The opposite of a close-up lens?

"Lassi Hippeläinen" wrote in message
...
jjs wrote:

In article , (Bob
Monaghan) wrote:

The easiest solution would be to screw the lens farther back by a
millimeter or so (check with ground glass at focus point and loupe).

Which reminds me to ask: Just where is Infinity?

Nope, this is not an old stoned hippie asking for directions. I'm
looking
for a good rule of thumb. Isn't Infinity something like 200 times the
focal length?

Maybe we need a new definition: infinity is so far that you can't get a
corrective lens that would improve things. The weakest lens regularly
available seems to be a -0.25D eyeglass. I checked at home the 2003
catalog of Edmund Optics, and their weakest lens is a stunning -8D
(focal length -125mm)!

-- Lassi

P.S. Maybe Noblex could fix that focus, you they could be talked into
providing an infinity focus accessory lens for those that regret buying
the cheapo model...

There could be a problem. If the lens is adjusted then I expect the
magnification to change slightly, in which case if there was a large slit at
the back for the wider aperture then the image would not superimpose exactly
as the lens rotated and the image would get blurred.

"MikeWhy" wrote in message
om...
"brian" wrote in message
...
(Ralf R. Radermacher) wrote in message
...
brian wrote:

I assume that you've got a "normalish" lens such as a Tessar with a
modest field of view.

It is in fact a Tessar type but at a focal length of 50 mm for a
medium-format camera it isn't exactly what one would call "normalish".

Ralf

Since the lens only has to cover the short side of the format it is
optically "normal" even though it produces an ultrawide panorama. The
vertical coverage would be about 54 degrees. If, as is discussed
below, the hyperfocal distance is set to about 30 feet, then you would
only need -1/10 diopter of correction to focus at true infinity. The
resulting astigmatism introduced by such a weak plano-concave negative
lens would be negligible compared to the inherent zonal astigmatism of
a Tessar. Alternatively, you could attempt to move the lens back by
about 0.25mm. Do you know where the focal distance is set?

1/100 of an inch is about two layers of bond paper. A layer of duct tape
on
the pressure plate ought to do it. I think Littleboy suggested that some
fifty posts ago.

That seems the best idea. Pad up the back that the film runs over so the
film is further away from the lens. Perhaps some strips of Dynotape label
without any writing on. Could be called something different wherever you are
but the plastic tape you press characters into so they turn white and are
raised and then you stick them on things.

"Roland" wrote:
"MikeWhy" wrote in message
"brian" wrote in message
(Ralf R. Radermacher) wrote in message
brian wrote:

I assume that you've got a "normalish" lens such as a Tessar with
a
modest field of view.

It is in fact a Tessar type but at a focal length of 50 mm for a
medium-format camera it isn't exactly what one would call
"normalish".

Ralf

Since the lens only has to cover the short side of the format it is
optically "normal" even though it produces an ultrawide panorama. The
vertical coverage would be about 54 degrees. If, as is discussed
below, the hyperfocal distance is set to about 30 feet, then you would
only need -1/10 diopter of correction to focus at true infinity. The
resulting astigmatism introduced by such a weak plano-concave negative
lens would be negligible compared to the inherent zonal astigmatism of
a Tessar. Alternatively, you could attempt to move the lens back by
about 0.25mm. Do you know where the focal distance is set?

1/100 of an inch is about two layers of bond paper. A layer of duct tape
on
the pressure plate ought to do it. I think Littleboy suggested that some
fifty posts ago.

That seems the best idea. Pad up the back that the film runs over so the
film is further away from the lens.

Oops. Wrong direction: you need to get the film closer to the lens.

Perhaps some strips of Dynotape label
without any writing on. Could be called something different wherever you
are
but the plastic tape you press characters into so they turn white and are
raised and then you stick them on things.

FWIW, this is NOT what I suggested (which was to get a repair person to
adjust the lens position); this sounds like a screamingly bad idea. Getting
tape adhesive anywhere near a camera's internals is not something I'd even
think about doing. (Other than Holgas, where it's necessaryg.)

David J. Littleboy
Tokyo, Japan

the slit width would mainly change the "shutter speed" or duration of
exposure, acting like a focal plane shutter, right?

In this case, a 1/4 millimeter shift in position isn't going
to change much except the focus point wide open to include infinity ;-)

grins bobm
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************

(Bob Monaghan) wrote in message ...
the slit width would mainly change the "shutter speed" or duration of
exposure, acting like a focal plane shutter, right?

In this case, a 1/4 millimeter shift in position isn't going
to change much except the focus point wide open to include infinity ;-)

grins bobm

On the contrary, you will have changed the magnification factor by
about 1% by moving the lens closer to the film. The maximum size slit
for the Horizon camera, for example, is about 1 cm (from memory). The
change of 1% in the magnification will mean that the image will be 1%
narrower (or wider - I forget) and so an image on the film plane will
move by about 1/10th mm as the 1cm wide slit swings round, whereas
before it would have stayed in the one place. 1/10th mm may not sound
like much but when you enlarge then it will show up as a horizontal
blur. This thread was referring to the Noblex roll fim camera which
might have a different swing speed for each exposure, for all I know.
If the slit stays narrow then this effect with a 1% change of
magnification will be less.

I took my Widelux F8 into a shop once and asked them if it would be
worth moving the lens towards the film (I think it is set to focus at
8 ft, or at least it seems that way to me) and the guy explained to me
about the magnification and if how this changes then it affects the
sharpness of the image.

(Bob Monaghan) wrote in message ...
the slit width would mainly change the "shutter speed" or duration of
exposure, acting like a focal plane shutter, right?

In this case, a 1/4 millimeter shift in position isn't going
to change much except the focus point wide open to include infinity ;-)

grins bobm

Here is another way of explaining what I already said in reply to
Bob's post. Hasn't showed up yet for me. Here goes:

Reducing the distance between the lens and the film would get the far
distance in focus. There's no argument there. But there is a
difference between moving the lens backwards and moving the film
forwards. It is moving the film forwards that would solve the problem,
not moving the lens backwards. If you imagine the cylinder the film
wraps around then if we could shrink the cylinder to 99% of its radius
then everything would work fine. Note that the length of film would be
99% of what it was before if we could do that. But if we move the lens
back towards the film then the film length stays the same. So there is
a difference. Since the film is longer in the second case and yet
covering the same angle of view, then it stands to reason that in the
second case the images have been shifted horizontally by about 1%. In
the original configuration, then if we had an image of a pole then as
the lens swings round the pole image stays in the one place on the
film. If we shift the lens backwards to 99% of the distance then the
image is 99% of the size and so as the lens swings round then the
position of the pole image when it first comes into view is not the
same as the position of the pole image as it goes out of view. If the
slit is narrow at the back then it does not matter but if the slit is
wide, like for a longer exposure, then it does. You will have got the
sharpness at the expense of a smaller image that shifts horizontally
very slightly. On the other hand, if the cylinder were shrunk to 99%
then the image of the pole would stay on the same place on the film as
it came into view and as it went out of view and it would be perfectly
sharp. So shifting the lens backwards is not a good cure for the
distance being out of focus.

RolandRB wrote:

So shifting the lens backwards is not a good cure for the
distance being out of focus.

In real life, this doesn't appear to be that much of a problem, since
the 150U can be focussed to various settings from a few meters to
infinity. And no, they don't do this by shrinking and expanding the rear
of the camera.

Ralf

--
Ralf R. Radermacher - DL9KCG - Köln/Cologne, Germany
private homepage: http://www.fotoralf.de
manual cameras and photo galleries - updated March 30, 2004
Contarex - Kiev 60 - Horizon 202 - P6 mount lenses

(Ralf R. Radermacher) wrote in message ...
RolandRB wrote:

So shifting the lens backwards is not a good cure for the
distance being out of focus.

In real life, this doesn't appear to be that much of a problem, since
the 150U can be focussed to various settings from a few meters to
infinity.

And no, they don't do this by shrinking and expanding the rear
of the camera.

I suggested at no point that this was a mechanical possibility.
Instead I put it forward as an ideal (non-practical) scenario that
would circumvent the magnificationm problem that would occur if the
rear slit were large. But then you snipped nearly all my post without
acknowledging it.

I looked at your web site and saw that you had taken a night-time
photograph with your Horizon 202. There was some sky on the right left
over from a setting sun but the exposure on the left that was more
like night conditions and was correctly exposed. Did you not reply in
a previous message to mine, when I suggested that the Horizon was
capable of taking night shots, that in your 4 years of using the
camera you must have missed something, because you could not find a
setting to take exposures longer than 1/2 a second? Perhaps you forgot
about that photo you took.

RolandRB wrote:

Perhaps you forgot
about that photo you took.

No, not at all. This picture has been taken at f2.8 with 1/2 sec on ISO
800 film. The whole thing only worked because the surface of the street
is rather bright and there are lots of artificial light sources. Still
it's underexposed by at least two stops and quite grainy and it wouldn't
lend itself to any enlargement bigger than 15 cm in the longest
direction. If this is acceptable quality to you, it isn't to me.

Ralf

--
Ralf R. Radermacher - DL9KCG - Köln/Cologne, Germany
private homepage: http://www.fotoralf.de
manual cameras and photo galleries - updated March 30, 2004
Contarex - Kiev 60 - Horizon 202 - P6 mount lenses

(Ralf R. Radermacher) wrote in message ...
RolandRB wrote:

So shifting the lens backwards is not a good cure for the
distance being out of focus.

In real life, this doesn't appear to be that much of a problem, since
the 150U can be focussed to various settings from a few meters to
infinity. And no, they don't do this by shrinking and expanding the rear
of the camera.

Ralf

I've just found a Noblex site with some information on that I can use
in a few calculations. What I will show you is that moving the lens
backwards to achieve focus at infinity will cause horizontal
resolution problems. But focussing at infinity can be achieved by
adjusting the focal length of the lens in any case so long as the
effective distance from the film will not change. But first I will
show you how moving the lens back causes a problem.

I will use this model for my calculations:
http://www.whistlerinns.com/noblex/150_hs.htm

First I need to calculate the radius of the drum inside the camera
(from centre to film surface) according to the figures provided on
that page. The horizontal 135 degrees is on 120mm of film so the
radius is given by:

(360*120)/(135*2*pi)mm = 50.93 mm

The lens is quoted as having a focal length of 50.75 mm so you see
that they are using a lens of slightly shorter focal length than the
centre to film radius. The effect of this is to have a distance closer
than infinity in focus and they quote this distance as 10.4 meters. I
will now use that figure of 10.4 meters to accurately calculate the
lens centre to film surface distance.

1/f = 1/objectdistance + 1/imagedistance

1/imagedistance = 1/50.75 - 1/10400 = 0.019608

....therefore image distance = 51 mm

So the distance of the optical centre of the lens to the film surface
is 51 mm and I will assume that this is what 50.93 should have been if
the figures for the angle and film length were given more accurately.
So what we have is a 50.75 mm focal length lens at the centre of a
51mm radius and the effect of that slightly shorter focal length is to
have objects at 10.4 meters in focus rather than objects at infinity
in focus.

I will show you how the horizontal resolution is affected as the lens
is moved backwards. Moving back from 51mm to 50.75mm to put the
distance in focus will change the magnification to 1.005 of what it
was before. Half a percent. First I will make a guess of the resolving
power of such a lens. This will be very high due to the simple design
that is not required to "get into the corners" as flat film cameras
do. I will assume a resolving power of 100 line pairs per millimeter,
except maybe at the top and bottom of the image. Now supposing the
slit at the back were 1 mm in width, then the 0.5% change in
magnification would cause an upright image to move by 1/200th of a
millimeter horizontally. This would reduce the horizontal resolution
from an initial assumption of 100 lp/mm to a guesstimate (done by me
drawing) of 66 lp/mm (not a problem). But if the rear slit were 2mm
wide then the horizontal resolution would be reduced to about 33 lp/mm
(a slight problem but such a poor resolution is, I believe, not
uncommon for medium format cameras) and if the slit were 3mm wide then
the horizontal resolution would be reduced to 17 lp/mm (a problem). If
the slit were 5mm wide then the horizontal resolution would be about 4
lp/mm and this would be in stark contrast to the vertical resolution
still at 100 lp/mm.

Now I believe from reading some Internet sites that the Noblex 120
film models have a fixed slit at the back and different exposures are
achieved by using different rotational speeds. I have no confirmation
of this, though. So if the focussing at infinity was achieved by
moving the lens backwards then there is no way this problem can be
escaped and the resolution would be affected as I have indicated. If
the slit were 2mm wide or less then I wonder if that problem would be
noticed. It wouldn't be noticed if the slit were as narrow as 1mm.

So what I am suggesting is that if the slit at the back is indeed
fixed and is wider than 2mm then they might be achieving the change in
focus by means other than moving the lens forwards and backwards.
There is nothing to stop them changing the focal length of the lens
using a correcting lens so long as the optical centre of the
arrangement was still 51mm from the film and therefore the
magnification did not change. If the combined lens had a focal length
of 51mm instead of 50.75 mm and the optical centre was 51mm from the
film then the distance would be in focus and there would be no loss of
resolution. But there is a much easier way to achieve this without a
correcting lens. If the lens is a Tessar type then they could adjust
the air gap between the front converging element and the strongly
diverging element behind it. Ths was done on front-cell focussing
folding cameras. This changes the focal length of the lens and could
probably be done in such a way as to leave the optical centre of the
lens arrangement the same distance from the film. I guess it would be
just as easy or even easier to move the front element as move the
whole lens.

It would be interesting to hear from somebody who knows the optical
construction of these focussing Noblex 120 film cameras. If somebody
out there has one then can they confirm that the slit at the back is
fixed and if so what width is it?