Nicholas F. Jones
23-Aug-2001, 16:42
Hi Everybody,
The spec sheet on my Tachihara triple extension 8X10 gives the "minimum extensio n" as 135mm (maximum, 840mm). (The owner's manual for the selfsame camera gives 105mm as the "bellows extension", but this is presumably a misprint). Since it is my belief that, for a given lens, an extension equal to the focal length is required in order to focus at infinity, I assumed until recently that my camera would accommodate a 135mm lens (albeit with loss of some movements).
To test this assumption I placed a 300mm lens on the camera, focused at infinity , then tried to find a measurement corresponding to this lens' focal length. It is approx. 300mm from the location of the film in the film holder to the plane of the lens board. But when I racked the bellows all the way in to its stopping point (at the front swing plate), the same measurement gave not 135mm but 150mm .
Since I want to determine with some accuracy the shortest lens that can be focus ed at infinity on my camera, my questions are:
1) How exactly is the relevant measurement taken? My manuals mention the node o r the diaphragm of the lens as one terminus, the film plane as the other.
2) Can users of the Tachihara 3X 8x10 shed light on my problem by drawing my att ention to features of the camera (or its descriptive lit.) that so far have esca ped my notice?
3) And, while we're at it, can someone define for me "flange focal length" and c omment on its bearing, if any, on this question.
All the best, Nick.
The spec sheet on my Tachihara triple extension 8X10 gives the "minimum extensio n" as 135mm (maximum, 840mm). (The owner's manual for the selfsame camera gives 105mm as the "bellows extension", but this is presumably a misprint). Since it is my belief that, for a given lens, an extension equal to the focal length is required in order to focus at infinity, I assumed until recently that my camera would accommodate a 135mm lens (albeit with loss of some movements).
To test this assumption I placed a 300mm lens on the camera, focused at infinity , then tried to find a measurement corresponding to this lens' focal length. It is approx. 300mm from the location of the film in the film holder to the plane of the lens board. But when I racked the bellows all the way in to its stopping point (at the front swing plate), the same measurement gave not 135mm but 150mm .
Since I want to determine with some accuracy the shortest lens that can be focus ed at infinity on my camera, my questions are:
1) How exactly is the relevant measurement taken? My manuals mention the node o r the diaphragm of the lens as one terminus, the film plane as the other.
2) Can users of the Tachihara 3X 8x10 shed light on my problem by drawing my att ention to features of the camera (or its descriptive lit.) that so far have esca ped my notice?
3) And, while we're at it, can someone define for me "flange focal length" and c omment on its bearing, if any, on this question.
All the best, Nick.