As already discussed, focus is fundamental to photography, both in terms of what is - and what is not in focus. The rangefinder camera determines what should be in focus without actually demonstrating the degree (depth of field). The TLR goes one step further, by using a second viewing lens with identical characteristics to the taking lens. An image is formed on a ground glass screen by the viewing lens, which is completely independent from the one that takes the picture, this image is viewed from above by the user directly or through a magnifier. This allows the user to view the picture almost exactly as the taking lens will. However the image will be mirrored in the viewfinder and depth of field will only relevant if the taking lens is not stopped down. A significant proportion of TLRs are fitted with ring set diaphragm shutters, those that are equipped with flash will synchronize at all speeds. Whilst the TLR does address the focussing issue, they don't generally have interchangeable lenses, close up photography can be awkward, are bulky and the view is mirrored. At one point almost all camera makers had a TLR in their catalogues - almost invariably 6x6cm format, but the design doesn't fully address the focus or view issues and is now all but extinct. The last ones in production are the Chinese Seagulls, the current model being the late series 4A, a significant improvement over our own early Seagull 4 of the mid 1960s.

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