projector lens selection, curved field or flat field?
Rob Hasker
>A week ago, I posted an article asking for suggestions for projector lenses.
>Currently, I use a Kodak "curved field lens" and have problem with
>uneven focus: unable to simultaneously focus both the center and edges
>of the projected picture. Thanks again for the follow ups and replies.
>-- Shun Cheung, AT&T Bell Laboratories, Middletown, New Jersey, USA
I've become pretty convinced that Kodak is talking through its hat as well
but I must admit I haven't actually tried another piece of glass (this will
have to wait until I graduate :-(. But does anyone else make curved-field
lenses? It seems to me that if no one else makes them, something must be
wrong (or even criminal) with Kodak.
Anyways, back when I was getting very frustrated with trying to focus on my
slides, I finally thought of building a projector stand so that the lens
points at the exact middle of the screen. This resulted in a dramatic
improvement. I knew I would get better results if I didn't have the
projector pointing up at an angle, but I hadn't realized just how much.
Now just the extreme edges won't come into focus. Of course, this will
probably be obvious to most readers, but I offer it as a suggestion to
those still setting their projector on a chair.
--
Rob Hasker University of Illinois at Urbana-Champaign
has...@cs.uiuc.edu {uunet|convex|pur-ee}!uiucdcs!hasker
David Dyer-Bennet
: *All* photographic lenses are of `flat-field' design. None are
:perfectly flat, but all designers aim for it. A lens that is noticably
:`nonflat,' or `curved,' is completely useless.
This is arrant nonsense. (also errant nonsense :-)
The standard Kodak projection lenses are curved field, and the taking
lenses in the early Kodak 110 cameras were also widely speculated to
be deliberately of curved field design. In both cases, it was because
the film they were working with was not being held flat.
As you say, the curve of a slide in cardboard mount (or a 110
cartridge) is not precisely the same each time. But it's close enough
that a lens computed for the average works better most of the time.
For precision work (multi-image shows with panoramas, for example),
pin-registered slide mounts, probably with glass, are desirable, and
in that case flat-field projection lenses would be appropriate.
--
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Jan Brittenson
sh...@cbnewsh.att.com (shun.cheung) writes:
> Curved field lenses are designed to compensate for the non-flat
> surface of slides under the heat of projection lamps.
.flame
What makes you think that the heat from the lamp is constant, that
the film curvature while the slide sits in the light is constant, or
that different film bases and different individual slides, are
identical or identically mounted? You can't assume that a slide is
bent, curved, or out of shape in any way. The only assumption you can
make is that a slide is _flat_, which it will be to greater extent if
it's mounted in a glass mount.
.nflame
> The October 1990 issue of PopPhoto has a short article on this ...
I buy one or two issues a year to keep myself a jour with
mail-order prices. I've a long time ago decided that the magazine in
general doesn't question marketing strategies, and so degenerate
itself to, sort of, an independent glossy. Occasionally there is
something worth reading of course, but in my opinion it's not worth
the cents. I can't say I'm too surprised this round of humbug
originates from PopPhoto.
This sounds a lot like the Instamatic Hype of the 1970s.
"Technology has gotten so far now, that people don't _need_ such big
old films." "You have to be an expert to see the difference between
110 and older films." "The price/performance ratio is outstanding."
"Technological breakthroughs..." It all amounts to having people pay
proportionally more for less - if you can't sell oranges because
they're too small, market them as `orangettes' at twice the unit
price.
> Besides Kodak, Leica and Schneider, among other brands, make this type
> of lenses. How well these "curved field" projector lenses work is a
> different problem.
No Leitz projector/lens I've ever used has called itself "curved
field." I'm quite surprised to hear that Leitz has jumped on the fad
train, and would like to know how they have implemented perspective
control (vertical lens adjustment) without a very flat field.
> I have literally thousdans of slides (3000-4000) in paper mount. Is
> there any convenient and inexpensive way to switch to other mounts?
Inexpensive, yes. Difficult, no. Convenient, no. But that doesn't
make paper mounts any better. With respect to flatness, they're
several magnitudes worse than glass mounts. If I were you, I'd start
remounting the most important ones immediately. Or remount a dozen and
see what improvements I get. I'd guess the lens is a greater factor,
though, and it's more trivially remediable (read: $$$).
shun.cheung
Currently, I use a Kodak "curved field lens" and have problem with
uneven focus: unable to simultaneously focus both the center and edges
of the projected picture. Thanks again for the follow ups and replies.
Most of my slides are processed by Kodak/Kodalux and have paper (cardboard)
mounts. Kodak introduced curved field lenses in the '70's for slides
in this type of mounts. However, some people feel that this lens
design is actually the source of my problem while some other replies
suggest that curved field lenses are more suitable for paper mount slides.
Any further suggestion on what type of lens I should get to improve
my results? Thanks again.
--
-- Shun Cheung, AT&T Bell Laboratories, Middletown, New Jersey, USA
electronic: sh...@hou2d.att.com
voice: (908) 615-5135
shun.cheung
> In article <1990Nov29.1...@cbnewsh.att.com>
> sh...@cbnewsh.att.com (shun.cheung) writes:
>
> > Curved field lenses are designed to compensate for the non-flat
> > surface of slides under the heat of projection lamps.
> .flame
^^^^^^
> What makes you think that the heat from the lamp is constant, that
^^^^^^^^^
> the film curvature while the slide sits in the light is constant, or
> that different film bases and different individual slides, are
> identical or identically mounted? You can't assume that a slide is
> bent, curved, or out of shape in any way. The only assumption you can
> make is that a slide is _flat_, which it will be to greater extent if
> it's mounted in a glass mount.
> .nflame
These are good questions. However, this is usually a flameless news group,
and I certainly don't want to break this "tradition" which I treasure
very much. Please read what I typed carefully before flaming. I was merely
repeating (from my poor memory) what Kodak claimed to be their objective
and advantage when they introduced curved field lenses some ~13 years ago.
I never said I thought that objective was ever achieved, precisely due to
the problems you mentioned above. As a matter of fact, my curved field lens
is showing poor performance, possibly due to its design objective.
Let me quote a few lines (without permission :-)) from the Leica P2000
catalog I received earlier this month about their curved field lens:
"... the Leica Colorplan-P CF was developed. CF is the abbreviation
for `curved field.' Because of its special optical characteristics,
this lens renders a focused projection of unglazed slides even if they
are warped."
Again, I am merely repeating what Leica says, if you don't believe
their claims, please contact Kodak or Leica directly.
John Meixner
mounts, but as a matter of practical fact the great majority of them will
end up in cardboard or plastic mounts. (How much do you think it would cost
me to have my 25,000 plus slides remounted in glass?) I agree with Jan that
lens designers aim for 'flat-field' focus (unattainable in practice, however),
but I never regarded Kodak's Ektanar C curved field lenses as a marketing
ploy aimed at making a virtue out of a defect. I have experimented with
both flat and curved field lenses (all Kodak), and there is no doubt in my
admittedly subjective judgement that the curved field lenses deliver a better
projected image for slides mounted in plastic or cardboard. When a new
slide is FIRST projected, the center will appear to be in focus while the
edges are blurry, but after the slide 'pops' (about 30 seconds of cooking
for a new slide), the whole image is in reasonably good focus (acutally the
center pops out of focus but when you refocus the image there is a reasonable
consistency of focus from center to edge. There are, of course, exceptions
with some poorly mounted slides. With a flat field projection lens, the
reverse seems true -- Overall good focus at first, but uneven focus after
cooking. If you like to project your slides often, and for more than a
few seconds at a time, I say go for the curved field lenses. In fact, I am
now considering Schneider's Cinelux-AV 90mm/2.5 CF (I wish they made a zoom)
which B&H quoted me as $209 yesterday by telephone.
While I manage to make a few bucks from photography, I am a logician by
profession, and nothing would please me more than to see several followup
posts explaining where and why my arguments fail. All I'm after is the
truth, and, of course, the best possible viewing of my photos.
Regards to a top-notch discussion group... JM
-----------------------------------------------------------------------------
John Meixner | "The major cause of man's unhappiness is that he has
3IU...@CMUVM.BITNET | not learned to sit quietly in his room." --Pascal
-----------------------------------------------------------------------------
Jan Brittenson
In article <1990Nov29....@cbnewsh.att.com>
sh...@cbnewsh.att.com (shun.cheung) writes:
> A week ago, I posted an article asking for suggestions for projector
> lenses. Currently, I use a Kodak "curved field lens" and have problem
> with uneven focus:
*All* photographic lenses are of `flat-field' design. None are
perfectly flat, but all designers aim for it. A lens that is noticably
`nonflat,' or `curved,' is completely useless.
> Most of my slides are processed by Kodak/Kodalux and have paper
> cardboard) mounts. Kodak introduced curved field lenses in the '70's
> for slides in this type of mounts.
If this is true, then it's the marketing ploy of the century!
Cardboard mounts are poor, in the sense that it's impossible to
predict how the film is going to curve and bend when mounted. Ideally
speaking, since the film is a flat surface, it should be mounted flat,
and projected with a flat lens. Anything else is bunk.
Ronald B.J. Wisner eloquently expresses it in the Jul/Aug 1990
issue of _Darkroom & Creative Camera Techniques_, page 22, "Myths of
`Flat-Field' Lenses":
If ever there were an optical topic rife with mythology and
misinformation, it is the so-called ``flat-field'' lens. Some
of thse myths have become so much a part of the language of
photography they've taken on the ring of truth.
To summarize: buy as `flat' a lens as you can afford, and avoid
cardboard mounts.
shun.cheung
> *All* photographic lenses are of `flat-field' design. None are
> perfectly flat, but all designers aim for it. A lens that is noticably
> `nonflat,' or `curved,' is completely useless.
>
> > Most of my slides are processed by Kodak/Kodalux and have paper
> > cardboard) mounts. Kodak introduced curved field lenses in the '70's
> > for slides in this type of mounts.
>
> If this is true, then it's the marketing ploy of the century!
> Cardboard mounts are poor, in the sense that it's impossible to
> predict how the film is going to curve and bend when mounted. Ideally
Curved field lenses are designed to compensate for the non-flat surface
of slides under the heat of projection lamps. The October 1990 issue of
PopPhoto has a short article on this, along with a long artcile on
projector comparison. Besides Kodak, Leica and Schneider, among other
brands, make this type of lenses. How well these "curved field" projector
lenses work is a different problem.
> To summarize: buy as `flat' a lens as you can afford, and avoid
> cardboard mounts.
I have literally thousdans of slides (3000-4000) in paper mount.
Is there any convenient and inexpensive way to switch to other mounts?
--
Carl Madson
Hey, Jan Brittenson....
LAY OFF!!!
Shun made none of the explicit 'assumptions' you attribute to him, nor is he
short-sighted enough to generalize in such a way. He is merely trying to sort
out some of the confusion in flat vs. curved-field lenses, and your flames on
irrelevant points don't help him or any of the rest of us at all. Launching an
attack on the (improved, in case you hadn't noticed) Pop Photo doesn't solidify
your points, and the thought of netters rushing out to re-mount their slides
based on YNSHO (your...opinion) leaves me cold.
When people on rec.photo ask for help, they deserve it -- along with a bit of
understanding, even if you find their requests sophmoric. When someone makes
too broad of a statement, they should also be corrected, because it helps us
all get a bit closer to the 'truth', if there be so. Help, don't harrass.
--Carl Madson, SRI International, Menlo Park, CA
Darrell Raymond
I would advise using plastic mounts, if only because of the very
sharp borders they provide, which give your slides a more professional
look. Carboard always has ragged edges. I like Gepe 7001's, which come
in a box of 100 and cost about about $9.00 (Canadian). They are resuable
and the two sides are of different colours, which helps you figure out how
to stick the slide in the tray. Typically I have my slides developed
without mounting, thus saving me the lab's mounting charge - it's also easier
to preview the results this way than when the slides are all mounted.
It'd be tedious to remount thousands of slides, but I usually save the job
for some evening that would otherwise be devoted to mindless TV watching.
And then I do both.
Hudson Leighton
I personaly use curved field lens with my plastic mounted slides
The slide mounts that my lab uses hold onto the film with pins through
the sproket holes. I have looked at the slide and do not see much of a
bow and my projected image is sharp all across the screen.
A friend who will only let Kodalux process her pictures slides
Her slides exhibit a signifcant bow to the and we have trouble
getting the corners sharp.
Another friend (hi Dan) has his film processed wherever its cheapest
and his slides show different states of bow with acompaning focus
problems.
Another big problem is is the projector square to the screen
Is the screen Flat?
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Robert Claeson
>I've become pretty convinced that Kodak is talking through its hat as well
>but I must admit I haven't actually tried another piece of glass (this will
>have to wait until I graduate :-(. But does anyone else make curved-field
>lenses? It seems to me that if no one else makes them, something must be
>wrong (or even criminal) with Kodak.
Leica makes them, and Zeiss (under the name Colorplan). But I'd suggest
anyone interested in a Leica projector to look at the Zeiss Royal AF
Selectiv (the name has presumably been changed to something more 'nifty'
in the U.S., but this is the name under which it is sold in Europe). Lower
temperature, better and more even ligh, and a lower price tag are some
of the advantages. The Colorplan lens, which is standard in Zeiss'
projectors, is of the same quality as the one in Leica's projectors.
But I'd suggest the Zeiss P-Sonnar 90mm/2.5 T* instead. It is the
finest projection lens for any 35mm slide projector I know of.
--
Robert Claeson |Reasonable mailers: rcla...@erbe.se
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