Double or Multiple Exposure
We conceptually think of a photograph being made by taking a single image, its a frozen piece of time with a definable aperture, and colour balance.
Suppose however that we combine two or more pieces of time with perhaps different arrangements, then we have an array of creative possibilities. Many of the effects we can create are different from those that would be recorded in a single image.
I am not limiting it to just two photos, many cameras have the ability built in to take a umber of photos and combine the results. The Nikon D80 for example can take 3, and the D200, D300, D2 and D3 cameras can take ten that can be combined. In each of these cases you have the option to select the number of shots and decide if you want the gain set automatically, or you are going to make the settings for each shot yourself. This is on the shooting menu under multiple exposures. Some cameras within other makes will have a similar arrangement.
The auto gain feature just divides up the exposure equally amongst the number of exposures so that they add up to the same exposure that you would have got from a single photograph. This is useful when just shooting the image in a number of combined shots, for example so as to avoid long exposure noise. Its also quite useful for fun shots like photographing children as ghosts. It's however not always ideal when using the multiple exposure technique to get an effect that you want.
Lets look at some ways we could use this:-
Equivalent of a hard grad. Set the camera on a tripod to take two images, and auto gain on, put a piece of black card over the brightest part of the image, usually the sky, you can just hold it or cut it to slip into a filter holder. Take one shot with it in then take it out and take the second shot. You have a 1 stop difference between the two parts, and the edge should be sufficiently out of focus to not be visible. Some like to tear rather than cut the card, to get a less precise edge and some like to tear a specific card for a scene, making the shape of the edge of the graduation match their needs. this is the equivalent of a ND2, to get two stops, the equivalent to a ND4, set the camera to take three shots, and have the card in for two of these, so as to get slightly more graduation slide the card up slightly after the first shot.
Similar to a soft grad. Similar to the above but you use a larger number of shots, and either turn auto gain off and work out the exposures or have a number of steps and then the same number of exposures with no card at all, producing a very similar result to a soft ND2. Card in for 6 shots, at different places and out completely for 3, to get the same as a soft ND4. Again you can tear the card to give you a specific shape. The larger the aperture (smaller f number) the better this works.
Multiple angle grad effects. As above either with a holder and several pieces of card or you can do it with a simple holder and having a card in all the time but from different directions between shots. Just remember that the effect is the combination of the exposures, see below.
Multiple exposures of different exposures. The effect of a multiple exposure is to combine the images rather than just sum the exposures. The effect is not the same and with a little experimentation you can get a greater tonal range via adding images together than you would by taking a single exposure. So for example taking a photo in a room with a window, you would either normally have a black room and see through the window or a room exposed correctly and the window overexposed. To get them both is tricky, but possible using multiple exposures but you may still need to so some editing to get the ideal results. This works as you are adding the images together and not the exposures. The effect can also produce similar results to HDR editing. Don't expect to master this on the first attempt, it takes practice and a lot of thinking about.
Multiple coloured grad effects. Similar to the multi angle grad effect above but using coloured grads.
Selective coloured grad effects. Another variation but using a black card torn to block out some of the exposure or all of the exposure while the coloured grad is in place.
Reverse grads, ND or colours. Using the same effect as the hard or soft grad but with the grad in for more time, but the grad upside down so that rather than going from the soft side, you have the top edge in the centre of the image area.
Producing ghosts. Put the camera on a tripod, point it at a child in a chair, set the camera to take 3 multiple exposure shots, and gain to auto shots. Take one shot of the child in the chair then get them to move away without disturbing the chair and take the other two shots.
Multiple copies of a person in a night shot, for example a person pushing themselves on a swing with three other copies of themselves watching. Set the camera on a tripod in a dark garden, using a flash determine the correct exposure for the image when the flash is a set distance from the subject, (fire the flash manually or from the camera in commander mode). knowing the exposure you can set the camera on manual, and the multi exposure setting for the number of shots and auto gain to off. You can separate the camera from the flash and set it off in a number of ways. You could get an assistant to hold the flash, use a second tripod or lighting stand or use a remote release or self timer. You move the person to a new location and set the flash the same distance from the person for each shot, in the sequence. Try not to have anything between the subject and the camera, if you use an assistant it may help if they are dressed in black.
Two halves of a picture. Two exposures, one with a black card one way and one the other, but with some change between, perhaps its the direction of the subjects eyes allowing a picture with two eyes pointing outwards, then join up the nose, or perhaps a split focusing arrangement where the lower part of the image is focused near and the top part focused further away. Although mostly done with two images, if each individual image was composed of several shots with a slight move of the card between, the join would be far less obvious.
Light people. Similar to the ghost images but using a larger number of shots, the people should be lighter, but as the images rather than the exposures are added together they will still be visible, where on film they would not have been. This may make an interesting shot for a tourist site or similar. To make the people completely disappear use a high value ND filter allowing a time exposure instead.
Waterfall magic, with a longer time exposure you get the misty water effect, with short exposure you get detailed water. Combine them so that they form a single picture and it appears that you have more water, the mist of the longer exposure, but some detail from the shorter one.
These are a few ideas, you may be able to come up with others.
You could also experiment with scissors with shaped edges, you can get cheap ones at art shops, or better made ones but in less variety from sewing shops.
You could also make up a set of cards with holes cut out, the possibilities are endless.
Points to remember
See also: Filter Section for more articles.