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

Discussion in 'Digital Photography' started by Bruce, Feb 4, 2012.

  1. Bruce

    Trevor Guest

    "Bruce" <> wrote in message
    news:p...
    >>
    >>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.
     
    Trevor, Feb 9, 2012
    #41
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  2. Bruce

    Bruce Guest

    "Trevor" <> wrote:
    >"Bruce" <> wrote in message
    >news:p...
    >>>
    >>>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.



    This is like trying to have a conversation with a wasp.
     
    Bruce, Feb 9, 2012
    #42
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  3. Bruce

    RichA Guest

    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.
     
    RichA, Feb 9, 2012
    #43
  4. Bruce

    Bruce Guest

    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.

    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.
     
    Bruce, Feb 9, 2012
    #44
  5. 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 pissed.
    Python Philosophers (replace 'nospam' with 'kennedym' when replying)
     
    Kennedy McEwen, Feb 9, 2012
    #45
  6. Bruce

    Pete A Guest

    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.
     
    Pete A, Feb 9, 2012
    #46
  7. Bruce

    Mike Guest

    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
     
    Mike, Feb 9, 2012
    #47
  8. 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 pissed.
    Python Philosophers (replace 'nospam' with 'kennedym' when replying)
     
    Kennedy McEwen, Feb 9, 2012
    #48
  9. Bruce

    Rol_Lei Nut Guest

    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...
     
    Rol_Lei Nut, Feb 9, 2012
    #49
  10. Bruce

    Pete A Guest

    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.
     
    Pete A, Feb 9, 2012
    #50
  11. Bruce

    Pete A Guest

    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/telecentric-lenses-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 :)
     
    Pete A, Feb 11, 2012
    #51
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