The principal advantages of electronic flash are that it is fast and cool, contrasting favorably with an equivalent tungsten array, which would need slow shutter speeds and produce considerable heat. Even though high output flash produces relatively slow discharges, these rarely last long then 1/1000 or 1/500 of a second, making it possible to handle most regular action, such as the limb movements of a fashion model or a splashing liquid. The low output of heat is an important advantage not simply because it makes for more pleasant working conditions in a small studio, but because it allows lamp heads to be fitted inside enclosed area lights. This type of diffusion is heavily used in still life photography, and would be impossible with tungsten lamps. Moreover, many still life sets are lit from a very close distance to achieve maximum diffusion from an area light, and many subjects – food for instance – would suffer damage and heat.

All of these advantages, however, depend on having a sufficient power output, and this depends on both the type of photography and the camera format. With continuous lighting, such as daylight or tungsten, exposure can be controlled by adjusting the aperture, shutter speed or the light. As the aperture and lighting position also affect the appearance of the image, this means in practice that changing the shutter speed is the most useful method – as long as the subject is static; it makes no difference to the style of the shot.

A flash discharge, however, is more or less instantaneous, and so should be capable of allowing the ideal aperture for a typical shot. The factors involved are: the minimum lens, working distance for the lights, and lighting attachments. In all of this, film speed is not normally an area of choice, as fine grained images are standard in most studio work.

Studio flash is rated, not by guide number, but by the electrical input. The measurements used are joules, or watt-seconds, and the most common size or power-pack is rated at about 1000 joules. Typically, such power packs can be linked in series to deliver a much higher total output, while single large power packs are available up to 5000 joules. At the lower end of the scale, units of about 200 joules and 400 joules are useful for small format cameras.

Separate power packs offer considerable flexibility, as there is usually provision for feeding a number of flash heads from one console, and the ratio can be altered. In other words, 1000 joules could be split in several ways between a few heads: 750 joules and 250 joules, or 500 joules, 250 joules and 250 joules. An alternative system is the type of flash unit that combines power pack and head in one; in some ways these are neater and have fewer trailing cables, although they are bulky.

The power output needed can only be worked out by experiment, knowing the typical conditions. For example, heavily diffused close range still life sets would need minimum aperture settings, and if an area light was typically used from two or three feet, the power needed would be in the region of 500 joules for 35mm. for a head and shoulders portrait lit with an umbrella, the power requirements would be in the order of 1,000 joules with a medium speed film. Much depends on the depth of field needed and maximum aperture of the lens but this rating will be generally satisfactory for small and medium-format cameras.