Minimum bellows extension and shortest focal at infinity

[Nicholas F. Jones]

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.

[Jorge Gasteazoro]

are you sure the back element did not hit the glass?

[neil poulsen]

I don't know about this camera, but does it have front axis and base tilts? My point being, is it possible to tilt the front back at the base, and then right the lens stage to vertical by tilting it at the axis? My old Deardorff could do this, and one could thereby achieve a smaller "minimum bellows".

[Dave Willison]

Nick:

The technical data listed on the Midwest Photo web site confirms that the minimum extension on the Tachihara Triple Extension 8x10 is 135mm. (Specifically, they indicate that the measurment is 13.5cm or "just under" six inches.)

As the post above notes, mounting a lens of the same focal length as the minimum bellows extension sometimes requires camera movements. I have a 4x5 Calumet Wood Field XM which is essentially the same camera as the Tachihara 4x5. The minimum bellows extension is 65mm. To use a 65mm lens at infinity, I simply tilt the front standard backwards and adjust the rise/fall to center the lens. Note, however, that this arrangement basically limits any further camera movements to make adjustments for DOF, perspective control, etc.

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[Chris Ellinger]

"Flange focal length" is the distance from the shutter mounting flange (front surface of the lens board) to the film plane at infinity. This is often different than the optical focal length.