I've been working on my little project some more this
weekend, I think I've gotten somewhere. This note is long
and has very little stereo content. If you don't have a
desire to fit wider lenses to your TL-120, you should
probably just skip to the next message.



My current plan puts the flange of the lens .64" from the
shutter frame of the camera. Since the rear element of the
lens extends .62" back from the flange, that leaves .02"
between the rear element and the shutter frame. This puts
the focus at about 3.4m which should work well for f/22
fixed-focus shooting.

Here's what I can offer. It's kinda like a group-buy. I'll
have the boards made and sell one to anyone who wants one.
The cost will be the total order cost divided by the number
of boards (rounded up to nearest dollar). To that will be
added actual shipping cost to you plus one dollar. (If it
costs me $600 to have six made, each will cost
$100+shipping+$1.)

I offer drawings of what I'm ordering if you want to check my
shapes or dimensions. I'm open to suggestions if you find
something wrong or just want something changed.

Things you should be aware of:

The outline may not be perfect. The lens board has complex
curves on it. I expect the final product to require a little
file-work on the corners to fit neatly.

I think the board is thin enough that the stock screws can be
reused to attach it to the camera. It's hard to say for certain.

I've only put four screw-holes in the board. If you have
newer five-hole lens board, you will just ignore the enter one.

What next:

If you are still interested after all that, here's what I
need. Send me a mail message off-list. If you want to review
my drawings, just ask. I'll share. Cost depends on volume,
so after a few days, I'll count up how many people are
interested, and let you know the cost of the boards. If at
that point you are still interested I need 25% of that amount
sent to me as a deposit.

When the boards arrive, you send me the balance+shipping
costs and I'll send out the boards.

If the boards turn out to be non-functional, you are not
obligated to pay the balance or accept delivery. The
deposit, however, is mine.

If the boards are functional, you are obligated to pay the
balance and accept delivery. You can't change your mind and
leave me with a pile of boards.

It all hinges, I guess, on how we define functional. I
expect there may be some filing around the edges or a little
shimming under the lens flanges. That's part of the job. If
I've put the lens holes in the wrong place, however, that's
non-functional. In the end, I'll defer to your good judgment
and our community's supportive nature. I'm not going to
argue with anyone who says it's fatally flawed and is
refusing delivery.

I've updated the picture at
http://www.alaska.net/~thurston/images/frontofboard.gif

Some sample costs our real costs may be different, but this
will put you in the ballpark:
8 boards $710
6 boards $600
4 boards $430
2 boards $350
1 board $340

If you have questions or comments, contact me off-list.
Details of this transaction really don't need to clutter up
the list.
________________________________________
John Thurston
Juneau, Alaska
http://stereo.thurstons.us

John Thurston writes:

> If you have questions or comments, contact me off-list.

As has been pointed out one of the steps in this process is to get
matching lenses. If everyone was to send two pair of lenses to a single
person they could go through them all and get the actual fl and send
matched sets back out. Having a larger pool of lenses would make the
process much more acurate.

Just a suggestion.



David W. Kesner

--- In MF3D-group@yahoogroups.com, "David W. Kesner" David
writes:

> As has been pointed out one of the steps in this process is to get
> matching lenses. If everyone was to send two pair of lenses to a

single

> person they could go through them all and get the actual fl and send
> matched sets back out. Having a larger pool of lenses would make the
> process much more acurate.
>
> Just a suggestion.
>

Hi David,

Yes, that may be a good idea if that person has the equipment to do
the matching. It would be sheer luck to find a perfectly matched pair
from among only four lenses. I guess it depends much on how well
matched one wants the lens pair to be.

> It would be sheer luck to find a perfectly matched pair
> from among only four lenses. I guess it depends much on how well
> matched one wants the lens pair to be.

From experience from testing "modern" 35mm camera lenses (not medium
format), it is rare to find a pair of fixed FL lenses that is not well
matched for my standards.

Definition of "my standards": I just put the film chips in a stereo mount
and look up and down. I make sure that the images are aligned at the
bottom and then I look at the top to see if they are aligned there.

I have only seen mis-matched film chips when looking at pictures from zoom
lenses (linked zoom lenses, but obviously not perfectly linked). Usually,
when I can see the difference in the top/bottom test, I can also see the
stereo window not being perpendicular to the camera axis (i.e. equally
spaced on the right and left sides) but tilted. That bothers me to no
end, that's why I only use fixed FL lenses in my 35mm stereo cameras.

I consider myself rather picky in these things so it is possible that for
other people this mismatched is no big deal. But the bottom line is that
I don't remember ever had a mismatched fixed FL lens. So I question the
statement that " It would be sheer luck to find a perfectly matched pair
from among only four lenses" unless if things are different in Medium
Format lenses or if your standards are much higher then mine and you have
a different way of measuring this, than looking at the pictures
themselves.

George Themelis

So I question the statement that " It would be sheer luck to find a
perfectly matched pair from among only four lenses" unless if things are
different in Medium Format lenses or if your standards are much higher
then mine and you have a different way of measuring this, than looking
at the pictures themselves.



Hey Doc..... When shooting a rotational panoramic
camera, its imperative to know the exact fl of the lens, at least down
to .x mm. Years ago, when fine tuning my rotational panoramic camera,
Seitz provided a means to check the fl of lens. We used to shoot a
square and enter ranges of fl's, such as 77, 78, 79, 80, 81, 82, 83, 84,
into the brain box. We viewed the film through a 10x loupe (typical
enlargement amount) and evaluated which fl produced the truest square.
Then, if 83mm produced the best square, we shot one more roll, 82.5,
85.6..... etc. After two rolls, we could nail down every lenses fl
within .x mm fl. (at least to the standard of the camera)


The lens types and actual fl vs. nominal fl's were
given to Seitz. They used to provide a list of the findings when you
purchased their cameras. Remarkably, the same lenses, of the same
batch, often were all over the map. This included 35mm lenses from
Nikon, Zeiss, Canon, Leica....and MF lenses from Zeiss, Mamiya, Hassy,
Pentax. I once had two identical 55mm nominal MF fl lenses that
measured approx. 51mm and the other 59mm. When I order paired 47mm
Digitar lenses from Schneider, at times I would have to wait 6 months
till they could get a tight match. Other times, I got lucky, they
would just finish big run of the lenses, and would get a pair.... of
course, I raised my standards a bit. Recently, I just waited over 3.5
months for a paired set of 35mm fl Digitars.



I think the group has the right idea, bundle the
lenses, do a rough focus check using a dial indicator, and pair the
lenses accordingly. Maybe buy more lenses than you need, then re sell
the additional lenses. The actual fl is not an issue, only the proper
pairing of them. It's possible the design of the 55mm lenses being used
will have close fl tolerance by nature of their design. I never
researched the details of what variables within the lens design allows
for this. In addition to the lens design, the spherical and spacing
tolerances also plays a big part. The less expensive the lens, the
more loose the tolerances. This is true with all optics, not just
camera lenses. A dial indicator will let you know the variance between
the lenses and get you the best matched pairs.


As for viewing tolerances of mis magnified
images.... there is a relationship between viewing time and the amount
of mis-magnification error of the final retinal images. Mis
magnification errors is another form of binocular rivalry, something we
rarely experience with normal vision.


However, in the real world, we develop this same
binocular rivalry through refractive variances of our eyes. Or, if we
already have corrective eye-wear, and our refractive errors change vs.
the correction in our prescribed eyewear, then we again develop
refractive mismatch errors. When the error becomes great enough, this
condition brings on headaches / dizziness, usually by the end of the
day. The greater the errors become, the earlier in the day the
problems surface. I have had many friends that know immediately when
their refractor errors no longer match their current eye-ear
prescriptions, as this annoying feeling overcomes them every day....most
wait till the dizziness or headaches creep down till mid day, then, they
can't tolerate it anymore....they get their eyes re-examined, and use
the new corrective eye-wear, which always solves the problem.


Using a stereo viewer with mismatched fl lenses, or
stereo images shot with mismatched fl lenses has the exact same
effect. The degree of error between the displayed images on the
retina, often dictates how long one can view before these annoying
effects begin to surface. Then of course, we are all different, so
therefore, we all have different tolerances to these visual oddities.
I always believed that many stereo enthusiast have very high tolerance
to these problems vs. the general public, hence why they became
enthusiasts..... as if they constantly got dizzy, they would never take
up the hobby.


The good news is, for MF stereo, if you get the
taking lenses relatively close in fl, the errors are only magnified 3x
in the viewer. With 35mm viewers, the errors are magnified 5x - 6x, or
maybe greater, so the taking lenses need to be better matched by a
factor of 2x, for the same end result. Luckily, simple viewer lenses,
such achromat doublets have fl's that are relatively close.... again,
this is based on tolerances, but usually within a few %. So the bigger
potential for variances lies in the more complex taking lenses.

Hi Bill,

> Remarkably, the same lenses, of the same
> batch, often were all over the map. This included 35mm lenses from
> Nikon, Zeiss, Canon, Leica....and MF lenses from Zeiss, Mamiya, Hassy,
> Pentax. I once had two identical 55mm nominal MF fl lenses that
> measured approx. 51mm and the other 59mm.

That's crazy!

> When I order paired 47mm
> Digitar lenses from Schneider, at times I would have to wait 6 months
> till they could get a tight match. Other times, I got lucky, they
> would just finish big run of the lenses, and would get a pair.... of
> course, I raised my standards a bit. Recently, I just waited over 3.5
> months for a paired set of 35mm fl Digitars.

So, what were your standards? And what do you think a reasonable matching
of FL for MF shooting is?

From what you are saying, I must have been very lucky in matching Minolta,
Pentax and Contax lenses without any problems. I usually buy 3 identical
lenses only to find that all 3 are matched well, so I keep a pair and sell
the 3rd lens. As I said, I did not measure anything, just looked at the
mounted stereo images, paying attention to the top and bottom edge of the
mount.

In an attempt to offer a minimum standard, here is my experience from 35mm
photography: The vertical adjustment in RBT mounts is based on steps of
0.1mm. The resolution of the system is half this amount, 0.05mm. I
consider this error to be negligible.

So a mismatch in vertical size by 0.05mm in 35mm film is acceptable. This
is equal to 0.05mm/24mm = 0.2% difference in size on-film. I believe this
is the same as the difference in Focal lengths (the size of an object on
film is approximately equal to the actual size x Focal length / Distance
of object from camera)

So acceptable match for a 50mm lens is +/- 0.1mm. (A far cry from your 51
to 59mm range!)

How does that sound? Too strict? And how do we measure this with some
accuracy?

George

>> I once had two identical 55mm nominal MF fl lenses that
>> measured approx. 51mm and the other 59mm
> That's crazy!
>

after reviewing 40+ results of the biggest
and most expensive 35 and MF lenses, I found out, this is NOT crazy, its
normal....

>
> So, what were your standards? And what do you think a reasonable matching
> of FL for MF shooting is?
>

It's all relative.... but I shoot for very high
standards now, I match to a hundredth of a mm taking lenses and the same
for viewer lenses....in some studies I have read, this will match human
vision on average, assuming the viewer lenses are well matched. I
figure, if I am spending this much on lenses, why not get it right...

> >From what you are saying, I must have been very lucky in matching Minolta,
> Pentax and Contax lenses without any problems. I usually buy 3 identical
> lenses only to find that all 3 are matched well, so I keep a pair and sell
> the 3rd lens. As I said, I did not measure anything, just looked at the
> mounted stereo images, paying attention to the top and bottom edge of the
> mount.
>

If you don't measure them, its hard to
determine their variance, as our brains will fuse just about anything in
stereo, but it comes at a price.... again, you probably have a very high
tolerance for binocular rivalries....

> In an attempt to offer a minimum standard, here is my experience from 35mm
> photography: The vertical adjustment in RBT mounts is based on steps of
> 0.1mm. The resolution of the system is half this amount, 0.05mm. I
> consider this error to be negligible.
>

Vertical adjustment, and magnification
mismatch are two different forms of rivalry.

> So a mismatch in vertical size by 0.05mm in 35mm film is acceptable. This
> is equal to 0.05mm/24mm = 0.2% difference in size on-film. I believe this
> is the same as the difference in Focal lengths (the size of an object on
> film is approximately equal to the actual size x Focal length / Distance
> of object from camera)
>

Think of two round spheres..... mis alignment
will produce binocular rivalry in positioning, very common in most
binoculars..... however, the fl mismatch will produce two different size
spheres, which your brain will fuse, but it comes at a price.... so in
this case, you can have two forms of rivalry. If you view the slides
for a few minutes and the errors are not huge, its a moot point. It
comes down to viewing time vs. % error.

> So acceptable match for a 50mm lens is +/- 0.1mm. (A far cry from your 51
> to 59mm range!)
>

Fully agreed, specially considering the cost of
the lenses and their limited availability...

> How does that sound? Too strict? And how do we measure this with some accuracy?
>

Use a dial indicator on the ground
glass of view camera. A dial indicator for $40 will measure down to
.01mm. Just focus at infinity, record dial indicator position for
each lens....when done, pair em up as best you can. This won't tell ya
the fl, but it will tell you the % difference in fl between the lenses.
(which is all that matters) A lens that focuses at .3mm at infinity
(relative value on indicator), vs. the next lens at 2.3mm, has a
difference in fl of 2mm. 2/55 = 4% fl variance (close enough for this
exercise). Under these random conditions, I would consider matches
below 1% to be excellent, 2% good, 3% fair / poor.... etc. Again, its
all relative .... till someone tests some of these, you won't find out
how much leeway exists in the lens design. Since they are simple
lenses, at normal fl, this helps....but I would think tolerances are
lax, which would could effect fl.


Remember, lenses are made for 2d photography, the
actual fl vs. the nominal fl is not of concern to lens manufacturers.
Most 200mm lenses are barely ~195mm.... the 200mm signature is more for
marketing, not for exact accuracy. You will never see a 195mm lens for
sale. Even Schneider does not use the final fl of a lens as a
specification for final QC. It's not an important issue to anyone,
except us.....


Bill

> If you don't measure them, its hard to
> determine their variance, as our brains will fuse just about anything in
> stereo, but it comes at a price.... again, you probably have a very high
> tolerance for binocular rivalries....

The way I compare the lenses visually has nothing to do with fusing the
pair. I am comparing the image with a fixed frame of reference. This
frame of reference is the stereo mount. I use either Albion aluminum or
RBT plastic mounts. I am definitely able to detect a 0.1mm mismatch in
image size. Anyone who has ever flipped the pin bars in RBT mounts knows
that 0.1mm displacement is easily detected.

I also mentioned that if the lenses are mismatched, I can see a tilted
stereo window. Considering that a 35mm stereo pair from the stereo
realist with an object depth range from 7 ft to infinity, gives a total
deviation of 1.2mm, 0.1mm is a rather signficant fraction of it (8% to be
exact). If you take a picture of far away objects with two lenses
mismatched by 0.1mm difference in image size, and them mount them so that
the right side window is at infinity, the left side will be at 24 1/2
meters, which can be easily seen.

> Vertical adjustment, and magnification
> mismatch are two different forms of rivalry.

The vertical adjustment I am talking about is a result of different
magnfication = different on-film size of the same object.

> Remember, lenses are made for 2d photography, the
> actual fl vs. the nominal fl is not of concern to lens manufacturers.
> Most 200mm lenses are barely ~195mm.... the 200mm signature is more for
> marketing, not for exact accuracy. You will never see a 195mm lens for
> sale. Even Schneider does not use the final fl of a lens as a
> specification for final QC. It's not an important issue to anyone,
> except us.....

I am still a bit skeptical about this. Here, I have two projection lenses
by Docter. These lenses have the exact (I assume!) focal lenth marked
both in the lens and the box.

Lens1: http://home.att.net/~drt-3d/ProjectionLens1.jpg
60mm nominal length, marked 59.9

Lens2: http://home.att.net/~drt-3d/ProjectionLens2.jpg
150mm nomical length, marked 150.9

The % difference is 0.2% in the first lens and 0.6% in the second lens.
This is just a random sample of two projection lenses for which the
manufacturer supplies the measured FL and it is rather small. Maybe they
pay more attention to projection lenses because they are might be matched
in multi-projector shows?

George Themelis

When you measure the focus distance with a dial indicator you are really
measuring the distance between the lens flange and the ground glass. Is this
really the same as measuring the focal length? Consider that the factory may
adjust the flange to focal point distance to ensure uniformity of infinity
focus.

> -----Original Message-----
> From: MF3D-group@yahoogroups.com [mailto:MF3D-group@yahoogroups.com] On
> Behalf Of Bill G
> Sent: Sunday, February 17, 2008 5:23 PM
> To: MF3D-group@yahoogroups.com
> Subject: Re: [MF3D-group] lens matching
>
>
> >> I once had two identical 55mm nominal MF fl lenses that
> >> measured approx. 51mm and the other 59mm
> > That's crazy!
> >
> after reviewing 40+ results of the biggest
> and most expensive 35 and MF lenses, I found out, this is NOT crazy, its
> normal....
>
>
>
>
> >
> > So, what were your standards? And what do you think a reasonable
> matching
> > of FL for MF shooting is?
> >
> It's all relative.... but I shoot for very high
> standards now, I match to a hundredth of a mm taking lenses and the same
> for viewer lenses....in some studies I have read, this will match human
> vision on average, assuming the viewer lenses are well matched. I
> figure, if I am spending this much on lenses, why not get it right...
>
>
>
>
>
> > >From what you are saying, I must have been very lucky in matching
> Minolta,
> > Pentax and Contax lenses without any problems. I usually buy 3 identical
> > lenses only to find that all 3 are matched well, so I keep a pair and
> sell
> > the 3rd lens. As I said, I did not measure anything, just looked at the
> > mounted stereo images, paying attention to the top and bottom edge of
> the
> > mount.
> >
> If you don't measure them, its hard to
> determine their variance, as our brains will fuse just about anything in
> stereo, but it comes at a price.... again, you probably have a very high
> tolerance for binocular rivalries....
>
>
>
>
> > In an attempt to offer a minimum standard, here is my experience from
> 35mm
> > photography: The vertical adjustment in RBT mounts is based on steps of
> > 0.1mm. The resolution of the system is half this amount, 0.05mm. I
> > consider this error to be negligible.
> >
> Vertical adjustment, and magnification
> mismatch are two different forms of rivalry.
>
>
>
> > So a mismatch in vertical size by 0.05mm in 35mm film is acceptable.
> This
> > is equal to 0.05mm/24mm = 0.2% difference in size on-film. I believe
> this
> > is the same as the difference in Focal lengths (the size of an object on
> > film is approximately equal to the actual size x Focal length / Distance
> > of object from camera)
> >
> Think of two round spheres..... mis alignment
> will produce binocular rivalry in positioning, very common in most
> binoculars..... however, the fl mismatch will produce two different size
> spheres, which your brain will fuse, but it comes at a price.... so in
> this case, you can have two forms of rivalry. If you view the slides
> for a few minutes and the errors are not huge, its a moot point. It
> comes down to viewing time vs. % error.
>
>
>
>
> > So acceptable match for a 50mm lens is +/- 0.1mm. (A far cry from your
> 51
> > to 59mm range!)
> >
> Fully agreed, specially considering the cost of
> the lenses and their limited availability...
>
>
>
>
> > How does that sound? Too strict? And how do we measure this with some
> accuracy?
> >
> Use a dial indicator on the ground
> glass of view camera. A dial indicator for $40 will measure down to
> .01mm. Just focus at infinity, record dial indicator position for
> each lens....when done, pair em up as best you can. This won't tell ya
> the fl, but it will tell you the % difference in fl between the lenses.
> (which is all that matters) A lens that focuses at .3mm at infinity
> (relative value on indicator), vs. the next lens at 2.3mm, has a
> difference in fl of 2mm. 2/55 = 4% fl variance (close enough for this
> exercise). Under these random conditions, I would consider matches
> below 1% to be excellent, 2% good, 3% fair / poor.... etc. Again, its
> all relative .... till someone tests some of these, you won't find out
> how much leeway exists in the lens design. Since they are simple
> lenses, at normal fl, this helps....but I would think tolerances are
> lax, which would could effect fl.
>
>
> Remember, lenses are made for 2d photography, the
> actual fl vs. the nominal fl is not of concern to lens manufacturers.
> Most 200mm lenses are barely ~195mm.... the 200mm signature is more for
> marketing, not for exact accuracy. You will never see a 195mm lens for
> sale. Even Schneider does not use the final fl of a lens as a
> specification for final QC. It's not an important issue to anyone,
> except us.....
>
>
> Bill
>
>
>
>
>
>
>
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>

> When you measure the focus distance with a dial indicator you are really
> measuring the distance between the lens flange and the ground glass.

Yes, this is true..... assuming you pick the
flange as the reference point.

> Is this really the same as measuring the focal length?

No, it was not our desire to measure fl, only
differences in fl. That is all that matters in stereo, i.e. a matched
pair.

> Consider that the factory may adjust the flange to focal point distance to ensure uniformity of infinity
> focus.
>

That is a great point Ken.... if this
is the case, you need to find another point on the lens to measure
from. I have no experience with these lenses, maybe others who know
this lens better can offer advise. There is always some point on the
lens you can use a reference point. Sometimes, they will shim the
lenses to achieve infinity focus. But more commonly today, if focus is
achieved via a helical focuser, then, they simply calibrate the distance
scales to the proper focus distance, and all the focuser corrects for
all variances in fl. Schneider and Rodenstock do this with their
Helical Focusing Mounts..... if this calibration / adjustment means was
not offered, every lens would need to be identical fl, which is never
the case. I never used this Mamya camera or these lenses....


Bill

Inspired by John Thurston's fixed-focus 55-mm TL-120 project, I've
just finished putting together a spreadsheet that I've wanted to do
for a long time, but never got around to doing.

It can be used with a fixed-focus lens or when you want to setup your
camera to work quickly with an adjustable-focus wide angle lens that
does not have auto-focus.

http://home.globalcrossing.net/~zilch0/tools/FixedFocusDoF.xls

This spreadsheet creates a table of f-Numbers at the intersection of
various Near and Far subject distances. The trick here was to
accommodate the fact that the lens will be fixed at one focus
distance. So for each combination of Near and Far subject distances,
it calculates two f-Numbers: one that provides sufficient DoF for
the Near Distance and one that does so for the Far distance. But it
only displays the GREATER of the two f-Numbers. This takes care of
even those situations where the lens is focused at a distance outside
the distance range of the subject space, as well as those where the
focus is a lot closer to the Near or the Far than it would be ideally.

As downloaded, it's defaulting to values that would be correct for
John's 55-mm TL-120 project, where he intends to fix the focus at 3.4 meters.

Mike Davis

Hi Doc

> The way I compare the lenses visually has nothing to do with fusing the
> pair. I am comparing the image with a fixed frame of reference. This
> frame of reference is the stereo mount. I use either Albion aluminum or
> RBT plastic mounts. I am definitely able to detect a 0.1mm mismatch in
> image size. Anyone who has ever flipped the pin bars in RBT mounts knows
> that 0.1mm displacement is easily detected.
>

OK....

> I also mentioned that if the lenses are mismatched, I can see a tilted
> stereo window. Considering that a 35mm stereo pair from the stereo
> realist with an object depth range from 7 ft to infinity, gives a total
> deviation of 1.2mm, 0.1mm is a rather signficant fraction of it (8% to be
> exact). If you take a picture of far away objects with two lenses
> mismatched by 0.1mm difference in image size, and them mount them so that
> the right side window is at infinity, the left side will be at 24 1/2
> meters, which can be easily seen.
>

Easily be seen, how? By looking in the stereo
viewer?

>
> The % difference is 0.2% in the first lens and 0.6% in the second lens.
> This is just a random sample of two projection lenses for which the
> manufacturer supplies the measured FL and it is rather small. Maybe they
> pay more attention to projection lenses because they are might be matched
> in multi-projector shows?
>

It could be, they actually use the final fl as
part of the QC for other reasons with projector lenses. This is not the
case with most camera lenses. But as i mention, some camera lenses
designs, allow for very close fl's by default. Other lens designs are
very vulnerable to variance in fl's. The same is true with magnifiers.

Bill

One could avoid any issues relating to varying flange positions by measuring
the lens movement necessary to focus on a distant and then on a near object.

Although it would waste film, one could follow DrT's plan: accurately focus
and then photograph the same distant scene through each of the lenses being
tested. Superposition of pairs of the negatives and examination under a
microscope would accurately reveal any discrepancies in image size.

> -----Original Message-----
> From: MF3D-group@yahoogroups.com [mailto:MF3D-group@yahoogroups.com] On
> Behalf Of Bill G
> Sent: Monday, February 18, 2008 12:34 AM
> To: MF3D-group@yahoogroups.com
> Subject: Re: [MF3D-group] lens matching
>
>
> > When you measure the focus distance with a dial indicator you are really
> > measuring the distance between the lens flange and the ground glass.
> Yes, this is true..... assuming you pick the
> flange as the reference point.
>
>
>
>
> > Is this really the same as measuring the focal length?
> No, it was not our desire to measure fl, only
> differences in fl. That is all that matters in stereo, i.e. a matched
> pair.
>
>
>
>
> > Consider that the factory may adjust the flange to focal point distance
> to ensure uniformity of infinity
> > focus.
> >
> That is a great point Ken.... if this
> is the case, you need to find another point on the lens to measure
> from. I have no experience with these lenses, maybe others who know
> this lens better can offer advise. There is always some point on the
> lens you can use a reference point. Sometimes, they will shim the
> lenses to achieve infinity focus. But more commonly today, if focus is
> achieved via a helical focuser, then, they simply calibrate the distance
> scales to the proper focus distance, and all the focuser corrects for
> all variances in fl. Schneider and Rodenstock do this with their
> Helical Focusing Mounts..... if this calibration / adjustment means was
> not offered, every lens would need to be identical fl, which is never
> the case. I never used this Mamya camera or these lenses....
>
>
> Bill

> Although it would waste film, one could follow DrT's plan: accurately focus
> and then photograph the same distant scene through each of the lenses being
> tested. Superposition of pairs of the negatives and examination under a
> microscope would accurately reveal any discrepancies in image size.
>

Yep, I agree, shooting the same
subject, with different lenses, all set to the same focus distance, then
using either a microscope or even scanning the film will identify the
differences in image size. My method with the dial indicator was to
avoid exposing film. But, if you have all the lenses in a group,
ready to pair at once, the film method is a good alternative...