Re: Testing Focal Lengths of Lenses

[Bill Glickman <bglick@xxxxxxxx>]

Jook

> To recap, is it correct to say that a calibrated fl is only good at one
> focus setting? This is because the image size will change with the lens
> focus.

          It shouldn't.....  because as you change the focus distance in the
brain box, the film lenght changes accordingly...but what you need to
prevent is setting focus standards to high... such as attempting to keep
subjects very near AND very far  - which for both to have maximun sharpness
would require different film lengths.  I discussed this in a post a few
months back and have pasted it below for your review, it explains this in a
bit more detail.

> When I did lens testing, I looked for image sharpness and did not try to
> measure aspect ratios!

      Not quite as accurate but with a 10x loupe, it should get you close.
But remember, sharpness has many variables.  Using the aspect ratio method,
you are limiting the test to exactly what you are trying to perfect...i.e.
propler fl designation for each lens.

 I also set the 220VR to take 90 degrees so that I could fit
> many trials on a roll. But such short starts and stops may cause problems.

        This is very important, you must leave area wasted - specially
before each shot, to allow the motor to get up to speed...it can be stoped
much closer to the scenes end vs. the starting point.

Bill G

Here is the old post...

This is a response to questions I have been getting off list regarding
images not being as sharp as one would like on rotational cameras. It
discusses the necessary requirements for achieving the sharpest shots
possibly.  These include the need to use proper lens fl setting, and
consequences of focus distances and the Depth of Field one is trying to
attain in the shot.  I have been asked this question several times and
thought others on the list may benefit from the discussion.  These are my
comments after much research into the matter...  I use a Seitz roundshot
220VR, however, the same general principles apply to all rotational cameras.

       Point 1.  Determining exact lens fl.

 In theory every lens has only one fl.   Whether this is a good match with
what's in the Seitz manual would be a guess, however it should be close.
Lens makers do  not produce exact fl lenses consistently, there is too many
variables to allow for this.  The book values are an excellent starting
point for knowing your lenses exact fl.  However fl vary from batch to
batch, as well as lens to lens.   How much this difference is unknown till
one tests the lens.  So if you want to be very sure,  shoot a brick wall or
something with known aspect ratio and try different fl's to see which one
most accurately reproduces that aspect ratio on film.  Be sure to enter the
proper distance to the wall!  Not using the exact fl of lens will cause very
small distortion in the image.

            Point 2.  Consequence of large DOF in a rotational camera.

When shooting something very near, say 3 meters, and 300 deg. rotation
with a 300 mm lens, the camera will use a film length of 1.78 meters.  Now,
when changing the focus distance in the camera to infinity, (leaving all
else the same)  the film length used changes to 1.58 meters, a difference of
.20 meters or 200mm!  This is a major difference in film length.  These
different film lengths are used to reproduce the aspect ratio of something
near vs. something far...the farther away something is, the smaller it gets,
hence less film is required to maintain the proper aspect ratio for a given
fl.

       Now, about soft pictures with rotational cameras...    using the
wrong film length in a shot will be interpreted first as soft, and as it
gets worse it will begin to look obviously distorted.  (stretched or
squashed)   So, if one shoots something at infinity only, it should be
sharp, and on the other hand if one shoots something within 20 meters only
in the scene, it too should be sharp.... in these examples, the camera is
providing the exact amount of film length for sharpness (distortion free)
for each shot.
However, when you try to shoot something at 5 meters and infinity in the
same scene, this is where a rotational camera struggles....  it can not
satisfy both "film length" requirements in the same shot.  The 220VR can
only use one film length, and this film length is calculated to produce
sharpness at the focus distance only!   The further objects get from the
focus distance in either direction, the more softness or possible distortion
they can suffer.  Non rotational cameras (with out movements) also suffer
from less sharpness when objects get further away from the point of exact
focus.  This can not be avoided on any camera / lens.   However, only
rotational cameras will suffer the distortion issues when moving further or
closer to the point of exact focus... which in small amounts is interpreted
by softness .

          As we know, a single lens with no movements, can only have one
focus distance...this is what is entered into the 220VR, however, we have in
our minds set this focus distance to encompass a near and a far distance
(DOF) that we want in sharp focus.  This assumes a given f stop and circle
of confusion to film.  But the camera does not know the near and the far
points, only the exact point of focus, and the film length offered by the
220 VR will provide objects at that focus distance sharp and distortion
free.  (this assumes all else is proper, such
as the exact fl is known)   But objects very far from this focus distance
are being recorded at the wrong film length and will become soft or if
really off, will seem stretched or squashed.

        So in addition to f stop and focus distance, rotational cameras have
a 3rd variable to deal with to satisfy our Depth of Field requirements -
film length.  And unfortunately, a rotational camera can only have one film
length for each shot.

       Therefore, we have to be a bit wiser in how we compose shots with
rotational
cameras and alter our expectations of that from a non rotational camera.  As
a general rule for sharp rotational camera pictures, I try to either shoot
scenes that are 1) either mostly all near, 2) all mid range, 3)  all far.  I
always
focus in the middle of this range via the gg and enter the same value in the
brain box.
 This greatly sharpens up ones images!

           Point 3.   After fully understanding the above limitations of
sharpness using rotational cameras..and one is still not happy with
sharpness, then one should pursue these variables next.

     Other softness factors include:

1.  gg / film alignment -  results should be consistently soft if out of
alignment.

2.  Acceptable film flatness at the slit opening.  Of course it will not be
perfectly flat, but it needs to be sufficiently flat to create the desired
cc
on film.  If not this effect will appear at a certain enlargement factor.

3.  Slit size - smaller slits produce sharper images!  But at the tremendous
expense of speed.

4.  Consistent speed of the film going across the slit.... potential
banding lines, or distortion. (stretch or squash)

5.    Type of lens used vs. film size.  Example, MF or LF lenses on 220 film
will always produce sharper images vs. using 35mm lenses.  The larger the
image circle, the larger the sweet spot, the sharper the image.  Remember,
using movements on any camera, including rotational camera can move the film
out of the desired sweet spot and into the less desirable area of the image
circle.  The larger the format a lens was designed for, the larger its image
circle  (rare exceptions)

Hope this benefits some users out their.

Regards
Bill G