Two problems with new Ansco 5x7

[William Whitaker]

Quite a conundrum you've got there, Brian-lad!

[djdister]

The Rear Element back:
Attachment 128588

So ... nothing immediately jumps out. There are two possible avenues to explore:

1. the inner element of each lens group could be reversed. You would need a reference lens to use as a guide.
2. the entire rear lens group is not matched to (not meant to be part) the front lens group. Are there any markings on the lens rear grouping such as serial number or other information that matches it to the front lens group? Although the rear lens group fits in the shutter, that doesn't prove that it is the proper rear lens group for a 180mm Sironar-N. Because the effective focal length of that assembled lens is so different from marked, it is a good bet that the whole rear lens group is from a different lens.

[brian steinberger]

Quite a conundrum you've got there, Brian-lad!

Yea tell me about it! At least I have a 14 day return policy!

[brian steinberger]

There is no markings on the rear element whatsoever.

So ... nothing immediately jumps out. There are two possible avenues to explore:

1. the inner element of each lens group could be reversed. You would need a reference lens to use as a guide.
2. the entire rear lens group is not matched to (not meant to be part) the front lens group. Are there any markings on the lens rear grouping such as serial number or other information that matches it to the front lens group? Although the rear lens group fits in the shutter, that doesn't prove that it is the proper rear lens group for a 180mm Sironar-N. Because the effective focal length of that assembled lens is so different from marked, it is a good bet that the whole rear lens group is from a different lens.

[brian steinberger]

Joe, do you mean to look through each element at a textured wall and see at what distance away from the wall it becomes sharp?

Can you determine the individual focal lengths of the front and rear groups by themselves by holding them away from a wall and seeing at what distance infinity comes into focus? I would assume the Sironar-N is a 180mm plasmat and would have element groups of roughly 420mm and 315mm if anything like a convertible Symmar 180mm. That might help you narrow the problem down and tell you if one of the groups is bad.

[William Whitaker]

Yea tell me about it! At least I have a 14 day return policy!

I don't see that anyone else has asked, but do you have another lens you could try? That would at least narrow it down to the lens. Or not.

How much time is left on the clock?

[brian steinberger]

Will, no I don't have another LF lens. Nor do I know anyone near me to borrow one. I have until next Friday to return, so plenty of time. If it's something I can fix I will, but if it's something like a Frankenstein lens then I'm sending it back!

I don't see that anyone else has asked, but do you have another lens you could try? That would at least narrow it down to the lens. Or not.

How much time is left on the clock?

[Joe Smigiel]

Joe, do you mean to look through each element at a textured wall and see at what distance away from the wall it becomes sharp?

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.

[brian steinberger]

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.

Joe, this is great information. Thanks. I will try this experiment tomorrow when there's light.

[brian steinberger]

So I tried the wall trick. The lens as a whole focused at infinity measured 5 3/8" from the wall.

Front group only - 11 1/2" to center
Rear group only - 5" to center

This was holding the groups with larger side toward the window. Is this correct?