Multiple exposure is a photographic technique in which two or more separate exposures are recorded onto the same frame of film or the same digital image file, superimposing the images from each successive exposure on top of one another to create a single composite picture that combines elements from all of the individual exposures. The technique has been used creatively and technically since the earliest days of photography, and continues to offer a unique and distinctive approach to image making that produces results with a character and visual quality quite different from digitally composited images created in post processing.
The creative possibilities of multiple exposure are enormously varied and have been exploited by photographers across virtually every genre and discipline. One of the most classic applications is the creation of images in which the same subject - typically a person - appears two or more times within the same frame, achieved by photographing the subject in one position, asking them to move to a different part of the frame, and making a second exposure of the same frame without advancing the film. When executed carefully with appropriate attention to the tonal values of the background and subject, the result is a convincing image in which the subject appears to be present in two or more places simultaneously, an effect that was once achievable only in the darkroom through complex printing techniques but can be captured directly in camera using multiple exposure.
Another widely used creative application combines images made with lenses of very different focal lengths to produce surreal or dramatically scaled composite images that would be optically impossible to achieve in a single exposure. A classic example is the combination of a telephoto shot of a large, detailed moon with a wide angle landscape photograph, the two exposures together producing a single image in which an impossibly large and detailed moon dominates the landscape in a way that is visually compelling precisely because it exceeds the optical limitations of any single lens and exposure.
Multiple exposure can also be used for more subtle and abstract creative effects, including the layering of textures, patterns, and tonal gradients from different subjects to create complex and richly detailed images with a painterly or dreamlike quality. Long exposure star trail photography achieves its characteristic circular arcing trails through a form of sequential multiple exposure, stacking many individual exposures of the night sky made over the course of hours to accumulate the apparent movement of the stars across the frame into a single dramatic final image.
Achieving multiple exposures on film requires the camera to be capable of making a second exposure on the same frame without advancing the film between shots. On cameras with automatic film advance this presents a significant practical challenge, as the film advance mechanism is typically linked directly to the shutter cocking mechanism and advances the film automatically after every exposure. Some photographers overcome this by carefully holding the film rewind button while cocking the shutter manually to prevent the film from advancing, though this approach requires skill and practice to execute reliably. Recognising the creative demand for the technique, many camera manufacturers have incorporated a dedicated multiple exposure mode into their more advanced camera models, automating the process of making successive exposures on the same frame and in some digital implementations automatically blending the exposures as they are made to provide a real time preview of the accumulating composite image.
In digital cameras, multiple exposure modes offer additional creative control compared to their film counterparts, typically allowing the photographer to select from several blending methods - such as additive, average, bright, or dark blending - that determine how the pixel values of successive exposures are combined to produce the final composite. Some digital systems also display a semi-transparent overlay of the first exposure in the viewfinder or on the LCD screen while the subsequent exposures are being composed and made, greatly simplifying the precise alignment of overlapping elements that more complex multiple exposure compositions require.