a small stop used to bring as much of the rest as possible into focus.
With a scale focusing camera, all that can be done is to set the lens as for photographing in a plane mirror and then use the smallest practicable stop to extend the depth of field sufficiently to cover any errors.
Where the mirror has a regular curve, e.g: spherical or parabolic-the image can be focused directly by rangefinder or screen in the normal way. But scale focusing by any of the methods described above is useless because the image may lie farther away from or closer to the surface of the mirror than the object reflected. The position of the reflection can be found from the fundamental optical equation if the focal length of the mirror is known. The focusing scale can then be set for the total mirror-reflection and mirror-camera distances.