Probably the best-known film retriever is the model produced by Ilford, original supplies of which dried up when the original company ceased trading.
Why use a film retriever?
It is a situation which frequently faces professional and enthusiast photographers who need to swap films in mid roll - perhaps changing from black & white to colour, or from negatives to transparencies, and vice versa - in that having pressed the rewind button the film totally retracts into the cassette making it impossible to reload it when the need arises.
Given there is only one make of camera which provides the option of leaving the leader tongue outside the cassette when auto-rewinding, the majority of photographers are stuck with the task of either trying to open the cassette in a darkbag and rethreading it, or using a film retriever.
Making a simple job of an awkward situation, the Ilford retrievers easily locate and withdraw the leader tongue, thereby enabling the film to be reloaded in the normal way. After running on to the next unexposed frame (usually by firing the shutter release with the lens cap in place), the photographer is ready for the next masterpiece to be shot.
As is so often the case, it is the smallest elements which have the largest impact on what should be the most straightforward of tasks. Whilst those photographers who can afford it perhaps carry around more than one camera in order to accommodate their different film requirements, those in the know only need the one camera and the Ilford/Harman film retriever to cover all their shooting needs.