multiple times, negative exposure compensation must first be set to avoid overexposure. For example, to expose the frame twice with correct exposure, a −1 EV compensation have to be done, and −2 EV for exposing four times. This may not be necessary when photographing a lit subject in two (or more) different positions against a perfectly dark background, as the background area will be essentially unexposed. Medium to low light is ideal for double exposures. A tripod may not be necessary if combining different scenes in one shot. In some conditions, for example, recording the whole progress of a lunar eclipse in multiple exposures, a stable tripod is essential. More than two exposures can be combined, with care not to overexpose the film. Digital Digital technology enables images to be superimposed over each other by using a software photo editor, such as Adobe Photoshop or the GIMP. These enable the opacity of the images to be altered and for an image to be overlaid over another. They also can set the layers to multiply mode, which 'adds' the colors together rather than making the colors of either image pale and translucent. Many digital SLR cameras allow multiple exposures to be made on the same image within the camera without the need for any external software. And some bridge cameras can take successive multiple exposures (sometimes up to nine) in one frame and in one shot. It is the same with high-dynamic-range imaging, which takes multiple shots in one burst captures, then combines all the proper shots into one frame. Adoption in fine art In addition to direct photographic usage of the technique, fine artists' work has been inspired by the multiple exposure effect. Examples include Joan Semmel's oil on canvas "Transitions" from 2012, and Ian Hornak's acrylic on canvas "Hanna Tillich's Mirror: Rembrandt's Three Trees Transformed Into The Expulsion From Eden", from 1978 (depicted). Long exposures With traditional film cameras, a long exposure is a single exposure, whereas with electronic cameras a long exposure can be obtained by integrating together many exposures. This averaging also permits there to be a time-windowing function, such as a Gaussian, that weights time periods near the center of the exposure time more strongly. Another possibility for synthesizing long exposure from a multiple exposure is to use an exponential decay in which the current frame has the strongest weight, and previous frames are faded out