[MF3D.FORUM:525] Re: Exact fl match for paired lenses.

[Greg Erker <erker@xxxxxxxxxxxxxx>]

>     I conducted the test as follows.  I shot a grid on the full 6x7 frame.
>The grid was made with pen lines running vertical and horizontal.  I shot
>with the first lens, then without moving the camera, shot with the second
>lens.  I layed the chromes on top of one another on a light box and lined up
>the girds.  I lined the top grid lines up... but could not keep the bottom
>lines aligned.  (both horizontal and vertical would not align).  I looked
>through a 10x loue with a measuring gauge attached  determine how far off
>the lines were towards the bottom of the chrome.  The worst lines needed to
>come together by .15mm to be perfectly aligned.

  .15mm in 50 or 55mm is well under 1% so you should
probably be fine.

  By the way all my measurements were for a camera with
linked focusing. Assuming I built it without play in
the mechanism (and calibrated both lenses infinity focus)
then any image size missmatch is due to the lens FLs
being different.

  With two separate cameras the ability to focus them
accurately, repeatedly, comes into play. To check this
you might want to repeat you test and take several shots,
refocusing the cameras each time. If you get the same
0.15mm each time then your focusing accuracy is much
better than your lens match. But if you get a range
then you know the focusing plays a part too.

>    I shot some 3d shots with these lenses and I think that the corners go a
>bit soft... meaning the center was tack sharp, but in the corners, reading
>text on some boxes was more difficult to read then the box in the center,
>but still legible.  Of course I was afraid my mind was playing tricks with
>me after I already knew the grids did not match up perfectly.

  Most non-macro lenses aren't perfectly flat
field so they don't look great shooting charts
on the wall but work fine for 3d objects.
And lenses are usually sharper in the center.

  If the four corners are different in the amount
of unsharpness then likely you weren't perfectly
perpendicular to the wall. Less likely is camera
body damage (in an old camera) that makes the lens
mount and film plane non-parallel, or poor centering
of lens elements that would make one side sharper
than the other.

Greg E. (time to do some work :)