Exposure is to let the image passing onto the emulsion layer of film. The different brightness and color of the frame will make a latent image, which must be processed with the right kind of chemical to make it into postive or negative image.

The light passing through must be controlled to the proper quantity, too much will cause over-exposure, too less will cause under-exposure. Both will give an image without partial or all details. Over-exposure will make slide white out and negative darken; under-exposure will leave the slide dark and the negative blank. Exposure relies on aperture to control the volume of light and the shutter speed to control the timing.

The setting of these controls depend on estimation or metering. Estimation is built up by experience with reference to hints from film's guide. Since it requires lots of trial, almost no one would employ this method. Meter and camera metering system works easily and becomes the only method nowadays. Even if the result of metering is in EV, it is easy to convert into aperture and shutter speed. This is the most effective way, though we need a proper metering technique to make it right. Please refer to the Metering chapter, it will not be repeated here.

Since mechanical shutter speed may have error in timing, normally in long or very short time. Electronic controlled mechanical shutter (usually called electronic shutter) is much better, especially for longer time. In general, 1/60 and 1/125 sec. are the most accurate speed. Of course, the timing errors of shutter is usually less than half a stop. It alone will not affect the final image too much. But if there are other error, e.g. processing, the overall error may be greater.