A softbox is a light modifier consisting of a rigid or collapsible box-shaped housing with a reflective interior - typically lined with white or silver coated material - that attaches to the front of a studio flash head or continuous light source and incorporates one or more translucent diffusion panels across its front face through which the light exits towards the subject. By enclosing the flash head within a large, internally reflective chamber and forcing the light to pass through a broad, evenly illuminated diffusion panel before reaching the subject, the softbox transforms the small, intense, hard light source of the bare flash head into a large, soft, and even source of illumination whose quality and character is determined primarily by the size of the softbox's front diffusion panel relative to the subject being photographed.
The fundamental optical principle underlying the softbox is that the larger a light source appears relative to the subject it illuminates, the softer and more wrapping the quality of light it produces. A bare flash tube is an extremely small light source that produces hard, specular illumination with sharply defined shadow edges and high contrast between lit and unlit areas - a quality of light that can be dramatic and graphic but unflattering for most portrait and product applications. By enclosing the flash in a large softbox, the effective size of the light source is increased from the few centimetres of the flash tube to the full dimensions of the softbox front panel - which may be anything from thirty centimetres square for a small portable softbox to one metre, two metres, or larger for the biggest studio softboxes - dramatically changing the character of the light from hard and specular to soft and wrapping, with shadow edges that transition gradually and smoothly rather than abruptly, and illumination that follows the contours of the subject from multiple angles simultaneously rather than casting a single directional shadow.
The interior of a softbox is typically lined with a highly reflective white or silver material that bounces and redistributes the light from the flash head within the box, ensuring that the entire front diffusion panel is illuminated as evenly as possible before the light exits through the diffuser. Many softboxes incorporate an inner baffle - a secondary diffusion panel positioned within the box between the flash head and the front panel - that further homogenises the light distribution within the box and prevents the hot spot directly in front of the flash tube from showing as a brighter central area on the front diffusion panel. The quality and evenness of this internal light distribution is an important factor in the overall performance of a softbox, with better quality softboxes producing more perfectly even illumination across the front panel and therefore a more homogeneous, graduated quality of light on the subject.
One of the most recognisable and practically useful characteristics of softbox illumination is the quality of the highlights and reflections it produces on the subject. Unlike the irregular, amorphous highlights produced by an umbrella or other non-directional diffused source, a softbox produces a clean, well defined, rectangular or square catchlight in the eyes of a portrait subject that has a neat, professional, and aesthetically pleasing appearance. Similarly, reflections of a softbox in shiny surfaces - polished silverware, glassware, jewellery, automotive bodywork, and other reflective products - appear as clean, well defined rectangular forms of graduated tone that reveal the shape and curvature of the reflecting surface in a controlled and attractive way, making softboxes particularly well suited to product photography where the management of reflections is a primary concern.
Softboxes are available in a very wide range of sizes and shapes, each producing its own characteristic quality and coverage of illumination. Small softboxes of approximately 30 to 60 centimetres in their larger dimension produce a relatively concentrated soft light suitable for portrait headshots, small product photography, and beauty close-ups where a defined but soft source is required. Medium softboxes of 60 to 100 centimetres provide versatile, flattering illumination for three-quarter and full length portrait work and medium sized product photography. Large softboxes of one metre and above produce very soft, enveloping, and even illumination that wraps around subjects from multiple angles, reducing shadow intensity and providing the kind of broad, even lighting quality suited to group portraits, full length fashion and beauty work, and large product photography.
Octagonal softboxes - known as octoboxes - are a popular variant in which the front panel is octagonal rather than rectangular, producing a rounder, more naturally circular catchlight in the eyes that many portrait photographers prefer for its resemblance to the circular catchlight produced by a large window or skylight. Strip softboxes are an elongated rectangular variant - typically very narrow in one dimension and considerably longer in the other - that produce a strongly directional soft light suited to rim lighting, hair lighting, and the creation of graduated lighting on backgrounds, exploiting the narrow profile of the source to produce a well controlled beam of soft light that can be positioned and aimed with greater precision than a square or octagonal softbox.
The relationship between softbox size and the power of the flash head required to achieve adequate exposure is an important practical consideration in studio lighting. Larger softboxes with greater front panel areas absorb and scatter more of the flash output within the box before it reaches the subject, reducing the effective light output compared to smaller softboxes or unmodified flash heads. Very large softboxes therefore require correspondingly more powerful flash heads to achieve the aperture and depth of field settings required for a given subject, and are best paired with high output studio flash generators or monolights of adequate power. Reciprocally, the large, soft, and even quality of light produced by a large softbox often allows shadows to be adequately filled without the need for a separate fill light, reducing the total number of lights required for a balanced studio setup and partially offsetting the higher power requirements of the large modifier.