Lenses for digital cameras

Erik,

You and I live in different parts of the photographic realm. While it is true that most lenses for 35-mm SLRs and 35-mm point-n-shoot cameras are now autofocus, those for 35-mm rangefinder cameras (except Contax), and all lenses for medium and large-format cameras are still manual focus.

Recently, I read that most professional video photographers bypass the autofocus on their cameras. Perhaps the professional still photojournalists do, too. My own, very limited experience with autofocus has been that it can misread at the worst possible time and cost you an important image.

Too, on these forums I have read many postings by people who want, for one reason or another, to be able to focus the lenses on their digital cameras manually and wish for a more traditional lens with a helical focusing mount.

So I think that manual-focus lenses are still viable. Trouble is, it is the amateur snap-shooters who, by sheer number, dominate the camera market. Far more amateurs than professionals own even the high-end cameras (Hasselblad, Nikon F5, et cetera), so what we are being offered now is what will attract those amateurs.

I was putting forward the 10-mm SOM Berthiot f/1.9 lens as an example of one of many existing camera lenses that would cover the five-megapixel CCDs used in the current crop of 'consumer' digital cameras by Sony and others. The lens has a workable range of f/stops (down to f/22) and, on these CCDs, would yield the equivalent of a 40-mm lens on a 35-mm camera. I've never paid any attention to the technical specifications, such as line-pairs, on lenses so I can't help you there.

Must say that, as a working photographer (as opposed to an engineer), when I look at the raw image coming from the digital camera and see how fuzzy it is without any sharpening algorithms being applied, I can't help wondering just how important the sharpness of the lens is. Obviously, as CCD resolution increases, it will become more important. This is where a digital body capable of accepting the whole gamut of lenses (and not just those from one source) would be handy. Right now, the choice of algorithms may be more important.

All this is fun to discuss, but what is really great is taking digital pictures and then going into the virtual darkroom. I only regret that I probably won't live long enough to see (and be able to afford) a 20-megapixel six-by-six centimeter CCD.

Cheers,

Tyler Monson
Seattle, Washington

Erik,

You got me started on thinking about these autofocus lenses, and it's easy to see why the manufacturers are fond of them: they restrict you to using only their lenses on their cameras.

Over the years, I have used Nikon lenses on Contax cameras, Leica lenses on Hasselblad cameras, Hasselblad lenses on Pentax cameras, and just about every lens imaginable on sheet-film cameras.

Now, you buy a modern 35-mm SLR and you've made a big commitment; almost as big as marriage ... and as expensive to run away from.

Cheers,

Tyler

Yep, that is it exactly. Withthe older manual focus bodies, if
they changed a mount, within a week or so, someone was on
the market with an adapter so you could use the same lenses
you had been using. Not only that, the manual focus lenses
are just flat more flexible. My current target is two bird feeders
in the middle of a huge Lilac bush. Get an AF to look into the
bush to the feeders, and then get it to pick out the correct
bird on one of the feeders.

Tyler Monson said:

Erik,
You got me started on thinking about these autofocus lenses, and
it's easy to see why the manufacturers are fond of them: they
restrict you to using only their lenses on their cameras.
Over the years, I have used Nikon lenses on Contax cameras, Leica
lenses on Hasselblad cameras, Hasselblad lenses on Pentax cameras,
and just about every lens imaginable on sheet-film cameras.
Now, you buy a modern 35-mm SLR and you've made a big commitment;
almost as big as marriage ... and as expensive to run away from.

Cheers,

Tyler

Click to expand...

I saw a 16 MP back for Hasselblad on CeBIT a few weeks ago. The quality was incredible, but so was the price.... (about 10 000 $) I guess it will takes 2-3 years for the price drop at least 50%.....

Tyler Monson said:

Erik,
You and I live in different parts of the photographic realm. While
it is true that most lenses for 35-mm SLRs and 35-mm point-n-shoot
cameras are now autofocus, those for 35-mm rangefinder cameras
(except Contax), and all lenses for medium and large-format cameras
are still manual focus.
Recently, I read that most professional video photographers bypass
the autofocus on their cameras. Perhaps the professional still
photojournalists do, too. My own, very limited experience with
autofocus has been that it can misread at the worst possible time
and cost you an important image.
Too, on these forums I have read many postings by people who want,
for one reason or another, to be able to focus the lenses on their
digital cameras manually and wish for a more traditional lens with
a helical focusing mount.
So I think that manual-focus lenses are still viable. Trouble is,
it is the amateur snap-shooters who, by sheer number, dominate the
camera market. Far more amateurs than professionals own even the
high-end cameras (Hasselblad, Nikon F5, et cetera), so what we are
being offered now is what will attract those amateurs.

I was putting forward the 10-mm SOM Berthiot f/1.9 lens as an
example of one of many existing camera lenses that would cover the
five-megapixel CCDs used in the current crop of 'consumer' digital
cameras by Sony and others. The lens has a workable range of
f/stops (down to f/22) and, on these CCDs, would yield the
equivalent of a 40-mm lens on a 35-mm camera. I've never paid any
attention to the technical specifications, such as line-pairs, on
lenses so I can't help you there.
Must say that, as a working photographer (as opposed to an
engineer), when I look at the raw image coming from the digital
camera and see how fuzzy it is without any sharpening algorithms
being applied, I can't help wondering just how important the
sharpness of the lens is. Obviously, as CCD resolution increases,
it will become more important. This is where a digital body capable
of accepting the whole gamut of lenses (and not just those from one
source) would be handy. Right now, the choice of algorithms may be
more important.

All this is fun to discuss, but what is really great is taking
digital pictures and then going into the virtual darkroom. I only
regret that I probably won't live long enough to see (and be able
to afford) a 20-megapixel six-by-six centimeter CCD.

Cheers,

Tyler Monson
Seattle, Washington

Click to expand...

--Bjoernar

Bob Williams said:

We are not talking about surface mounted
particles in a medium that light can travel thru anymore. In
fact, the depth that light can penetrate the sensors is quite
small. Rays of light at steep angles are blocked or partially
blocked from hitting the sensors at the edges of the circle
of focus, unless that path has been corrected to meet the
requirements of the medium.

Click to expand...

I see this claim a lot, but I'm not sure it really applies to the cameras and lenses we're talking about. It is true that the photodiodes on CCDs cannot properly record light striking the CCD at sharply oblique angles, and this is often stated as the reason why we don't yet have 35mm-sized CCDs in cameras designed to use those lenses. People claim that at the edges of a 35mm-sized sensor, where the angle of incidence would be highest, incomplete recording of light would result in darker exposures and color aliasing. And here you are claiming that the same reason would prevent lens designers from simply scaling existing lens designs down to fit the smaller sensors now in use.

But I have never seen anyone demonstrate that the angles of incidence at the image plane that are typically produced by lenses for 35mm cameras are actually too oblique for the CCDs used in digital SLRs. And one optical physicist who posts regularly on the Nikon SLR forum physically calculated the ray angles for several lenses to debunk a claim based on this same kind of claim a few weeks back. In that message he also noted that he had seen images shot with a 17mm lens (made for 35mm cameras) on a Horseman Digiflex, which uses a 24 x 36mm Phillips CCD. He says he saw no light fall-off problems in those images. The message is here:

http://www.dpreview.com/forums/read.asp?forum=1021&message=2175093

Again, I don't know for sure what the real answer is here, but I've never seen anyone actually demonstrate that this problem really exists for 35mm format lenses and the sensors typically used in digital SLRs. I think it might -- just might -- be an urban myth.

Tyler Monson said:

You and I live in different parts of the photographic realm.

Click to expand...

Maybe. I still have more MF cameras and lenses that I do AF. Someone in another thread had this observation and it matches my experience: while scanning (literally) my old slides, I've noticed that the percentage of losers due to quality of focus issues has gone way down since I replaced my FD camera with an EOS camera. It may not be perfect, but darn if it isn't a lot better. I always thought that I was pretty good with focus too. There is a real reason AF has taken over the market.

Member said:

and all lenses for medium and large-format cameras
are still manual focus.

Click to expand...

Then why does B&H list Contax, Fuji, Mamiya, and Pentax MF autofocus cameras for sale if there are no lenses for them?

Member said:

Perhaps the professional still photojournalists do, too.

Click to expand...

If they do, then Nikon and Canon wasted a lot of money building fancy predictive AF systems in their pro-line cameras.

Member said:

Too, on these forums I have read many postings by people who want,
for one reason or another, to be able to focus the lenses on their
digital cameras manually and wish for a more traditional lens with
a helical focusing mount.

Click to expand...

Yes, it's a nice feature. If they really want it, they can buy a Nikon D1 series that is still capable of using the old MF lenses.

Member said:

So I think that manual-focus lenses are still viable. Trouble is,
it is the amateur snap-shooters who, by sheer number, dominate the
camera market.

Click to expand...

They (the amateurs) pay the piper, so they get call the tune. If you and a bunch of rich friends can convice a camera mfg that's there money to be made, then do it. Just don't expect it to be at mass-market prices.

Member said:

Far more amateurs than professionals own even the
high-end cameras (Hasselblad, Nikon F5, et cetera),

Click to expand...

However, I don't see most pros shunning the Canon EOS and Nikons.

Member said:

I was putting forward the 10-mm SOM Berthiot f/1.9 lens as an
example of one of many existing camera lenses that would cover the
five-megapixel CCDs used in the current crop of 'consumer' digital
cameras by Sony and others.

Click to expand...

The reason I was asking about resolution is that 5MP Sony needs to resolve about 600 line/pairs on an 7mm high sensor. That's 85 lp/mm which is an exceedingly sharp lens. I'd be surprised if a lens design for a moving-frame camera has anywhere near that resolution as it's just not needed. I could be wrong, but it looks like sensor technology has left these old lenses behind.

The point of my postings has been to convey some idea as to why the state of the digital market is what it is and what obstacles may exist to it moving closer to what you would like it to be. I truly hope that someone someday makes a camera that you would like to use and can afford.--ErikFree Windows JPEG comment editor http://home.cfl.rr.com/maderik/edjpgcom

peripheralfocus said:

http://www.dpreview.com/forums/read.asp?forum=1021&message=2175093

Click to expand...

OK, here is what he said:

brian said:

1) The maximum ray incidence angle on axis for the 50mm/1.4 or any
f/1.4 lens is about 21 degrees, not 31 degrees. (formula is
ASIN(1/(2*f#))

2) In the 50mm f/1.4 lens the maximum ray incidence angle in the
corner ray bundle is about 24 degrees, not 48 degrees. I base this
on ray tracing.

Click to expand...

If you look at the sensor rumoured to be used in the new Olympus:
http://www.kodak.com/country/US/en/digital/ccd/kaf5100ce.shtml

and find the spec sheet PDF, you will see a graph that the sensitivity to green light at 21 and 24 degrees horizontal is about .5 to .33. Or 1 to almost 2 stops.

peripheralfocus said:

Again, I don't know for sure what the real answer is here, but I've
never seen anyone actually demonstrate that this problem really
exists for 35mm format lenses and the sensors typically used in
digital SLRs. I think it might -- just might -- be an urban myth.

Click to expand...

The truth is out there. You decide.

--ErikFree Windows JPEG comment editor http://home.cfl.rr.com/maderik/edjpgcom

Erik Magnuson wrote:
(snip)

Member said:

and find the spec sheet PDF, you will see a graph that the
sensitivity to green light at 21 and 24 degrees horizontal is about
.5 to .33. Or 1 to almost 2 stops.

Member said:

Again, I don't know for sure what the real answer is here, but I've
never seen anyone actually demonstrate that this problem really
exists for 35mm format lenses and the sensors typically used in
digital SLRs. I think it might -- just might -- be an urban myth.

Click to expand...

The truth is out there. You decide.

Click to expand...

Thanks for the pointer to that PDF, Erik. It's very interesting.

Unfortunately, I'm not really sure how to interpret that graph. It's beyond my limited knowledge of CCD physics.

I'm venturing into murky waters for me with what follows, because I am not an optical physicist nor an electronic engineer, but here goes anyway ...

Even if the Kodak PDF really does say that, at a ray incidence angle of 24 degrees, the CCD's sensitivity to green light goes down almost 2 stops, I don't think that fact necessarily adds up to a meaningful problem. Why? Because what's actually important is the difference between the total exposures recorded by the CCD for on-axis and off-axis object points. That difference is what would cause lower exposure levels, or "light fall-off", at the edges of the picture.

So then: an on-axis object point is exposed onto the CCD by a collection of light rays, some of which have an angle of incidence of 21 degrees (with the lens we're using as an example) but the vast majority of which have an angle of incidence less than 21 degrees. An off-axis object point is exposed onto the CCD by a collection of light rays, some of which have an angle of incidence of 24 degrees but the vast majority of which have an angle of incidence less than 24 degrees.

So the actual difference in exposure between an on-axis image point and an off-axis image point is whatever results from a roughly 2/3 stop difference (i.e. the difference between angle sensitivity at 21 degrees and 24 degrees, according to Kodak's graph) between the CCD's sensitivity to a small fraction of the total number of light rays striking the CCD at each image point.

For the sake of argument, let's say that, for both the on-axis and off-axis object points, 20% of the ray bundle is composed of rays striking the CCD at the maximum incidence angle. A 2/3rd stop difference for 20% of the light ray bundle equals a total exposure difference between on-axis and off-axis image points of about 1/8th of a stop. (The math here is very rough; obviously the distribution of incidence angles between the two points would not be 80% identical, then 20% different by 3 degrees.) 1/8th of a stop ain't much -- most people can't see it and many, many lenses exhibit much greater edge fall-off than that. Besides that, it's not clear to me that the camera manufacturers couldn't compensate for much of it with a little creative signal processing.

Anyway, the point of all this is not to deny that the problem really does exist. I just don't think I've seen a convincing proof -- even after reading the PDF you pointed to.

In any event, it's easier to test it with real lenses and a real camera than to theorize about it. As Brian pointed out in the thread I referenced, the Horseman Digiflex makes a perfect test bed for this question -- at least for one CCD. Maybe I'll see if I can get my hands on one by hook or by crook.

Erik Magnuson said:

Tyler Monson said:

and all lenses for medium and large-format cameras
are still manual focus.

Click to expand...

Then why does B&H list Contax, Fuji, Mamiya, and Pentax MF
autofocus cameras for sale if there are no lenses for them?

Click to expand...

Okay, change "all" to "practically all". Approximately 10% of the lenses listed by B&H for medium-format cameras are noted as autofocus. I haven't kept up on the latest, since everything I do now is digital.

Member said:

Member said:

Perhaps the professional still photojournalists do [use manual, not autofocus], too.

Click to expand...

If they do, then Nikon and Canon wasted a lot of money building
fancy predictive AF systems in their pro-line cameras.

Click to expand...

But are they really making those cameras for the 'pros', if the vast majority are sold to amateurs -- either advanced or with just too much money ? The true professional market is so small that prices are astronomical. For example, the $20,000 Better Light digital scanning back, or Kodak's recently introduced 16-Mpixel single-shot back, at about the same price.

Member said:

They (the amateurs) pay the piper, so they get [to] call the tune. If
you and a bunch of rich friends can convice a camera mfg that's
there money to be made, then do it. Just don't expect it to be at
mass-market prices.

Click to expand...

Erik, I am unsure of the point you are trying to make here. I hold the amateur status in highest regard, because, by definition, this is a person who does it for love, not money. And I've known amateurs who were extremely capable photographers (and all too many professionals who weren't).

All of us, professional or amateur, have to use whatever equipment is available (that we can afford), so we are all at the mercy of the manufacturers. Cameras and lenses without all this built-in automation are a lot easier to modify to suit individual tastes and needs but, as you've pointed out, more and more cameras have more and more automation built-in.

Member said:

The reason I was asking about resolution is that 5MP Sony needs to
resolve about 600 line/pairs on an 7mm high sensor. That's 85
lp/mm which is an exceedingly sharp lens.

Click to expand...

Remembering that I am just a poor user of cameras, not an engineer, why is it that the raw output (before sharpening algorithms) from these cameras is so fuzzy?

Member said:

I truly hope that someone someday makes a camera that you would like to use
and can afford.

Click to expand...

Thank you, Erik, but rest assured that, until then, I am having a great time with my little digital camera, and never leave the house without it. And I think that what you can do with images in Photoshop is just the neatest thing since sliced bread! Still, I can't help dreaming of better digital quality, but that's my personal nemesis.

Cheers,

Tyler Monson
Seattle, Washington

Tyler Monson said:

But are they really making those cameras for the 'pros', if the
vast majority are sold to amateurs

Click to expand...

Either it's important to some significant class of photographers or they wasted money. I'll leave the definition of which class of photographers it's significant to to others.

Member said:

Member said:

They (the amateurs) pay the piper, so they get [to] call the tune. If
you and a bunch of rich friends can convice a camera mfg that's
there money to be made, then do it. Just don't expect it to be at
mass-market prices.

Click to expand...

Erik, I am unsure of the point you are trying to make here. I hold
the amateur status in highest regard, because, by definition, this

Click to expand...

I was responding to your comment that some of the features were oriented towards a certain market segment. Since you have to appeal to a large market segment to recoup your investment, that is going to influence the feature set. Again, "pro" or "amateur" is irrelevant as the cameras seem to be well received by both groups.

Member said:

Remembering that I am just a poor user of cameras, not an engineer,
why is it that the raw output (before sharpening algorithms) from
these cameras is so fuzzy?

Click to expand...

Compromises in the interpolation algorithms and optical filtering to reduce other image artifacts. Since they do sharpen up quite nicely, I don't worry about it any more than the fact that you have to enlarge a 35mm frame somehow (even if it's just a loupe) to make IT useful.
--ErikFree Windows JPEG comment editor http://home.cfl.rr.com/maderik/edjpgcom

Eamon Hickey said:

An off-axis object point is exposed onto the CCD by a
collection of light rays, some of which have an angle of incidence
of 24 degrees but the vast majority of which have an angle of
incidence less than 24 degrees.

Click to expand...

Think about a point at the corner of the image sensor. What percentage of the light will come from less than 24 degrees? Only that light that comes from an arc that intersects the axis. (Think of two overlapping circles of the same radius, one at centered at the midpoint and one at the corner. Only the areas that overlap would be less than 24 degrees.) This is not the "vast majority". It's not even the simple majority.

OK, if you don't want to believe me, try
http://www.optics-online.com/literature/CCDlens.htm

and read the section called "Relative illumination and telecentricity "--ErikFree Windows JPEG comment editor http://home.cfl.rr.com/maderik/edjpgcom

Erik Magnuson said:

Think about a point at the corner of the image sensor. What
percentage of the light will come from less than 24 degrees? Only
that light that comes from an arc that intersects the axis. (Think
of two overlapping circles of the same radius, one at centered at
the midpoint and one at the corner. Only the areas that overlap
would be less than 24 degrees.) This is not the "vast majority".
It's not even the simple majority.

OK, if you don't want to believe me, try
http://www.optics-online.com/literature/CCDlens.htm
and read the section called "Relative illumination and
telecentricity "

Click to expand...

Well, you've lost me with the above. I can't quite figure out what you're getting at. One of us is fundamentally misunderstanding something important -- but I'm not sure what it is, and I'm not saying it's you that's misunderstanding it.

As to your reference to the optics online web site, I'll just repeat that I never denied that telecentricity and CCD acceptance angles were an important issue -- I just said that I'm not sure that lenses for 35mm film cameras are actually insufficiently telecentric for 24 x 36mm CCDs to work adequately in digital SLRs. The web site you referenced doesn't address that question.

But again, the best way to find out what's really going on is to test it with real cameras and lenses, like Nikon lenses on the Horseman DigiFlex. And I think I'll let that be my last word on the topic for this thread at least.

You have light rays hitting the sensor from the center of the
lens at almost a zero angle. The exposure is set to this value.
You have light at the edges of the frame losing half or more
of that light value. In other words, if the center exposure is
20, the edge exposure hovers around 10, if those figures are
right. You are not talking about losing part of a stop. You
are talking about losing half or more of the light gathering
ability, and introducing distortion, if those figures are correct.
Of course you could set the exposure somewhere other than
the center, or use a full frame average. Then the center
gets clipped highlights, and the edge gets clipped shadows.
Either way, this is not progress. In a best case example, you
would enlarge the mount, increase the exit lens size, and continue
to use the present cropped system to keep the levels correct
across the image. Lets don't even discuss curved imagers.

Eamon Hickey said:

Erik Magnuson said:

Think about a point at the corner of the image sensor. What
percentage of the light will come from less than 24 degrees? Only
that light that comes from an arc that intersects the axis. (Think
of two overlapping circles of the same radius, one at centered at
the midpoint and one at the corner. Only the areas that overlap
would be less than 24 degrees.) This is not the "vast majority".
It's not even the simple majority.

OK, if you don't want to believe me, try
http://www.optics-online.com/literature/CCDlens.htm
and read the section called "Relative illumination and
telecentricity "

Click to expand...

Well, you've lost me with the above. I can't quite figure out what
you're getting at. One of us is fundamentally misunderstanding
something important -- but I'm not sure what it is, and I'm not
saying it's you that's misunderstanding it.

As to your reference to the optics online web site, I'll just
repeat that I never denied that telecentricity and CCD acceptance
angles were an important issue -- I just said that I'm not sure
that lenses for 35mm film cameras are actually insufficiently
telecentric for 24 x 36mm CCDs to work adequately in digital SLRs.
The web site you referenced doesn't address that question.

But again, the best way to find out what's really going on is to
test it with real cameras and lenses, like Nikon lenses on the
Horseman DigiFlex. And I think I'll let that be my last word on the
topic for this thread at least.

Click to expand...

Sitting in the yard yesterday, 20 feet from the feeders, with
an old Yashica manual focus 35 mm SLR on a tripod, focused
on the feeder. This is a perfect setup for this use. When the
big hawk was seen later, I grabbed the camera off the tripod,
ran it out to infinity, and as the bird flew over, tried to keep
up with the focus by hand as he got inside the infinity range
of the setup. For this use, a Canon Rebel would have been
much better than anything in the manual focus range, no matter
the cost. The best of both worlds would be a G2 type
camera with true manual focus caps, and one of the longer
range IS stabilized lenses, or a DSLR with a good lens on it, and
a good manual focus screen. Focusing thru the branches to
the feeder is manual focus work. The Hawk was AF work.
That is the problem. When do we get a DSLR that has a true
manual focus screen, or a higher end consumer camera with
one? With the setup I was shooting, at 20 feet, a Cardinal
is hard to get completely into the frame. This would not be
practical with a fixed lens design unless the lens had a very wide
zoom range, like one of the 35-300 type lenses. I still say that
a decently built DSLR with a true manual focus screen, able
to use one of todays lens systems and a very accurate metering
system, would sell a lot of units, if the price was in the high end
consumer range.

Erik Magnuson said:

Tyler Monson said:

But are they really making those cameras for the 'pros', if the
vast majority are sold to amateurs

Click to expand...

Either it's important to some significant class of photographers or
they wasted money. I'll leave the definition of which class of
photographers it's significant to to others.

Member said:

Member said:

They (the amateurs) pay the piper, so they get [to] call the tune. If
you and a bunch of rich friends can convice a camera mfg that's
there money to be made, then do it. Just don't expect it to be at
mass-market prices.

Click to expand...

Erik, I am unsure of the point you are trying to make here. I hold
the amateur status in highest regard, because, by definition, this

Click to expand...

I was responding to your comment that some of the features were
oriented towards a certain market segment. Since you have to appeal
to a large market segment to recoup your investment, that is going
to influence the feature set. Again, "pro" or "amateur" is
irrelevant as the cameras seem to be well received by both groups.

Member said:

Remembering that I am just a poor user of cameras, not an engineer,
why is it that the raw output (before sharpening algorithms) from
these cameras is so fuzzy?

Click to expand...

Compromises in the interpolation algorithms and optical filtering
to reduce other image artifacts. Since they do sharpen up quite
nicely, I don't worry about it any more than the fact that you have
to enlarge a 35mm frame somehow (even if it's just a loupe) to make
IT useful.

--
Erik
Free Windows JPEG comment editor
http://home.cfl.rr.com/maderik/edjpgcom

Click to expand...

Eamon Hickey said:

I just said that I'm not sure
that lenses for 35mm film cameras are actually insufficiently
telecentric for 24 x 36mm CCDs to work adequately in digital SLRs.

Click to expand...

A few more issues come to mind (unfortunately only as more questions without answers.)

• Not all sensors have the same incidence profile. What are the tradeoffs in noise, sensitivity, cost, etc. for increasing the acceptance angle? What about X3 designs?

• One of the reasons many people want full frame sensors is to use wide angle lenses. What are the numbers for wides as opposed to a 50mm?(Here might be a case where the large backfocus due to the mirror assembly clearence of SLRs is a "good" thing. Imagine the incidence angle problems for non-retrofocus lenses.)

• Ultra-wide lenses already have a problem with optical vignetting. Even if the incidence problem is normally quite limited, would the cumulative effect restrict the usefulness of these lenses anyway?

--ErikFree Windows JPEG comment editor http://home.cfl.rr.com/maderik/edjpgcom

Bob Williams said:

You have light rays hitting the sensor from the center of the
lens at almost a zero angle.

Click to expand...

This is true only for a point at the center of the image. For an off-axis image point the light rays that strike the sensor with a zero incidence angle will be coming from an off-axis location on the exit pupil. Here's a cool little Java applet that illustrates the formation of an image by a converging lens. Specifically, it shows the refraction of light rays coming from a single off-axis object point to its corresponding image point. As you can see from that applet, if you placed a sensor at the image point the light rays that would strike the sensor with a zero angle of incidence are coming from the "lower" part of the lens, not the center. Note, too, that each light ray would strike the CCD at a different angle of incidence -- and only one of the rays illustrated would strike the CCD from the maximum incidence angle. (In real life of course, each image point is formed by a three-dimensional cone of light rays coming from the lens exit pupil, so it gets a little more complex, but the principle holds.):

http://www.physics.uoguelph.ca/applets/Intro_physics/kisalev/java/clens/index.html

Well, I said I'd leave this topic alone, and here I am posting on it again. I'm really gonna' shut up now.