Olympus OM enthusiasts' digital prayers have been answered ...

In article 2012020819270899904-pete3attkins@nospamntlworldcom, Pete A
writes
On 2012-02-08 16:30:15 +0000, Kennedy McEwen said:

In article , Rol_Lei Nut
writes
On 2/7/2012 14:12, Kennedy McEwen wrote:

OM lenses work just as well on digital as they did on film, which
doesn't mean telecentric lenses can't work better, but the argument was
false to begin with and was merely an attempt by Olympus to justify
their investment in cheap chips.
I have a Panasonic M43 and use *lots* of made for film lenses on it
(M42, Zeiss, Leica both M & R, M39 & others).
Some lenses, especially fast one, often don't work well at their
faster apertures (by "not well" I mean visibly less well than on a
film camera).
Most lenses will exhibit vignetting when wide open. That has
nothing whatsoever to do with the sensor, whether digital or film,
but due to the exit pupil being truncated. You can see this
happening just by looking through the lens while tilting it - that
circular aperture wide open closes down to a "cat's eye" when
significantly off axis. The "cat's eye" is the main cause of the
corner vignette. Stop the lens down a little and tilting it has no
effect on the aperture.
This was just the same with film as it is on a digital sensor -
whether you noticed it or not. Furthermore, stopping the lens down
doesn't change its telecentricity - so the very fact that you get a
different level of vignette fully open demonstrates that
telecentricity isn't the issue that Olympus claim(ed) it to be.

If a lens is rear telecentric then its rays are parallel i.e. the angle
of incidence is zero.

How can an image form with parallel rays? It is the *principle* rays
which are parallel in a telecentric lens, not all the rays and certainly
not the peripheral rays which determine the f/#.

If what you claimed was true then any pinhole lens would be telecentric
- which is certainly not the case.

Stopping it down cannot reduce the angle of incidence any further. For
a non-telecentric lens, of course stopping it down reduces the angle of
incidence thereby making it _nearer to_ rear telecentric than when it
is wide-open.

Stopping a non-telecentric lens down does not change the angle of
incidence of the principle rays to any point on the focal plane. Thus,
by definition, stopping a lens down does *NOT* make a lens "nearer to"
telecentric in any way.

Lots of compact, allegedly useless on digital because they aren't
telecentric, OM lenses are used on Canon and other FF digital cameras
without problem, hence their used prices are holding up quite well.
Only this morning I was shooting with the Zuiko 18mm f/3.5, one of
the least telecentric lenses on the OM lineup due to the tiny rear
pupil,

Hang on a minute - a tiny rear exit pupil ensures that rays will not
strike the sensor/film at an oblique angle

Only close to the optic axis, ie. in the centre of the frame. A tiny
exit pupil ensures that the principle rays at the corner of the frame
strike the focal plane at a *much* more oblique angle than those at the
centre of the frame. To be telecentric, the principle rays for each
image point in the frame are parallel - a small rear element, only
slightly larger than the on-axis pupil, ensures that they are not.

, therefore how can it be one of the least telecentric lenses in the OM
lineup?
That paradox in your theory is something for you to ponder. There is no
paradox in mine.

One of the benefits of telecentricity is that geometric distortion is
minimised and objects remain the same size as the focus travels through
them. The design of the OM series Zuiko 18mm maximises both of these
effects, the object field magnifies at close focus and shrinks as focus
moves to infinity. Exaggerated geometry, not to be confused with the
exaggerated perspective due to the short focal length, was a selling
feature of that lens - achieved by its extremely non-telecentric design.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

"Bruce" wrote in message
news

I only suggested it didn't match my experience and you have now explained
why. Thank you!


You're welcome. But I didn't add anything that wasn't already in the
post you decided not to read. ;-)

So show me where you previously mentioned the use of Kodak sensors which my
Canon doesn't have, and possibly explains why I don't have the problems you
are sure exist with the Olympus cameras.

Trevor.

In article , Bruce
writes

The suggestion that the least telecentric lenses are always those with
tiny rear elements is completely risible.

Nobody said that is *always* the case.
I gave an example, which is certainly the most common situation, where
it *IS* the case.


In some cases, they are
among the *most* telecentric!

That is a completely ridiculous statement - we can all restrict our
comparisons to the "some cases" which *can* be worse. In most cases
that is NOT the case. I referred to the entire OM range.

There are many aspects of the OM 18mm which demonstrate it's poor
telecentricity. However poor rear telecentricity is guaranteed
*because* it has a small rear element. That, together with the proximity
of the rear element to the focal plane (it projects into the lens mount
and just clears the mirror!), restricts the angle of incidence of the
principle rays at the corners of the focal plane to very oblique angles:
the very problem that Olympus claim makes such a non-telecentric lens
unsuitable for digital. Yet it works extremely well and is one of the
most sought after OM Zuikos for FF dSLRs!

The implied converse, that a larger rear element means the lens is
closer to telecentric is *complete nonsense*.

That is *not* the implied converse. Only an idiot thinks that "all cats
are furry animals" implies "all furry animals are cats"!

The converse is that telecentricity, more specifically
rear-telecentricity which is the version under discussion here,
*requires* a large rear element!

Should Bruce Almighty dispute that, I am sure he can point us to a ray
diagram which shows otherwise. (That doesn't mean a diagram showing that
telecentricity *can* be worse on a large rear element design, that is
obvious with the standard textbook design of a front-telecentric lens
being such an example.)
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

On 2012-02-08 23:44:00 +0000, Kennedy McEwen said:

In article 2012020819270899904-pete3attkins@nospamntlworldcom, Pete A
writes
On 2012-02-08 16:30:15 +0000, Kennedy McEwen said:

In article , Rol_Lei Nut
writes
On 2/7/2012 14:12, Kennedy McEwen wrote:

OM lenses work just as well on digital as they did on film, which
doesn't mean telecentric lenses can't work better, but the argument was
false to begin with and was merely an attempt by Olympus to justify
their investment in cheap chips.
I have a Panasonic M43 and use *lots* of made for film lenses on it
(M42, Zeiss, Leica both M & R, M39 & others).
Some lenses, especially fast one, often don't work well at their faster
apertures (by "not well" I mean visibly less well than on a film
camera).
Most lenses will exhibit vignetting when wide open. That has nothing
whatsoever to do with the sensor, whether digital or film, but due to
the exit pupil being truncated. You can see this happening just by
looking through the lens while tilting it - that circular aperture wide
open closes down to a "cat's eye" when significantly off axis. The
"cat's eye" is the main cause of the corner vignette. Stop the lens
down a little and tilting it has no effect on the aperture.
This was just the same with film as it is on a digital sensor - whether
you noticed it or not. Furthermore, stopping the lens down doesn't
change its telecentricity - so the very fact that you get a different
level of vignette fully open demonstrates that telecentricity isn't the
issue that Olympus claim(ed) it to be.

If a lens is rear telecentric then its rays are parallel i.e. the angle
of incidence is zero.

How can an image form with parallel rays? It is the *principle* rays
which are parallel in a telecentric lens, not all the rays and
certainly not the peripheral rays which determine the f/#.

Indeed, the principle ray contains no light whatsoever because it's
infinitely thin. Yes, the aperture affects the cone of peripheral rays.


If what you claimed was true then any pinhole lens would be telecentric
- which is certainly not the case.

A pinhole camera exhibits cos^4 light falloff at the film. Having a
very small aperture results in a correspondingly small cone therefore
it is primarily the angle of the principle ray that causes the light
falloff in such a camera. At 33 degrees the falloff is 1 f-stop.

With an image-side (rear) telecentric lens, it is only the angle of the
cone that causes light falloff. The cone does not vary across the image
plane therefore the image doesn't suffer from corner vignetting. If the
lens is badly designed then mechanical vignetting may cause light
falloff in the image corners.

Very wide-angle lenses suffer from light falloff on the object side,
but this is a separate issue.

In any mirror-less system, the lens designer is free to position the
exit pupil very close to the image plane, which places an unrealistic
demand on the design of image sensors; especially Bayer CFA sensors.
Specifying "near-telecentric" simply means placing the exit pupil
_reasonably_ far away from the sensor. So, it is a big issue and
perfectly warranted in a mirror-less system specification.


Stopping it down cannot reduce the angle of incidence any further. For
a non-telecentric lens, of course stopping it down reduces the angle of
incidence thereby making it _nearer to_ rear telecentric than when it
is wide-open.

Stopping a non-telecentric lens down does not change the angle of
incidence of the principle rays to any point on the focal plane. Thus,
by definition, stopping a lens down does *NOT* make a lens "nearer to"
telecentric in any way.

Literally, you are correct. In essence, stopping down reduces the rays
that are problematic to the sensor.


Lots of compact, allegedly useless on digital because they aren't
telecentric, OM lenses are used on Canon and other FF digital cameras
without problem, hence their used prices are holding up quite well.
Only this morning I was shooting with the Zuiko 18mm f/3.5, one of the
least telecentric lenses on the OM lineup due to the tiny rear pupil,

Hang on a minute - a tiny rear exit pupil ensures that rays will not
strike the sensor/film at an oblique angle

Only close to the optic axis, ie. in the centre of the frame. A tiny
exit pupil ensures that the principle rays at the corner of the frame
strike the focal plane at a *much* more oblique angle than those at the
centre of the frame. To be telecentric, the principle rays for each
image point in the frame are parallel - a small rear element, only
slightly larger than the on-axis pupil, ensures that they are not.

The tiny exit pupil you mentioned is not the problem. The distance of
the exit pupil from the image plane is the problem.


, therefore how can it be one of the least telecentric lenses in the OM lineup?
That paradox in your theory is something for you to ponder. There is
no paradox in mine.

There is no paradox, you misstated cause and effect. That isn't a
criticism - I frequently write things that make sense only to me.


One of the benefits of telecentricity is that geometric distortion is
minimised and objects remain the same size as the focus travels through
them. The design of the OM series Zuiko 18mm maximises both of these
effects, the object field magnifies at close focus and shrinks as focus
moves to infinity. Exaggerated geometry, not to be confused with the
exaggerated perspective due to the short focal length, was a selling
feature of that lens - achieved by its extremely non-telecentric design.

That sounds like a very interesting lens.

Why would a user of the 24x36mm OM-1/2/3/4 be anxious for a 1/4 frame
(17.3 x 13 mm) look-a-like?


--
Mike

In article 2012020918152437271-pete3attkins@nospamntlworldcom, Pete A
writes
On 2012-02-08 23:44:00 +0000, Kennedy McEwen said:

In article 2012020819270899904-pete3attkins@nospamntlworldcom, Pete
A writes

If a lens is rear telecentric then its rays are parallel i.e. the
angle of incidence is zero.
How can an image form with parallel rays? It is the *principle*
rays which are parallel in a telecentric lens, not all the rays and
certainly not the peripheral rays which determine the f/#.

Indeed, the principle ray contains no light whatsoever because it's
infinitely thin. Yes, the aperture affects the cone of peripheral rays.

Of course it does, but those peripheral rays don't affect the
telecentricity. It is the angle of the principle rays which determine
that.

If what you claimed was true then any pinhole lens would be
telecentric - which is certainly not the case.

A pinhole camera exhibits cos^4 light falloff at the film. Having a
very small aperture results in a correspondingly small cone therefore
it is primarily the angle of the principle ray that causes the light
falloff in such a camera. At 33 degrees the falloff is 1 f-stop.

With an image-side (rear) telecentric lens, it is only the angle of the
cone that causes light falloff. The cone does not vary across the image
plane therefore the image doesn't suffer from corner vignetting.

Exactly what I said previously! A pinhole lens is NOT telecentric - the
narrow cone angle of the high f/# does not affect the telecentricity of
a lens, which is determined by angle of the principle rays to each point
on the image plane.

In any mirror-less system, the lens designer is free to position the
exit pupil very close to the image plane, which places an unrealistic
demand on the design of image sensors; especially Bayer CFA sensors.
Specifying "near-telecentric" simply means placing the exit pupil
_reasonably_ far away from the sensor. So, it is a big issue and
perfectly warranted in a mirror-less system specification.

We are discussing the use of OM series lenses in particular, which were
not mirrorless cameras, and lenses from other film SLR cameras in
general. So the issue of "placing an exit pupil close to the image
plane" is irrelevant. Nevertheless, the same constraints on lens
designs existed for mirrorless film cameras, such as Leica rangefinders.
Furthermore, the lens specification for the 4-turds standard required
compatibility with dSLRs as well, so that standard has the same
restriction on rear element position as well. Olympus claimed they had
to use small sensors because digital sensors in dSLRs required
telecentric optics to work properly and the lenses designed for film
would not cope with full size digital sensors. That was a lie and
remains demonstrably so by anyone with an OM adapter and a FF dSLR.

Stopping a non-telecentric lens down does not change the angle of
incidence of the principle rays to any point on the focal plane. Thus,
by definition, stopping a lens down does *NOT* make a lens "nearer to"
telecentric in any way.

Literally, you are correct. In essence, stopping down reduces the rays
that are problematic to the sensor.

It reduces just as many rays that are less oblique, and thus *less
problematic*, as it does rays that are more oblique than the principle
ray - it has no effect on the telecentricity or the response of the
sensor. The change in light fall-off when stopping down is almost
entirely a consequence of the exit pupil being partially obscured when
fully open, not the angle of incidence of the principle rays - they
remain fixed.

The tiny exit pupil you mentioned is not the problem. The distance of
the exit pupil from the image plane is the problem.

You can't place a larger rear element any closer to the focal plane on
an SLR without obstructing the mirror, consequently the 18/3.5 is "one
of the least telecentric lenses in the OM series" due to its small rear
element. If the lens design utilised a larger convergent rear element
it could be placed at exactly the same distance with better
telecentricity - so it *is* the size that makes this lens "one of the
least telecentric lenses in the OM lineup".

, therefore how can it be one of the least telecentric lenses in the
OM lineup?
That paradox in your theory is something for you to ponder. There is
no paradox in mine.

There is no paradox, you misstated cause and effect.

I have re-read exactly what I wrote and I certainly did NOT mis-state
cause and effect. On the contrary, you claimed that "a tiny rear exit
pupil *ensures* that rays will not strike the sensor/film at an oblique
angle" which is not only confusing cause and effect but also wrong!
However, this shouldn't degrade to a "he said, she said" issue, the fact
remains that telecentricity or the lack of it in lenses designed for
film has no more effect on the solid state sensors in modern digital
cameras than it did on film.

One of the benefits of telecentricity is that geometric distortion is
minimised and objects remain the same size as the focus travels
through them. The design of the OM series Zuiko 18mm maximises both
of these effects, the object field magnifies at close focus and
shrinks as focus moves to infinity.

That sounds like a very interesting lens.

All non-telecentric lenses exhibit this effect to some degree. In fact
it is one of the simplest tests of rear telecentricity - fill the frame
with a subject and pull focus from closest point through to infinity. If
the object remains the same size independent of the focus position (the
distance of the lens from the focal plane) then the lens is rear
telecentric. The more that image size changes, and hence geometry is
distorted, with focus the less telecentric the lens is.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

On 2/9/2012 19:50, Mike wrote:
Why would a user of the 24x36mm OM-1/2/3/4 be anxious for a 1/4 frame
(17.3 x 13 mm) look-a-like?


Not anxious, but there are things to like: "Free film" (well, once
you've more or less heavily invested in equipment and any necessary
peripherals), "instant developing" (though if you shoot raw that also
takes some time & most images can use some adjusting before printing or
viewing), some quite decent lenses (some of the very few WA zooms I've
found to be good - I normally use Zeiss & Leica with film) and very
compact and light (even compared to the original OM series).

What I like less it that the "form follows function" idea got lost in
translation and the useless false pentraprism adds quite a bit to the
effective size of the camera.
Also the hand grips seem to be a lame attempt to label the camera as a
"pro" model (marketing predominating over content).
Perhaps Olympus should introduce some oversized white telephoto lenses
which can be parked at the side of sport fields, then they'd have
succeeded in emulating the ultimate marketing brand....

That said, if the image quality is as good as it should be and the
camera seems tough enough, I'll probably end up buying one when they get
affordable. A water resistant, tough (hopefully) camera with decent (or
hopefully more than decent) quality and tiny but good lenses is
something to like.

But I'll still use my film Leicas and Rolleiflexes when I want to do
something special...

On Thu, 09 Feb 2012 14:55:37 +0000, Bruce
wrote:

RichA wrote:
On Feb 8, 11:30*am, Kennedy McEwen wrote:
In article , Rol_Lei Nut
writes

On 2/7/2012 14:12, Kennedy McEwen wrote:

OM lenses work just as well on digital as they did on film, which
doesn't mean telecentric lenses can't work better, but the argument was
false to begin with and was merely an attempt by Olympus to justify
their investment in cheap chips.

I have a Panasonic M43 and use *lots* of made for film lenses on it
(M42, Zeiss, Leica both M & R, M39 & others).

Some lenses, especially fast one, often don't work well at their faster
apertures (by "not well" I mean visibly less well than on a film
camera).

Most lenses will exhibit vignetting when wide open. *That has nothing
whatsoever to do with the sensor, whether digital or film, but due to
the exit pupil being truncated. *You can see this happening just by
looking through the lens while tilting it - that circular aperture wide
open closes down to a "cat's eye" when significantly off axis. *The
"cat's eye" is the main cause of the corner vignette. *Stop the lens
down a little and tilting it has no effect on the aperture.

This was just the same with film as it is on a digital sensor - whether
you noticed it or not. *Furthermore, stopping the lens down doesn't
change its telecentricity - so the very fact that you get a different
level of vignette fully open demonstrates that telecentricity isn't the
issue that Olympus claim(ed) it to be.

Lots of compact, allegedly useless on digital because they aren't
telecentric, OM lenses are used on Canon and other FF digital cameras
without problem, hence their used prices are holding up quite well.

Only this morning I was shooting with the Zuiko 18mm f/3.5, one of the
least telecentric lenses on the OM lineup due to the tiny rear pupil,
and it is extremely flat to the corners when stopped down to f/5.6 -
less than quarter a stop.

I think there is more to this than just telecentricity. I tested an
85mm f2.0 OM lens on a m4/3rds camera and it vignetted, right down to
f5.6.
If an 18mm OM doesn't vignette horribly on a FF, then something else
is at work.


I find this discussion highly entertaining because it is the blind
leading the blind.

http://www.opto-engineering.com/tele...-tutorial.html may
be of some help.

The suggestion that the least telecentric lenses are always those with
tiny rear elements is completely risible. In some cases, they are
among the *most* telecentric!

The implied converse, that a larger rear element means the lens is
closer to telecentric is *complete nonsense*. It is extremely naive
to assume that because the size of the rear element is closer to the
size of the sensor, that the rays must be emerging almost parallel to
each other and therefore perpendicular to the sensor. Once again, the
truth can be the exact opposite, because a larger rear element can
allow light rays to emerge at *more* oblique angles!

That might appear counter-intuitive to some, but that merely
demonstrates that intuition is highly subjective, and often wrong.



Regards,

Eric Stevens

On 2012-02-09 19:27:46 +0000, Kennedy McEwen said:

In article 2012020918152437271-pete3attkins@nospamntlworldcom, Pete A
writes
On 2012-02-08 23:44:00 +0000, Kennedy McEwen said:

In article 2012020819270899904-pete3attkins@nospamntlworldcom, Pete A
writes

If a lens is rear telecentric then its rays are parallel i.e. the angle
of incidence is zero.
How can an image form with parallel rays? It is the *principle* rays
which are parallel in a telecentric lens, not all the rays and
certainly not the peripheral rays which determine the f/#.

Indeed, the principle ray contains no light whatsoever because it's
infinitely thin. Yes, the aperture affects the cone of peripheral rays.

Of course it does, but those peripheral rays don't affect the
telecentricity. It is the angle of the principle rays which determine
that.

If what you claimed was true then any pinhole lens would be telecentric
- which is certainly not the case.

A pinhole camera exhibits cos^4 light falloff at the film. Having a
very small aperture results in a correspondingly small cone therefore
it is primarily the angle of the principle ray that causes the light
falloff in such a camera. At 33 degrees the falloff is 1 f-stop.

With an image-side (rear) telecentric lens, it is only the angle of the
cone that causes light falloff. The cone does not vary across the image
plane therefore the image doesn't suffer from corner vignetting.

Exactly what I said previously! A pinhole lens is NOT telecentric -
the narrow cone angle of the high f/# does not affect the
telecentricity of a lens, which is determined by angle of the principle
rays to each point on the image plane.

In any mirror-less system, the lens designer is free to position the
exit pupil very close to the image plane, which places an unrealistic
demand on the design of image sensors; especially Bayer CFA sensors.
Specifying "near-telecentric" simply means placing the exit pupil
_reasonably_ far away from the sensor. So, it is a big issue and
perfectly warranted in a mirror-less system specification.

We are discussing the use of OM series lenses in particular, which were
not mirrorless cameras, and lenses from other film SLR cameras in
general. So the issue of "placing an exit pupil close to the image
plane" is irrelevant. Nevertheless, the same constraints on lens
designs existed for mirrorless film cameras, such as Leica
rangefinders. Furthermore, the lens specification for the 4-turds
standard required compatibility with dSLRs as well, so that standard
has the same restriction on rear element position as well. Olympus
claimed they had to use small sensors because digital sensors in dSLRs
required telecentric optics to work properly and the lenses designed
for film would not cope with full size digital sensors. That was a lie
and remains demonstrably so by anyone with an OM adapter and a FF dSLR.

Stopping a non-telecentric lens down does not change the angle of
incidence of the principle rays to any point on the focal plane. Thus,
by definition, stopping a lens down does *NOT* make a lens "nearer to"
telecentric in any way.

Literally, you are correct. In essence, stopping down reduces the rays
that are problematic to the sensor.

It reduces just as many rays that are less oblique, and thus *less
problematic*, as it does rays that are more oblique than the principle
ray - it has no effect on the telecentricity or the response of the
sensor. The change in light fall-off when stopping down is almost
entirely a consequence of the exit pupil being partially obscured when
fully open, not the angle of incidence of the principle rays - they
remain fixed.

The tiny exit pupil you mentioned is not the problem. The distance of
the exit pupil from the image plane is the problem.

You can't place a larger rear element any closer to the focal plane on
an SLR without obstructing the mirror, consequently the 18/3.5 is "one
of the least telecentric lenses in the OM series" due to its small rear
element. If the lens design utilised a larger convergent rear element
it could be placed at exactly the same distance with better
telecentricity - so it *is* the size that makes this lens "one of the
least telecentric lenses in the OM lineup".

, therefore how can it be one of the least telecentric lenses in the OM lineup?
That paradox in your theory is something for you to ponder. There is
no paradox in mine.

There is no paradox, you misstated cause and effect.

I have re-read exactly what I wrote and I certainly did NOT mis-state
cause and effect. On the contrary, you claimed that "a tiny rear exit
pupil *ensures* that rays will not strike the sensor/film at an oblique
angle" which is not only confusing cause and effect but also wrong!
However, this shouldn't degrade to a "he said, she said" issue, the
fact remains that telecentricity or the lack of it in lenses designed
for film has no more effect on the solid state sensors in modern
digital cameras than it did on film.

One of the benefits of telecentricity is that geometric distortion is
minimised and objects remain the same size as the focus travels through
them. The design of the OM series Zuiko 18mm maximises both of these
effects, the object field magnifies at close focus and shrinks as focus
moves to infinity.

That sounds like a very interesting lens.

All non-telecentric lenses exhibit this effect to some degree. In fact
it is one of the simplest tests of rear telecentricity - fill the frame
with a subject and pull focus from closest point through to infinity.
If the object remains the same size independent of the focus position
(the distance of the lens from the focal plane) then the lens is rear
telecentric. The more that image size changes, and hence geometry is
distorted, with focus the less telecentric the lens is.

The axe that you are grinding is with Olympus. My condoning of Olympus
recommendations for practical reasons obviously irritates you - this is
obvious not by your replies, it is obvious from what you snip from my
replies.

Enjoy your photography.

On 2012-02-09 20:12:27 +0000, Eric Stevens said:

On Thu, 09 Feb 2012 14:55:37 +0000, Bruce wrote:

RichA wrote:
On Feb 8, 11:30*am, Kennedy McEwen wrote:
In article , Rol_Lei Nut
writes

On 2/7/2012 14:12, Kennedy McEwen wrote:

OM lenses work just as well on digital as they did on film, which
doesn't mean telecentric lenses can't work better, but the argument was
false to begin with and was merely an attempt by Olympus to justify
their investment in cheap chips.

I have a Panasonic M43 and use *lots* of made for film lenses on it
(M42, Zeiss, Leica both M & R, M39 & others).

Some lenses, especially fast one, often don't work well at their faster
apertures (by "not well" I mean visibly less well than on a film
camera).

Most lenses will exhibit vignetting when wide open. *That has nothing
whatsoever to do with the sensor, whether digital or film, but due to
the exit pupil being truncated. *You can see this happening just by
looking through the lens while tilting it - that circular aperture wide
open closes down to a "cat's eye" when significantly off axis. *The
"cat's eye" is the main cause of the corner vignette. *Stop the lens
down a little and tilting it has no effect on the aperture.

This was just the same with film as it is on a digital sensor - whether
you noticed it or not. *Furthermore, stopping the lens down doesn't
change its telecentricity - so the very fact that you get a different
level of vignette fully open demonstrates that telecentricity isn't the
issue that Olympus claim(ed) it to be.

Lots of compact, allegedly useless on digital because they aren't
telecentric, OM lenses are used on Canon and other FF digital cameras
without problem, hence their used prices are holding up quite well.

Only this morning I was shooting with the Zuiko 18mm f/3.5, one of the
least telecentric lenses on the OM lineup due to the tiny rear pupil,
and it is extremely flat to the corners when stopped down to f/5.6 -
less than quarter a stop.

I think there is more to this than just telecentricity. I tested an
85mm f2.0 OM lens on a m4/3rds camera and it vignetted, right down to
f5.6.
If an 18mm OM doesn't vignette horribly on a FF, then something else
is at work.


I find this discussion highly entertaining because it is the blind
leading the blind.

http://www.opto-engineering.com/tele...-tutorial.html may
be of some help.

Thanks Eric.

Some may find these interesting:

http://www.pco.de/fileadmin/user_upload/db/download/pco_cooKe_kb_shading_0603_s.pdf


http://en.wikipedia.org/wiki/Vignetting

http://toothwalker.org/optics/vignetting.html



The suggestion that the least telecentric lenses are always those with
tiny rear elements is completely risible. In some cases, they are
among the *most* telecentric!

The implied converse, that a larger rear element means the lens is
closer to telecentric is *complete nonsense*. It is extremely naive
to assume that because the size of the rear element is closer to the
size of the sensor, that the rays must be emerging almost parallel to
each other and therefore perpendicular to the sensor. Once again, the
truth can be the exact opposite, because a larger rear element can
allow light rays to emerge at *more* oblique angles!

That might appear counter-intuitive to some, but that merely
demonstrates that intuition is highly subjective, and often wrong.

I totally agree with Bruce. A true bi-telecentric (afocal) lens demands
a front element somewhat larger than the object and a rear element
somewhat larger than the sensor; a near-telecentric image-space lens
has no such demands. Very few people can reverse-engineer a lens just
by looking at it and jumping to intuitive conclusions :-)