When we shoot IR with a TTL camera (be it analog or digital) the problem is that the viewfinder is blacked out by the IR filter in front of the lens, and therefore we can neither frame nor manually focus. The reason for having this filter on the lens is indeed that of blocking all visible light! There are two alternatives to circumvent this problem, neither one particularly effective nor elegant. The first one is to use an external viewfinder mounted on the flash hot shoe. The framing (apart from parallax errors) may be accurate enough but manual focusing is still not possible and we have to rely on the autofocus (we shall see in the second part of the article that this may or may not represent a wise choice). The second alternative is to mount the camera on the tripod, remove the filter, frame and focus, put the filter back on the lens and shoot. This requires a tripod but, considering that long exposures are the norm anyway, a tripod is required no matter what. It is a rather cumbersome and slow procedure, though.
For the above reasons serious IR photographers have always preferred twin-lens reflex (e.g., Rolleiflex) or rangefinder cameras (e.g., Leica in the 35mm format or Fuji or Mamiya in the medium format). Point-and-shoot cameras with a separate viewfinder can also be a solution, although the cheapest ones may not have a way to mount the IR filter on the lens.
Separate viewfinder to be mounted on the flash hot shoe to allow the framing in a TTL reflex camera when an IR filter is present.
High-end digital point-and-shoot. The optical viewfinder (top right) allows us to frame the picture and shoot even though an IR filter has been mounted on the lens.
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