[MF3D.FORUM:499] Re: Exact fl match for paired lenses.

["David Lee" <koganlee@xxxxxxxxxxxxx>]

Bill,

I would agree with most of what Greg has said about matching lenses. Having
matched 6 pairs of medium and large format cameras and lenses with mixed
success, I have sympathy for your plight. Especially considering the cost of
those lenses. I thought about buying a pair of those cameras but eventually
balked at the cost and ended up buying 2 large format lenses which the
Schneider people were kind enough to match for me at the factory. (They
measured them down to a tenth of a mm (123.6mm to be exact), which works out
to .1% difference at most.

As for the differences that the eye will tolerate, the researcher Bela
Julesz who wrote the seminal book, "Foundations of Cyclopean Vision," found
that the brain could fuse images (random dot in this case) up to something
like a 20% difference. So obviously one would have no trouble comfortably
fusing any difference in a modern lens with reasonable standards (as I have
no doubt that the Mamiya 7 has). But, as Greg said, the problem lies in the
differences at the edges of the mounts. I certainly won't tolerate much
myself, even though I mostly make prints and could actually vary the
enlarger position to make the images exactly the same size. Having done this
a few times, though, I have had enough of it and now insist on matched
lenses. It took me 5 Yashicamats to get close enough, although I had better
luck with the others.

Where I differ from Greg is that I would not even think of putting one of
the lenses out of focus to get the same image size. One only uses medium
format if one wants absolutely sharp images, and while depth of focus
(similar to, but not exactly the same as, depth of field) will cover it to
some degree, I can't imagine that you would find that satisfactory after
spending some $2000 dollars for the pair of lenses. And while one might
reasonably shim an old Ricoh or  Yashicamat lens, I can't imagine trying
that with a Mamiya 7 lens.

As for testing, I did very similar to what you are planning on, that is, I
shot a newspaper (to check the sharpness) with a MacBeath color checker (a
grid) on it. I then put one on top of the other and it was a simple matter
to see whether they were the same size. I can't imagine that you will see
any change in size with different f stops, but I have found with several
pairs that there is a difference in aperture size between cameras, and also
a difference in the same camera depending on whether you open up to get the
f stop, or close down to get to it. So I have a note on the back of the
cameras telling me how much more to open or close a particular aperture, and
if they are the same, I have gotten into the habit of either opening or
closing both of them, as opposed to opening one and closing the other to get
to the same aperture.

As for interpreting the results of the test, you might, on the same rolls,
shoot some actual stereo shots, mount them, and see if they are acceptable
to your eye. If they are not, I would cross that bridge when I came to it
and hope for the best in the meantime. By the way, I suspect that if you
simply got one additional lens it would very likely be acceptably close to
one of the ones you have. We are, after all, talking about some of the
sharpest and best lenses ever made. Then you could sell the odd one for  a
slight loss (or put it one eBay and make a profit :-))

David Lee

----- Original Message -----
From: Bill Glickman <bglick@xxxxxxxx>
To: Medium Format 3D Photography <MF3D.Forum@xxxxxxxxxxxxx>
Sent: Thursday, May 18, 2000 5:14 PM
Subject: [MF3D.FORUM:496] Re: Exact fl match for paired lenses.


> 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
> >
>