Re: Testing Focal Lengths of Lenses

[Bill Glickman <bglick@xxxxxxxx>]

George

RE: Optical bench testing lenses for roundshots.

           In my opinion, this is a "hit or miss" proposition.  Here is why
I feel this way.   There is two parts required for a rotational camera to
provide its maximum resolution on film.   This of course assumes that all
the procedures in my last email are being properly executed.  The two parts
are:

1.  exact fl of lens.
2.  film length actually used for each fl and slit size entered.

            If these two variables were independent of each other, optical
bench lens testing of lenses would be a great idea.  However these variables
are not independent of each other.  The optimal fl to enter into the brain
box is a combination of the lenses true optical fl, and any correction
factor of the film length used for each fl.  Remember, when changing the fl,
you are changing the amount of film being consumed.  Unless the actual
amount of film used matches EXACTLY the calculated length, then the exact
lens fl is NOT the optimal one to use.  From my experience, although the
Seitz is a super high precisioned piece of equipment, it is not perfect in
this area.  (absolute perfection may be in the $50k price range, not $10k)
The only way to insure fine tuning of the sharpness is by entering a fl
value that compensates for this potential minor shortcoming of the
mechanics.  (we are usually talking very minor corrections, like tenths of
mm of the fl)   So for each camera / lens as a system, the only true test
for optimal sharpness is shooting subjects of known aspect ratios (like a
rectangular building wall) and adjusting the fl till the aspect ratio on
film matches the aspect ratio of the subject.  Shooting items far away is
best to prevent distortion on lower fl lenses.    This is why I consider
optical bench testing "hit or miss."  You will hit it right, if the film
length used exactly matches the calc. length for the desired slit size.  If
it is not, then this fl determined by bench testing is not optimal.  The
Seitz book does offer good starting points for the lenses listed as these
are close to optical bench tested fl by the maker.  Of course each lens will
still vary from lens to lens and batch to batch.

       Every one always touts the incredible sharpness of fixed lens
Roundshots.  Super series cameras have one LF lens permenately affixed to
the camera.   Mr. Seitz confirmed way back, this increased sharpness of
these cameras occurs as a result of  thorough factory testing each
lens/camera, so that the optimal fl is used to provide for maximum
sharpness.   The larger image circle lenses can attest for additional
sharpness over 35mm lenses, but not over MF lenses which often have image
circles almost as large.  Most users that have both cameras always cameras
always claim how much sharper the super camera is.

       So the conclusion is this.... the only reason the brain box needs to
know the exact fl of the lens is to adjust the length of film being used for
that lens.  This is a simple mathematical equation.  If the film length used
does not EXACTLY match the film length calculated, then modification of the
lens fl value is the way to adjust for this shortcoming.  I considered
trying to confirm the film length for each shot matches the calculated
value, however, I don't think this is possible since the start and stop
points are kind of blurred a bit...so I am not sure where the actual
length's start and stop points should be measure from...this is why I
elected the aspect ratio test as the ultimate test for sharpness.  Mr. Seitz
agreed this is the only sure method.  Although the film length can be
adjusted in the service menu...without knowing the exact fl of the lens this
would be useless....and even if you did, you would still need to test
shooting aspect ratios to confirm it is correct.... so there seems not
getting around the testing!

       I have done a lot of this testing myself, and it is extremely labor
intensive.  Specially for you George, 10 lenses is a lot to test!   It is
important when doing this testing, you do it at shutter speeds and f stops
that would simulate your actual work.  Because testing in very low light and
getting complete accuracy would not guarantee the same would be as accurate
in very bright lighting conditions.   For a good average, run the test in
lighting which is in between your high and low light conditions.   It is
important to understand my last post to fully understand what type of
expectations one should have using a rotational camera.  The focus range
(near to far distance) is just as important as these factors we are
discussing.

        The one thing I would wish for to simplify this process....is some
type of Polaroid 220 film whereas you can test, read results, adjust, re
test, etc.  Of course no such product exist....so the only way I know to do
this - is burn real film and run it up and back to the lab!  The tedious
part is packing up your gear, going to lab, getting film processed and
starting all over again and again.  It can take many many trips....

Bill G






----- Original Message -----
From: "George Pearl" <alps007@xxxxxxxxxxxxxx>
To: <panorama-l@xxxxxxxxxxxxxxxxx>
Sent: Friday, December 08, 2000 4:30 PM
Subject: Testing Focal Lengths of Lenses


> Hello Roundshot VR camera owners!
>     I have made tentative special arrangements with an optical testing
company to test lens focal lengths for us once every 3 months at a price of
$50.00 per lens. It doesn't matter what kind of lens you send or how big of
a lens. You will receive the effective focal length of each lens back in
return with your lenses unharmed. We are looking for this to take about a
week or so per session as long as you give us your Fed-X number for shipping
/ insurance.
>
>     My question to the group is if any of you are interested in having
this done, and how many lenses you would want tested. If I could give the
testing lab some kind of idea of the amount of lens testing we would want,
they will tell me for sure if they want to provide this service for us. What
they do NOT want to do is to have to unpack and pack 20 different boxes
coming from all over the world. They have requested that I receive all
lenses and then pack them into one case for shipment to them. They in turn
will test all lenses in the case and return all of them to me at one time. I
in turn will reship all of the lenses to where they go. You will provide me
with a pre-addressed Fed-X label and a check made out to the optical company
for payment of the services. As long as I have pre-addressed labels, I
shouldn't have any problem with repacking you lenses and having Fed-X come
get um from me.
>
>     Please answer me directly at ALPS007@xxxxxxxxxxxxxx
>
> If any of you have someone who can professionally test our lenses for any
cheaper than this, please lets hear from you!
>
> I have 10 different lenses to test, and don't like the idea of shooting
lots of film and looking at tree branches off in the distance on rolls and
rolls of film. Why not just get the exact Focal Length from someone who does
that sort of thing, and be done with it?!!!
>
> My best,
> George S. Pearl, BCQDE, BCEP, FEPIC, QPP
> President,
> Atlanta Legal Photo Services, Inc. , dba:
> ALPS - Evidence & Photo
> 2139 Liddell Drive, NE
> Atlanta, GA 30324-4132
>
> Tel: 404/872-2577
> National: 1-800-USE-ALPS
> Fax: 404/872-0548
> Home: 404/634-1139
> Cell: 404/771-9121
> ALPS Website: "http://www.ALPSLABS.com"
> Atlanta Panorama is a division of ALPS...
> "http://www.AtlantaPanorama.com"
>
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