Abstract images comprising multiple exposures can be created in a number of ways. When using a traditional film camera, the film was simply rewound by one frame and a second exposure made over the first. The results were relatively unpredictable unless the photographer was experienced in the field, but that was not necessarily a disadvantage. Beautiful and somewhat unexpected results were sometimes obtained even though the final image may not have been what the photographer intended.
With digital cameras it is not possible to make double exposures in a comparable manner. Multiple-exposure images are therefore created using image-processing software such as Photoshop. Two or more images are overlayed as separate layers and then merged using any one of the numerous modes available. This digital process is much more controllable than it traditional film-based predecessor and consequently leaves a heavier burden of creativity in the hands of the photographer.
The overlayed images can also be created in various different ways. One approach is to combine images of quite different subjects, and another is to combine multiple slightly different images of the same subject. Images of different subjects are best limited to two or three. Place them in a multi-layer file ans play with the relevant opacities to see what effects can be achieved.
Similar images of the same subject can be created by shifting the camera slightly between exposures. Small movements of the camera have a significant effect upon the images, so the general advice is to start with very small amounts of movement. The technique works well with particular subjects so experience must be gained in this respect before the best results are achieved. Images containing lines tend to work well providing the lines are moved incrementally in the appropriate direction. Fewer images normally need to be combined when more complicated subjects are featured.
Another useful technique is to keep the position of the camera unchanged but zoom the lens in or out between exposures. Relatively small steps should be planned at regular intervals, and zooming on a subject centred in the frame tends to be more practicable than zooming out. However, both techniques can be used successfully.
A final technique involves rotating the camera by small increments between each exposure. To achieve this in a controlled manner, mount the camera on some sort of rotating mount and plan the stops appropriately. It is also worth combining some of the above techniques, perhaps zooming in on a subject as the camera is rotated. It gets more complicated of course, but there are no limits other than the photographer's imagination.