Tungsten lights, that is powerful filament bulbs of the type used in the domestic environment, have the major advantage of providing constant illumination. Unlike flash sources, the lighting can be turned on as the process of setting up a still-life subject progresses. This enables the photographer to see immediately the effects of small changes in set-up, composition and lighting - such as the direction and depth of shadows etc.
Still-life photographers typically struggle to achieve sufficient depth of field, particularly when working close to subjects such as jewellery. They also prefer to use slow ISO ratings to keep noise to a minimum. The consequence of working close to a subject at ISO 100 and F/45 is that exposures become very long and a high level of illumination is required. To make matters worse, light-absorbing diffusion screens and other light modifiers may be required, reducing the percentage of light output that actually reaches the subject. Fortunately, still-life subjects do not move, so long exposure times can be used. However there is a cost associated with providing a very high light output.
In the case of flash, all the light must be generated in a short period, and very powerful units tend to be expensive. However, when the light source is constant the output can be much lower because the effect accumulates during the exposure period. In summary, this means that constant light sources are much cheaper than their strobe equivalents.
Another compelling reason to use tungsten lighting is the vast range of products available and the extent to which they can be modified. Floodlights rated at 250W, 500W and 1,000 W lamps are available and do not have to be purchased from photographic suppliers. Nevertheless, tungsten lamps designed for photographic work are different from domestic bulbs. They use a halogen envelope system that prevents residue created by the filament out of harm's way. The lamps are thus prevented from darkening or changing in color temperature as they age. Life expectancy varies but may be 500 to 1,000 hours.
One aspect of using continuous lighting sources should not be ignored. Lamps with outputs of 500W or 1,000W emit a great deal of heat, and when they have been on for a few minutes they become very hot. Not only is there potential for the operator to get burnt, but also there is a real risk of damaging the subject. Food may be melted, flowers wilt and even plastic subjects may suffer. Take care!