Originally Posted by
Joe Smigiel
No. Hold the element group (or combined lens) in hand so that it projects the light coming from a window on the opposite side of the room onto the wall. Move the lens group manually towards and away from the wall and notice where the projected image comes into sharp focus on the wall. If the lens is configured correctly and has a combined focal length of 180mm, that's the distance away from the wall where the image ooutside the window should come into focus when the lens is assembled and both front and rear groups are present.
The definition of focal length is where an object at infinity (i.e., the bright area outside the window in this case) comes into sharp focus at the focal plane (i.e., the wall). Plasmat lenses like a Sironar-N or Symmar or old convertible lenses have a front and rear group that differ slightly in focal length but are approximately around twice the focal length of the combination. For the 180mm combined lens, the rear group is probably about 315mm focal length and the front group somewhat longer at 420mm (at least for the calculation I have for my Symmar 180mm). So, combined, the sharp image should appear around 180mm or 7" from the wall, with the rear only at 315mm or about 12 1/2" away, and with the front group only at about 420mm or roughly 16 1/2" away from the wall.
If the rear group only focuses at a considerable deviation from 315mm, as someone suggested earlier it may be an unmarked mismatched group from a different lens that just happened to fit the shutter and one that somebody mixed up in the past. Or it could be missing an element in the group, or have an element reversed, etc.
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