Canon 5D and limited lens choices

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In article <341_e.10010$w74.7940@trndny05>, SamSez
<samtheman@verizon.net> writes
>
>So then why do Sigma, Tamron, Nikon, and yes, [lower your head respectfully],
>even Canon, all use the phrase 'Optimized for digital SLR*' for SOME, but not
>all of their products???

Because most of the lenses bearing this tag are designed for a smaller
field coverage, eg. all of Canon's EF-S range, and therefore do not need
the overhead in glass, mount and movement that is requird to cover the
full field. Sure, some of them are allegedly closer to telecentric
design, but then most wide angle lenses for SLRs have to be closer to
telecentric than those for P&S cameras simply to provide enough room for
the mirror to flip out of the light path when the exposure is made. All
such inverse telephoto (where the distance from the optical centre to
the focal plane is longer than the focal length) designs are partially
telecentric - it is a consequence of the geometry. For focal lengths
greater than the mirror height, the non-telecentricity is negligible in
term of the ray-bundle offset between the filter and the silicon, which
gives rise to the CA you are sputtering about. In short, the problem is
negligible in an SLR design in any case, although it may well be a
problem with P&S cameras, particularly small pixel devices.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a ah heck when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

 

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Kennedy,

Thanks for the explanation. The only thing that still puzzles me is
this:
If a lens is fully telecentric, wouldn't that mean that a one foot
ruler
that was let's say 2 feet from the lens and a one foot ruler that was
4 feet from the lens would produced the same size image on the film
plane
(assuming they are within the depth of field)?
Wouldn't that lead to strange looking images? Or is it that when you
refer
to 'partially telecentric' you are saying that we are somewhere in
between
where perspective is maintained but the variation in angle of the
principle rays to the film plane is minimized?

Thanks again


Kennedy McEwen wrote:
> In article <1127941648.059540.293340@f14g2000cwb.googlegroups.com>,
> "winhag@yahoo.com" <winhag@yahoo.com> writes
> >Kennedy,
> >
> >A purely educational question. You talk about 'telecentric' lens
> >designs.
> >I had never heard of this and looked it up on the web. From what
> >I could understand, a telecentric lens maps objects of the same
> >dimension
> >to the same size on the film plane regardless of object distance from
> >lens (i.e.
> >no 'perspective distortion'). Is that what you are referring to or
> >something different?
> >
> Yes.
>
> If you draw out a simple lens then all of the principle rays (the
> principle ray is the central ray in the bundle) to every point between
> the object and its image on the focal plane passes through the centre of
> the lens. This means that there is only one point in the image where
> the principle ray is perpendicular to the focal plane - the centre of
> the focal plane. At every other point on the focal plane, the principle
> ray diverges from the centre. The easiest way to visualise this is just
> to draw a lens with an object and its image on a sheet of paper and then
> draw in the principle rays from the object to the image, making sure
> that they all pass through the centre of the lens. Now, if the angle of
> incidence at the focal plane is not 90deg, there is a chance that some
> of the light passing through the filter will actually land on an
> adjacent pixel rather than the pixel some small distance under the
> filter, and this causes colour distortions because the camera expects
> all of the light falling on certain pixels to have been filtered to the
> appropriate colour. Obviously the greater the angle from perpendicular
> of the principle ray, and the lower the f/# (ie. the wider the cone
> formed by the bundle of incident rays around the principle one), then
> the more light that can 'leak' into adjacent pixels this way.#
>
> With a telecentric lens, all of the principle rays reaching the focal
> plane are perpendicular to it and, since the same f/# must be produced
> across the focal plane, this means that each point of the image does not
> use all of the available lens aperture - so the rear lens elements are
> physically larger than would be required normally. Of course, that
> means that all of the rays incident at the corner of the focal plane are
> coming from the periphery of the lens, making it more difficult to
> control all aberations, not just chromatic. So telecentric lenses tend
> to have a poorer performance than conventional designs - or they are
> much more expensive for the same performance. The main reason for
> producing such a lens in pre-digital days was, as you have found in your
> net search, to ensure that images did not change size when the system
> was focus was moved. This would be important in instruments measuring
> dimensions directly from the image. However, since telecentricity means
> that the principle ray to every point in the image is perpendicular to
> the focal plane, there is an argument that it is ideal in digital
> sensors to ensure that the amount of leakage from one filter to adjacent
> pixels is fixed - although it does still change with f/# of course.
>
> In most cases though the CA from non-telecentric lenses argument is
> overhyped and you just need to examine the pixel dimensions to see how
> spurious it is. Based on the minimum back working distance to clear the
> mirror, which is proportional to the sensor size, the angle of incidence
> in the corner of the field for the principle ray from this worst case
> situation is almost the same in both formats - leaving only the pixel
> dimension as being the critical parameter. Smaller format sensors
> usually have smaller pixels and they consequently suffer more from this
> problem than larger sensors - completely contrary to the hype.
>
> However, putting this all into perspective, the semi-angle of the light
> cone itself with a fast optic is about the same angle as the worst case
> off-perpendiularity of the non-telecentric lens. So even a perfect
> "designed for digital" fully telecentric lens will produce the same
> colour distortion across the entire field as the worst case simple lens
> will produce at the corner. Added to which, that worst case only occurs
> at one particular focal length - shorter focal length lenses than this
> are partially telecentric because of their inverse telephoto design
> keeping the principle ray closer to perpendicular even in the corner of
> the frame, whilst longer focal length lenses produce principle rays
> closer to perpendicular in any case.
>
> In short, the entire CA argument (and this is one of the cornerstones of
> the entire 4/3 strategy!) is based on nothing more than hype. 4/3 and
> APS formats are there because they are cheaper to make, not because they
> or their optics are intrinsically superior.
>
> If there is CA in the image then it is almost entirely coming from the
> lens, just in the same way as it did on good old analogue colour film
> where, incidentally, the distance between the layers of emulsion isn't
> too different from the distance between the filter and silicon in a CCD
> or CMOS sensor.
>
> Yes, this issue will eventually become a problem, when pixel sizes get
> down to 3um or less on these larger focal planes, but obviously the
> smaller 4/3 and APS cameras will need to be there a long time before
> full frame cameras do, just on the pixel count wars alone. And to make
> it worse, these small size pixels will be restricted to faster minimum
> apertures (as they currently are in P&S cameras) to take full advantage
> of the pixel count in terms of real resolution, making the CA problem
> even more significant - telecentric lenses or not.
> --
> Kennedy
> Yes, Socrates himself is particularly missed;
> A lovely little thinker, but a ah heck when he's pissed.
> Python Philosophers (replace 'nospam' with 'kennedym' when replying)

 

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In article <1127941648.059540.293340@f14g2000cwb.googlegroups.com>,
"winhag@yahoo.com" <winhag@yahoo.com> writes
>Kennedy,
>
>A purely educational question. You talk about 'telecentric' lens
>designs.
>I had never heard of this and looked it up on the web. From what
>I could understand, a telecentric lens maps objects of the same
>dimension
>to the same size on the film plane regardless of object distance from
>lens (i.e.
>no 'perspective distortion'). Is that what you are referring to or
>something different?
>
Yes.

If you draw out a simple lens then all of the principle rays (the
principle ray is the central ray in the bundle) to every point between
the object and its image on the focal plane passes through the centre of
the lens. This means that there is only one point in the image where
the principle ray is perpendicular to the focal plane - the centre of
the focal plane. At every other point on the focal plane, the principle
ray diverges from the centre. The easiest way to visualise this is just
to draw a lens with an object and its image on a sheet of paper and then
draw in the principle rays from the object to the image, making sure
that they all pass through the centre of the lens. Now, if the angle of
incidence at the focal plane is not 90deg, there is a chance that some
of the light passing through the filter will actually land on an
adjacent pixel rather than the pixel some small distance under the
filter, and this causes colour distortions because the camera expects
all of the light falling on certain pixels to have been filtered to the
appropriate colour. Obviously the greater the angle from perpendicular
of the principle ray, and the lower the f/# (ie. the wider the cone
formed by the bundle of incident rays around the principle one), then
the more light that can 'leak' into adjacent pixels this way.#

With a telecentric lens, all of the principle rays reaching the focal
plane are perpendicular to it and, since the same f/# must be produced
across the focal plane, this means that each point of the image does not
use all of the available lens aperture - so the rear lens elements are
physically larger than would be required normally. Of course, that
means that all of the rays incident at the corner of the focal plane are
coming from the periphery of the lens, making it more difficult to
control all aberations, not just chromatic. So telecentric lenses tend
to have a poorer performance than conventional designs - or they are
much more expensive for the same performance. The main reason for
producing such a lens in pre-digital days was, as you have found in your
net search, to ensure that images did not change size when the system
was focus was moved. This would be important in instruments measuring
dimensions directly from the image. However, since telecentricity means
that the principle ray to every point in the image is perpendicular to
the focal plane, there is an argument that it is ideal in digital
sensors to ensure that the amount of leakage from one filter to adjacent
pixels is fixed - although it does still change with f/# of course.

In most cases though the CA from non-telecentric lenses argument is
overhyped and you just need to examine the pixel dimensions to see how
spurious it is. Based on the minimum back working distance to clear the
mirror, which is proportional to the sensor size, the angle of incidence
in the corner of the field for the principle ray from this worst case
situation is almost the same in both formats - leaving only the pixel
dimension as being the critical parameter. Smaller format sensors
usually have smaller pixels and they consequently suffer more from this
problem than larger sensors - completely contrary to the hype.

However, putting this all into perspective, the semi-angle of the light
cone itself with a fast optic is about the same angle as the worst case
off-perpendiularity of the non-telecentric lens. So even a perfect
"designed for digital" fully telecentric lens will produce the same
colour distortion across the entire field as the worst case simple lens
will produce at the corner. Added to which, that worst case only occurs
at one particular focal length - shorter focal length lenses than this
are partially telecentric because of their inverse telephoto design
keeping the principle ray closer to perpendicular even in the corner of
the frame, whilst longer focal length lenses produce principle rays
closer to perpendicular in any case.

In short, the entire CA argument (and this is one of the cornerstones of
the entire 4/3 strategy!) is based on nothing more than hype. 4/3 and
APS formats are there because they are cheaper to make, not because they
or their optics are intrinsically superior.

If there is CA in the image then it is almost entirely coming from the
lens, just in the same way as it did on good old analogue colour film
where, incidentally, the distance between the layers of emulsion isn't
too different from the distance between the filter and silicon in a CCD
or CMOS sensor.

Yes, this issue will eventually become a problem, when pixel sizes get
down to 3um or less on these larger focal planes, but obviously the
smaller 4/3 and APS cameras will need to be there a long time before
full frame cameras do, just on the pixel count wars alone. And to make
it worse, these small size pixels will be restricted to faster minimum
apertures (as they currently are in P&S cameras) to take full advantage
of the pixel count in terms of real resolution, making the CA problem
even more significant - telecentric lenses or not.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a ah heck when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

 

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Archived from groups: rec.photo.digital (More info?)

Kennedy,

Again thanks. I think I understand it now. I believe my confusion
was created by other sites which talked about lenses that were
telecentric in both the object and image plane. Your explanation
coupled with the explanation here:
http/www.edmundoptics.com/techsupport/DisplayArticle.cfm?articleid=261

helped clear it up. It sounds like you're talking about lenses which
are telecentric on the image side only.
Thanks again for taking the time!


Kennedy McEwen wrote:
> In article <1127960880.935146.226250@g43g2000cwa.googlegroups.com>,
> "winhag@yahoo.com" <winhag@yahoo.com> writes
> >Kennedy,
> >
> >Thanks for the explanation. The only thing that still puzzles me is
> >this:
> >If a lens is fully telecentric, wouldn't that mean that a one foot
> >ruler
> >that was let's say 2 feet from the lens and a one foot ruler that was
> >4 feet from the lens would produced the same size image on the film
> >plane
> >(assuming they are within the depth of field)?
> >Wouldn't that lead to strange looking images?
>
> Not really. Perspective is still present, but the lens is just designed
> so that the principle ray is perpendicular to the focal plane, so that
> as focus moves from one plane to another, the images that are out of
> focus don't change size when they become sharp.
>
> One way to think about this, although not particularly accurate, is that
> the telecentric lens is just a conventional lens with an additional
> group at the back which bends all of the ray bundles back towards the
> optic axis by an amount proportional to their distance from the axis.
> The inaccuracy lies in the fact that the additional group obviously has
> some optical power, since all it would be is a spherical surface element
> itself, so that has to be compensated in the main image forming part of
> the lens, but that is unnecessary to explain the basic principle. The
> ray bundles still all come to a focus at the focal plane, but they are
> all symmetrical around the perpendicular to the plane. So the image
> just looks like a normal image, with correct perspective, but things
> don't change size as they pass through focus.
>
> You have probably seen the non-telecentric effect in the past without
> thinking it was odd. Take a wide angle lens (where the effect is most
> significant) and view a wall through the finder, noting where the edges
> of the frame are on the wall. Now adjust the focus from infinity to the
> closest range. With a non-telecentric lens, the edge of the frame will
> move in relative to the wall as the lens is focussed closer. This is
> because the principle rays at the edge of the frame are not
> perpendicular to the focal plane, so when the lens moves further from
> the focal plane to focus closer the rays that previously reached the
> edge now fall outside of the image frame. A telecentric lens would not
> do this - as you focus from infinity down to the closest range the edges
> of the frame would not change position. Everything still goes in and
> out of focus and has the same perspective, but their size does not
> change as the focus changes.
> --
> Kennedy
> Yes, Socrates himself is particularly missed;
> A lovely little thinker, but a ah heck when he's pissed.
> Python Philosophers (replace 'nospam' with 'kennedym' when replying)

 

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Archived from groups: rec.photo.digital (More info?)

In article <1127960880.935146.226250@g43g2000cwa.googlegroups.com>,
"winhag@yahoo.com" <winhag@yahoo.com> writes
>Kennedy,
>
>Thanks for the explanation. The only thing that still puzzles me is
>this:
>If a lens is fully telecentric, wouldn't that mean that a one foot
>ruler
>that was let's say 2 feet from the lens and a one foot ruler that was
>4 feet from the lens would produced the same size image on the film
>plane
>(assuming they are within the depth of field)?
>Wouldn't that lead to strange looking images?

Not really. Perspective is still present, but the lens is just designed
so that the principle ray is perpendicular to the focal plane, so that
as focus moves from one plane to another, the images that are out of
focus don't change size when they become sharp.

One way to think about this, although not particularly accurate, is that
the telecentric lens is just a conventional lens with an additional
group at the back which bends all of the ray bundles back towards the
optic axis by an amount proportional to their distance from the axis.
The inaccuracy lies in the fact that the additional group obviously has
some optical power, since all it would be is a spherical surface element
itself, so that has to be compensated in the main image forming part of
the lens, but that is unnecessary to explain the basic principle. The
ray bundles still all come to a focus at the focal plane, but they are
all symmetrical around the perpendicular to the plane. So the image
just looks like a normal image, with correct perspective, but things
don't change size as they pass through focus.

You have probably seen the non-telecentric effect in the past without
thinking it was odd. Take a wide angle lens (where the effect is most
significant) and view a wall through the finder, noting where the edges
of the frame are on the wall. Now adjust the focus from infinity to the
closest range. With a non-telecentric lens, the edge of the frame will
move in relative to the wall as the lens is focussed closer. This is
because the principle rays at the edge of the frame are not
perpendicular to the focal plane, so when the lens moves further from
the focal plane to focus closer the rays that previously reached the
edge now fall outside of the image frame. A telecentric lens would not
do this - as you focus from infinity down to the closest range the edges
of the frame would not change position. Everything still goes in and
out of focus and has the same perspective, but their size does not
change as the focus changes.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a ah heck when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

 

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Archived from groups: rec.photo.digital (More info?)

Brian Baird <no@no.thank.u> wrote:
> In article <4337ab37$1@dnews.tpgi.com.au>, canvaspix@yahoo.com.au
> says...
>> The purple fringe problem occurs when adjacent photo detectors overload.
>> That is to say when the contrast range between the elements in a
>> photograph are outside that which the camera is capable of recording
>> detail in... Shooting a branch or building edge with bright sky will do
>> it as will shooting into the sun. Pretty much anything that will produce
>> CA in a lens, will add the sensor to the area of responsibility. CA from
>> the micro lenses is not yet fully understood (by me at any rate) but it
>> has been found to exist in laboratory tests.
>
> Yeah... no. "Blooming" doesn't occur with modern CCD or CMOS sensors...
> so, no... you lose, sorry.

Er, do you have something to support that? Granted, it generally
isn't much of an issue with a decent lens, but I've had signs of
blooming on my 20D even with the 85mm f/1.8, which is about as good a
lens and as modern a CMOS as you can get.

The primary culprit is usually small leaves against bright sky, and
under magnification blooming is visible all the way around the leaf,
showing that it isn't just CA.

Perhaps you meant to say that modern microlenses don't contribute
anything significant to the problem? That I find quite plausible.

--
Zed Pobre <zed@resonant.org> a.k.a. Zed Pobre <zed@debian.org>
PGP key and fingerprint available on finger; encrypted mail welcomed.