Hey Doc

In photography, multiple exposure can be done either in-camera or in the darkroom. Before the digital revolution in cameras, most of the film cameras I played with all have in-camera multiple exposure feature. This should be easy to do with the modern digital cameras but in thousands of digital cameras on the market, you get this feature only in few high-end DSLR cameras.

First thing of doing in-camera multiple exposure is to decide how many frames will be in this image. Because it has to do with the exposure compensation in each frame.

In film cameras:
For negative film
2 frames = use -1EV for each frame
4 frames = use -2EV for each frame
8 frames = use -3EV for each frame
For slide film
2 frames = use +1EV for each frame
4 frames = use +2EV for each frame
8 frames = use +3EV for each frame
In digital cameras:
I use Nikon D700 as example. In "shooting menu", select "multiple exposure", enter "number of shots", enable "auto gain" and shoot away.

In the darkroom, I mean Photoshop:
Expose each frame normally. Open all the images in Photoshop, "copy" and "paste" into one single PSD file. Each frame becomes a layer in that PSD file. The first frame will be the "background" layer, each consecutive frames will be labeled as "layer 1", "layer 2", and so on. Adjust each layer's opacity according to the table below:
background --- 100%
layer 1 --- 50%
layer 2 --- 33%
layer 3 --- 25%
layer 4 --- 20%
layer 5 --- 17%
layer 6 --- 14%
layer 7 --- 13%
layer 8 --- 11%
layer 9 --- 10%
layer 10 --- 9%

The above information was copied from a website which I have now forgot where I read it.

Have fun!