What is the key feature of a tintype photograph?

A tintype photograph is characterized by being a positive image created on a thin sheet of metal, specifically a tin plate. This technique was popular during the mid to late 19th century and stands out due to the way it captures images directly onto the plate without the intermediary of a glass negative. The uniqueness of the tintype lies in its durability and the distinctive aesthetic qualities that result from its production process. The metal plate is coated with a light-sensitive emulsion, and once exposed in the camera, it results in a direct positive image.

Other options describe features that do not align with the tintype method. For instance, a negative image pertains to traditional film photography, which inverts the colors and light values, while tintypes produce a direct positive. Extensive exposure time can apply to various early photographic techniques, but tintypes were notably quicker than some processes, often taking seconds rather than minutes— thus, it doesn't serve as a defining feature. The mention of a digital sensor pertains to modern photography, making it irrelevant in the context of tintypes, which are strictly analog.