Multiple exposure photography, as its name suggests, consists of superimposing a number of different images onto the same frame. A simple and often-used example is the addition of the Moon, usually much larger than its actual size, in the sky above a low-light scene such as an illuminated cityscape. There are four basic approaches to creating this type of image - in camera, in the darkroom, slide sandwiching, and digital manipulation.
To create multiple exposures in-camera it is most convenient to have a camera with a multiple exposure function. In a film camera, this allows the shutter to be reset and fired again without advancing the film. In a digital camera it is essential to have a multiple exposure facility.
It is important to plan carefully the whole sequence of exposures before beginning. It is worth drawing diagrams showing the location of each element of the final image, and also making notes of exposures settings and the sequence in which exposures must be made. In the example of adding the Moon to a night-time cityscape, it is essential to provide a sufficient area of clear dark sky within which the Moon can be located. The night sky needs to be almost completely black to avoid ghostly images of clouds being superimposed across the face of the Moon.
Basic exposure settings are calculated according to the number of images to be superimposed. Where two images are to be superimposed, the exposure of each should be halved - reduced by one stop. Where three images are to be superimposed, the exposure of each should be one third of the normal exposure - reduced by one-and-a-half stops. Where four exposures are used, the exposure of each shot should be reduced by two stops. However, this is only a general guide. Ideal settings will be determined by the subject matter and it is therefore wise to repeat the process several times with a range of exposure settings. Where a subject is superimposed on a completely black area of another scene, the exposure of each shot may remain as normal since no additional light is added to any exposed area of the image.
In the case of a digital camera featuring a multiple exposure setting, the photographer has less in-camera control and can do little more than compose the sequence of exposures and rely upon the manufacturer's instructions and electronics to do the job. However, users of digital cameras have the advantage of being able to view the result immediately and repeat the process if the result is unsatisfactory.
Multiple exposures can also be made in a traditional darkroom by printing more than one negative onto the same sheet of paper. The easiest method is to print the negatives in turn so that the images overlap, but it is also possible to use various masking techniques to prevent light from a particular image reaching selected areas of the paper.
Slide sandwiching involves using existing slides simultaneously to produce a single image. The technique has the advantage that the result can be seen immediately and adjusted to achieve the desired result. In most cases it is best to use a strong image such as a silhouette combined with weaker images such as those dominated by a seascape or an interesting sky or sunset. Slides with good colour saturation and a lot of detail are generally very difficult to combine.
Combining multiple images by digital manipulation is now by far the most commonly used technique. Software such as Photoshop is so versatile that a skilled worker can achieve almost any desired effect from a number of suitable images. This subject is vast and cannot be addressed here, but with the selective use of layers, masks and various blending techniques almost any desired result can be achieved.