[MF3D.FORUM:496] Re: Exact fl match for paired lenses.

["Bill Glickman" <bglick@xxxxxxxx>]

Greg

   These lenses are for the Mamiya 7 rangefinder camera.  They have leaf
shutters.  I will visit your site.

A suggestion that seemed sensible to me was to shoot a grid with each lens
and then lay the chromes on top of each other over a light box to tell if
the fl are identical.  I will try this at each f stop to be sure there is no
changes.  Of course if they are not, I don't know how to interpret the
shortcomings to know if it is acceptable.   What do you think about this?

Thanks you.

Bill G



> >Greg
> >
> >Thank you for the input.  How did you discover the % difference in fl's
of
> >the 5 lenses?   Did you shoot test targets without moving the camera
body?
> >What tolerance would you say would be acceptable after you advise me of
how
> >best to test the lenses?
>
>   Read my web page on how I compared the FLs (basically
> built a jig with shutter to hold the lenses and then
> projected a target onto a large sheet of paper).
>
>   I suspect everyones tolerance for missmatch is
> different. For me, how the scene is placed behind
> (or cutting) the window is very important to me
> (ie I look at the edges of the photo) so unequal
> cropping due to size missmatch is a big deal for
> me. People who just enjoy the 3d scene without
> looking too hard at the edges could tolerate and
> enjoy much more missmatch than me.
>
>
> >      If the fl's are off by more than the tolerable %, can I send them
off
> >to a lens shop and have them make the same adjustment you did?  If so,
does
> >anyone know of a lens shop that could accomplish such a task?  I can not
get
> >5 Mamiya lenses to choose from, I would be forced to buy them all then
sell
> >4 of them at a big loss!
>
>   Shimming one lens out to improve the size match
> has the sideeffect of moving the focus point.
> So the two lenses are no longer focused at the
> same distance. If you like to shoot wide open
> or have to shim a lot then this may not work
> for you.
>
>   In my case the 4 thou shim I moved makes the
> focus points mismatch by 3 (I think) inches at
> 2m distance. It was barely noticable in my test
> shots at f3.5 (newspaper laying on floor shot at
> 45 degrees) with a 10x loupe. At f8, the widest
> I'm likely to shoot for real photos I couldn't
> see the difference. And an MF viewer is only about
> 3x, not 10x.
>
>   But if you have significant size missmatch the
> focus shift from shimming may be greater than the
> DOF you have at semi-close distances and wider
> apertures.
>
>   I could crunch some numbers for you if you get
> a measurement for the missmatch (shoot some test
> rolls). And tell me what apertures you think you'll
> use.
>
>   I don't know how the Mamiya's are constructed but
> a camera tech could likely tweak it for you.
>
>   Are these Mamiya TLR's you have or SLRs?
>
> Greg
>