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Effect of aperture on chromatic aberation?
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Paul Ciszek
May 9, 2012, 7:54:59 PM5/9/12
to
Way back in undergraduate optics, I was told that reducing the aperture
of a lens mitigates various problems such as spherical aberration, and
of course it makes focusing less critical (aka depth of field). What I
am not sure of is whether a reduced aperture reduces chromatic
aberration. It would seem to me that if you make focal distance less
important, then the effect of red and blue light focusing at different
distances should also become less pronounced.
If I can get a hold of some solar film in time, I would like to attempt
to photograph the annular eclipse of the sun in May and the transit of
Venus in June. The way I see it, I should have plenty of light to
spare, and my camera is probably a long way away from being diffraction
limited, so why not use a small aperture?
The chromatic aberration issue with my camera is really only apparent
when attempting astrophotography. Here is one example:
http://www.flickr.com/photos/35853148@N05/4303433104/in/set-721576231290
10923/lightbox/
(view the 1024 pixel version for best results.)
--
Please reply to: | No nation is drunken where wine is cheap.
pciszek at panix dot com | --Thomas Jefferson
of a lens mitigates various problems such as spherical aberration, and
of course it makes focusing less critical (aka depth of field). What I
am not sure of is whether a reduced aperture reduces chromatic
aberration. It would seem to me that if you make focal distance less
important, then the effect of red and blue light focusing at different
distances should also become less pronounced.
If I can get a hold of some solar film in time, I would like to attempt
to photograph the annular eclipse of the sun in May and the transit of
Venus in June. The way I see it, I should have plenty of light to
spare, and my camera is probably a long way away from being diffraction
limited, so why not use a small aperture?
The chromatic aberration issue with my camera is really only apparent
when attempting astrophotography. Here is one example:
http://www.flickr.com/photos/35853148@N05/4303433104/in/set-721576231290
10923/lightbox/
(view the 1024 pixel version for best results.)
--
Please reply to: | No nation is drunken where wine is cheap.
pciszek at panix dot com | --Thomas Jefferson
Sam Wormley
May 9, 2012, 8:05:39 PM5/9/12
to
Paul Ciszek
May 9, 2012, 8:53:22 PM5/9/12
to
In article <uuWdneLeLfvOmzbS...@mchsi.com>,
Sam Wormley <swor...@gmail.com> wrote:
>
> See: http://www.youtube.com/watch?v=yH7rbRu7Av8
Thank you.
--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984
Martin Brown
May 10, 2012, 4:31:35 AM5/10/12
to
you aren't going to affect things all that much by stopping it down.
However, most of the chromatic abberation cyan fringe you have in the
sharper moon shot is a weak defocus and slightly under size red image.
Here is a quick ad hoc fix.
Separate the image to R, G, B.
Take the red image and rescale to 1026x644 pixels then crop to 1024x640
taking the centre part only. Apply a judicious amount of unsharp masking
to selectively enhance the resolution and then recombine to get an image
where the lateral chromatic abberation is largely gone.
You could tune the numbers to do better - left as an exercise.
The green channel is the sharpest so I would suggest you use that...
--
Regards,
Martin Brown
jwarner1
May 10, 2012, 11:22:00 PM5/10/12
to
or mirror, has to do with the statistical fact that most over or
under corrected zones are found at the edge of an optic - has
nothing to do with the optical theory of spherical or chromatic
aberration per se. Its just a statistcal production fact that most
departures from an ideal parameter in a circular optic are usually found near
the edge of an optic.
anorton
May 11, 2012, 2:03:35 AM5/11/12
to
"jwarner1" <jwar...@mchsi.com> wrote in message
news:4FAC85D7...@mchsi.com...
edge of an amateur-made telescope mirror, but it is not a consideration for
sizing the aperture of a commercial photographic lens. The design of the
lens is in fact the chief constraint on the minimum F/#, and it is the chief
thing that affects performance versus F/#. The image at small f-numbers is
not usually diffraction limited (as designed), and thus tolerances in
surface figure (turned egdes or otherwise) are not usually the worst errors.
Centering and spacing of elements typically end up being more important, but
in a good lens these will be kept small enough that they do not
significantly degrade the design performance of the lens. In any case, lens
element diameters are larger than needed optically so they can be held by
retaining rings and such, so any edge defects would be outside the active
area. In addition, a refractive lens element is usually made a little over
size, then its optical axis is carefully aligned to the axis of a spindle
where the outer edge is ground off to center the axis to the outer diameter.
Makers of commercial quantities of optics treat production as a process
control problem. If they start seeing significant turned edges during
polishing, it is a good indication that something has gone wrong (e.g. pitch
viscosity, slurry temperature, slurry concentration).
--
Adam Norton
Norton Engineered Optics
www.nortonoptics.com
(Remove antispam feature before replying)
anorton
May 11, 2012, 2:54:35 AM5/11/12
to
"Paul Ciszek" <nos...@nospam.com> wrote in message
news:jof04i$acd$1...@reader1.panix.com...
your resolution will start to get worse again due to diffraction effects.
At what aperture this occurs depends on your lens quality and pixel size.
Was the moon centered in your field when you took the photo on the left? I
see more color on one limb than the other. If the mage is not off center,
this indicates something in your lens may be off-center. Does this worst
edge rotated when you rotate the teleconverter? Is the teleconverter
mounted well and not cocked at an angle?
Generally I am surprised your teleconverter works as well as it does. If
the two images were both taken at the same f/#, the blown-up image on the
right looks worse because you are also blowing up the aberrations. Stiil,
the two images seem to show the teleconverter only introduces some extra
chromatic aberration on one edge.
Paul Ciszek
May 11, 2012, 12:12:16 PM5/11/12
to
In article <t1Lqr.595$x11...@newsfe21.iad>,
Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:
>
>Given that you are using a teleconverter on the front of the Lumix lens
>you aren't going to affect things all that much by stopping it down.
>
>However, most of the chromatic abberation cyan fringe you have in the
>sharper moon shot is a weak defocus and slightly under size red image.
>
>Here is a quick ad hoc fix.
I think this may be what the chromatic aberration tool does. The image
>
>Given that you are using a teleconverter on the front of the Lumix lens
>you aren't going to affect things all that much by stopping it down.
>
>However, most of the chromatic abberation cyan fringe you have in the
>sharper moon shot is a weak defocus and slightly under size red image.
>
>Here is a quick ad hoc fix.
you see there was, alas, already corrected using that tool. On the other
hand, I'm not sure I got it to work properly, or that it didn't make
things worse.
Paul Ciszek
May 11, 2012, 12:25:25 PM5/11/12
to
In article <IKidnd772Z9AKjHS...@earthlink.com>,
anorton <ano...@removethis.ix.netcom.com> wrote:
>
>Was the moon centered in your field when you took the photo on the left? I
Not as centered as I would have liked. I have no tracker, and at the time
>
>Was the moon centered in your field when you took the photo on the left? I
was using a crappy mount, and I had to wrestle the moon into the middle of
the field and take the shot, all the while *seeing* the moon move.
>see more color on one limb than the other. If the mage is not off center,
in Silkypics. I'm not sure I did it correctly, or that I didn't make
things worse.
>Generally I am surprised your teleconverter works as well as it does. If
some flowers that were about 5m inside an electric fence:
http://www.flickr.com/photos/35853148@N05/5927911703/in/set-72157627049229625/
From what I have read on the web, the Lumix FZ35 is the next best thing
to a digital SLR.
Maybe someone here can answer another important question: Given the
resolution you see in the moon picture, do I have a chance of getting
a recognizable silouette of Venus during the transit, if I can find a
way to mount the solar film?
Martin Brown
May 11, 2012, 12:51:45 PM5/11/12
to
On 11/05/2012 17:12, Paul Ciszek wrote:
> In article<t1Lqr.595$x11...@newsfe21.iad>,
> Martin Brown<|||newspam|||@nezumi.demon.co.uk> wrote:
>>
>> Given that you are using a teleconverter on the front of the Lumix lens
>> you aren't going to affect things all that much by stopping it down.
>>
>> However, most of the chromatic abberation cyan fringe you have in the
>> sharper moon shot is a weak defocus and slightly under size red image.
>>
>> Here is a quick ad hoc fix.
>
> I think this may be what the chromatic aberration tool does. The image
Quite likely - you generally have to get it exactly right to remove the
> In article<t1Lqr.595$x11...@newsfe21.iad>,
> Martin Brown<|||newspam|||@nezumi.demon.co.uk> wrote:
>>
>> Given that you are using a teleconverter on the front of the Lumix lens
>> you aren't going to affect things all that much by stopping it down.
>>
>> However, most of the chromatic abberation cyan fringe you have in the
>> sharper moon shot is a weak defocus and slightly under size red image.
>>
>> Here is a quick ad hoc fix.
>
> I think this may be what the chromatic aberration tool does. The image
fringes. An otherwise decent lens can have different effective focal
lengths for different wavelengths even at the same focal plane.
> you see there was, alas, already corrected using that tool. On the other
> hand, I'm not sure I got it to work properly, or that it didn't make
> things worse.
to use the sharp focussed green channel as luminance and then tint it
using the chrominance information from the red and blue channels.
--
Regards,
Martin Brown
Paul Ciszek
May 11, 2012, 1:42:42 PM5/11/12
to
In article <Dsbrr.5325$Nh4....@newsfe17.iad>,
It sounds like you are saying that even though the blue and red images
may extend beyond the edge of the moon as defined by the green image,
that information will be tossed--the image will be "trimmed" to fit the
green image.
Chris.B
May 11, 2012, 4:01:39 PM5/11/12
to
I think your moon images have more problems than mere chromatic
aberration. If you aren't using a driven mounting the image is
constantly moving across the sensor. (as you have already mentioned)
The telephoto device is simply making your lens far "slower" than it
already is. This makes your problems infinitely worse by optical
leverage/ magnification plus much longer exposures.
If the false colour was the only problem you could simply switch to a
B&W image to sharpen it nicely. Chromatic aberration, within reason,
has little or no effect on the visual sharpness of the image. My extra-
focal, hand-held images are often colour fringed. One or steps of
colour reduction in free image handling software, like PhotoFiltre,
will easily remove any criticism of false colour on the moon.
Image movement on the sensor is one of the main reasons I hate
"ABSOLUTE PROOF OF UFO'S EXISTENCE!!!" shots on YouTube. Had they
simply held the camera very still their results, though very small,
might still have some slight ephemeral interest. Instead of which they
zoom to the "max" and handhold their camera to destroy all chance of a
worthwhile video. Or even a frozen still image. You are, following
exactly the same route, I'm afraid.
If you want to improve your chances of capturing something more useful
you could build a dirt cheap, Poncet mounting/platform for your camera
and hand crank a screwed rod drive. Place the platform on a solid wall
and you should enjoy an improvement in your images. An alternative is
to buy a cheap secondhand, equatorially _driven_ equatorial mounting
to hold your camera. Ask you local astro society/club if there are any
such mountings going cheap or available to be borrowed. Or make one
with plywood and hinges and screwed rod drive.
aberration. If you aren't using a driven mounting the image is
constantly moving across the sensor. (as you have already mentioned)
The telephoto device is simply making your lens far "slower" than it
already is. This makes your problems infinitely worse by optical
leverage/ magnification plus much longer exposures.
If the false colour was the only problem you could simply switch to a
B&W image to sharpen it nicely. Chromatic aberration, within reason,
has little or no effect on the visual sharpness of the image. My extra-
focal, hand-held images are often colour fringed. One or steps of
colour reduction in free image handling software, like PhotoFiltre,
will easily remove any criticism of false colour on the moon.
Image movement on the sensor is one of the main reasons I hate
"ABSOLUTE PROOF OF UFO'S EXISTENCE!!!" shots on YouTube. Had they
simply held the camera very still their results, though very small,
might still have some slight ephemeral interest. Instead of which they
zoom to the "max" and handhold their camera to destroy all chance of a
worthwhile video. Or even a frozen still image. You are, following
exactly the same route, I'm afraid.
If you want to improve your chances of capturing something more useful
you could build a dirt cheap, Poncet mounting/platform for your camera
and hand crank a screwed rod drive. Place the platform on a solid wall
and you should enjoy an improvement in your images. An alternative is
to buy a cheap secondhand, equatorially _driven_ equatorial mounting
to hold your camera. Ask you local astro society/club if there are any
such mountings going cheap or available to be borrowed. Or make one
with plywood and hinges and screwed rod drive.
William Hamblen
May 11, 2012, 6:35:47 PM5/11/12
to
inversely with the square of the focal ratio which means for a constant
focal length inversely with the square of the diameter of the aperture.
For a given lens the blur circle due to chromatic aberration is a
small fraction of the diameter of the aperture. Statistics notwithstanding.
Bud
Thomas Womack
May 11, 2012, 7:32:07 PM5/11/12
to
In article <jojehl$ldr$1...@reader1.panix.com>,
transiting (it's 115 times smaller in diameter than the Sun, but three
times closer to Earth than the Sun is), about 60 arc-seconds across;
the image scale on your Moon is about 3.2 arc-seconds per pixel so
Venus will be 20px across, rather bigger than the obvious crater in
the middle of the mare at the right of your image.
http://www.chiark.greenend.org.uk/~twomack/transit-of-venus-2004.jpg
is eyepiece-projection of the last transit of Venus ( the right-hand
side is the limb of the Sun, the left-hand side is the limit of the
eyepiece, it's quite near the start of the transit ); it's a pretty
obvious event.
Paul Ciszek <nos...@nospam.com> wrote:
>Maybe someone here can answer another important question: Given the
>resolution you see in the moon picture, do I have a chance of getting
>a recognizable silouette of Venus during the transit, if I can find a
>way to mount the solar film?
Yes; Venus is about 1/40 the angular diameter of the Sun when
>Maybe someone here can answer another important question: Given the
>resolution you see in the moon picture, do I have a chance of getting
>a recognizable silouette of Venus during the transit, if I can find a
>way to mount the solar film?
transiting (it's 115 times smaller in diameter than the Sun, but three
times closer to Earth than the Sun is), about 60 arc-seconds across;
the image scale on your Moon is about 3.2 arc-seconds per pixel so
Venus will be 20px across, rather bigger than the obvious crater in
the middle of the mare at the right of your image.
http://www.chiark.greenend.org.uk/~twomack/transit-of-venus-2004.jpg
is eyepiece-projection of the last transit of Venus ( the right-hand
side is the limb of the Sun, the left-hand side is the limit of the
eyepiece, it's quite near the start of the transit ); it's a pretty
obvious event.
Chris.B
May 12, 2012, 1:18:11 AM5/12/12
to
On May 12, 1:32 am, Thomas Womack <twom...@chiark.greenend.org.uk>
wrote:
exposures are likely. (even with a solar foil, full aperture filter as
here)
http://4.bp.blogspot.com/_6k-wLqmhRSk/STqSV-M48qI/AAAAAAAAA_s/XX1hAJ_xq2M/s400/venus2+colr.jpg
Interestingly(?) you can buy a complete (and excellent) Bresser 70mm
refractor on an equatorial mounting for about the same as you pay for
a telephoto adaptor. Even less if you can find a used (but undamaged)
one.
This would give you many advantages over the telephoto adaptor. Except
that your camera is not ideal for making the best use of this little
telescope. A budget digital camera with a much smaller, small zoom
lens would be much better. A budget camera can also be had for the
same price as the telephoto adaptor. Make a simple digiscoping adaptor
and you have a lifetime's enjoyable imaging and experimentation ahead
of you. A driven mounting would be better. But the provided slow
motion worm/wheel could be driven with a synchronous motor bought on
eBay.
wrote:
>
> Yes; Venus is about 1/40 the angular diameter of the Sun when
> transiting (it's 115 times smaller in diameter than the Sun, but three
> times closer to Earth than the Sun is), about 60 arc-seconds across;
> the image scale on your Moon is about 3.2 arc-seconds per pixel so
> Venus will be 20px across, rather bigger than the obvious crater in
> the middle of the mare at the right of your image.
>
> http://www.chiark.greenend.org.uk/~twomack/transit-of-venus-2004.jpg
> is eyepiece-projection of the last transit of Venus ( the right-hand
> side is the limb of the Sun, the left-hand side is the limit of the
> eyepiece, it's quite near the start of the transit ); it's a pretty
> obvious event.
Fortunately the Sun is brighter than the Moon and much shorter
> Yes; Venus is about 1/40 the angular diameter of the Sun when
> transiting (it's 115 times smaller in diameter than the Sun, but three
> times closer to Earth than the Sun is), about 60 arc-seconds across;
> the image scale on your Moon is about 3.2 arc-seconds per pixel so
> Venus will be 20px across, rather bigger than the obvious crater in
> the middle of the mare at the right of your image.
>
> http://www.chiark.greenend.org.uk/~twomack/transit-of-venus-2004.jpg
> is eyepiece-projection of the last transit of Venus ( the right-hand
> side is the limb of the Sun, the left-hand side is the limit of the
> eyepiece, it's quite near the start of the transit ); it's a pretty
> obvious event.
exposures are likely. (even with a solar foil, full aperture filter as
here)
http://4.bp.blogspot.com/_6k-wLqmhRSk/STqSV-M48qI/AAAAAAAAA_s/XX1hAJ_xq2M/s400/venus2+colr.jpg
Interestingly(?) you can buy a complete (and excellent) Bresser 70mm
refractor on an equatorial mounting for about the same as you pay for
a telephoto adaptor. Even less if you can find a used (but undamaged)
one.
This would give you many advantages over the telephoto adaptor. Except
that your camera is not ideal for making the best use of this little
telescope. A budget digital camera with a much smaller, small zoom
lens would be much better. A budget camera can also be had for the
same price as the telephoto adaptor. Make a simple digiscoping adaptor
and you have a lifetime's enjoyable imaging and experimentation ahead
of you. A driven mounting would be better. But the provided slow
motion worm/wheel could be driven with a synchronous motor bought on
eBay.
Brad Guth
May 12, 2012, 8:50:08 AM5/12/12
to
using a projection lens as any magnifier, nor do they believe that
less aperture is ever good for anything, especially for looking at our
moon or even at Sirius.
http://groups.google.com/groups/search
http://translate.google.com/#
Brad Guth, Brad_Guth, Brad.Guth, BradGuth, BG / “Guth Usenet”
Martin Brown
May 15, 2012, 4:01:17 AM5/15/12
to
On 11/05/2012 21:01, Chris.B wrote:
> I think your moon images have more problems than mere chromatic
> aberration. If you aren't using a driven mounting the image is
> constantly moving across the sensor. (as you have already mentioned)
> The telephoto device is simply making your lens far "slower" than it
> already is. This makes your problems infinitely worse by optical
> leverage/ magnification plus much longer exposures.
His telephoto device is a front element multiplier - essentially a
> I think your moon images have more problems than mere chromatic
> aberration. If you aren't using a driven mounting the image is
> constantly moving across the sensor. (as you have already mentioned)
> The telephoto device is simply making your lens far "slower" than it
> already is. This makes your problems infinitely worse by optical
> leverage/ magnification plus much longer exposures.
dedicated x3 telescope fitted to the front lens so it doesn't alter the
lens speed at all. It does have a bit of chromatic aberration though.
>
> If the false colour was the only problem you could simply switch to a
> B&W image to sharpen it nicely. Chromatic aberration, within reason,
> has little or no effect on the visual sharpness of the image. My extra-
> focal, hand-held images are often colour fringed. One or steps of
> colour reduction in free image handling software, like PhotoFiltre,
> will easily remove any criticism of false colour on the moon.
Actually separating to RGB shows the green to be very good, blue not bad
> If the false colour was the only problem you could simply switch to a
> B&W image to sharpen it nicely. Chromatic aberration, within reason,
> has little or no effect on the visual sharpness of the image. My extra-
> focal, hand-held images are often colour fringed. One or steps of
> colour reduction in free image handling software, like PhotoFiltre,
> will easily remove any criticism of false colour on the moon.
and red a slightly different size and fuzzy.
>
> Image movement on the sensor is one of the main reasons I hate
> "ABSOLUTE PROOF OF UFO'S EXISTENCE!!!" shots on YouTube. Had they
> simply held the camera very still their results, though very small,
> might still have some slight ephemeral interest. Instead of which they
> zoom to the "max" and handhold their camera to destroy all chance of a
> worthwhile video. Or even a frozen still image. You are, following
> exactly the same route, I'm afraid.
>
> If you want to improve your chances of capturing something more useful
> you could build a dirt cheap, Poncet mounting/platform for your camera
> and hand crank a screwed rod drive. Place the platform on a solid wall
> and you should enjoy an improvement in your images. An alternative is
> to buy a cheap secondhand, equatorially _driven_ equatorial mounting
> to hold your camera. Ask you local astro society/club if there are any
> such mountings going cheap or available to be borrowed. Or make one
> with plywood and hinges and screwed rod drive.
His shots are not at all bad for a standard camera with an add on.
> Image movement on the sensor is one of the main reasons I hate
> "ABSOLUTE PROOF OF UFO'S EXISTENCE!!!" shots on YouTube. Had they
> simply held the camera very still their results, though very small,
> might still have some slight ephemeral interest. Instead of which they
> zoom to the "max" and handhold their camera to destroy all chance of a
> worthwhile video. Or even a frozen still image. You are, following
> exactly the same route, I'm afraid.
>
> If you want to improve your chances of capturing something more useful
> you could build a dirt cheap, Poncet mounting/platform for your camera
> and hand crank a screwed rod drive. Place the platform on a solid wall
> and you should enjoy an improvement in your images. An alternative is
> to buy a cheap secondhand, equatorially _driven_ equatorial mounting
> to hold your camera. Ask you local astro society/club if there are any
> such mountings going cheap or available to be borrowed. Or make one
> with plywood and hinges and screwed rod drive.
--
Regards,
Martin Brown
Paul Ciszek
May 16, 2012, 4:26:09 PM5/16/12
to
In article <Dsbrr.5325$Nh4....@newsfe17.iad>,
for a bit of chromatic aberration at the edges. Honestly, I expected
to look sightly yellowish with a bit of color difference around the
sunspots. Anyway, since there appeared to be no visible color to lose,
I said to heck with it and just isolated the green channel:
http://www.flickr.com/photos/35853148@N05/7207438758/sizes/l/in/photostream/
Chris.B
May 17, 2012, 1:27:52 PM5/17/12
to
On May 16, 10:26 pm, nos...@nospam.com (Paul Ciszek) wrote:
>
> Well, my first picture of the sun looked absolutely colorless except
> for a bit of chromatic aberration at the edges. Honestly, I expected
> to look sightly yellowish with a bit of color difference around the
> sunspots. Anyway, since there appeared to be no visible color to lose,
> I said to heck with it and just isolated the green channel:
>
> http://www.flickr.com/photos/35853148@N05/7207438758/sizes/l/in/photo...
>
> Well, my first picture of the sun looked absolutely colorless except
> for a bit of chromatic aberration at the edges. Honestly, I expected
> to look sightly yellowish with a bit of color difference around the
> sunspots. Anyway, since there appeared to be no visible color to lose,
> I said to heck with it and just isolated the green channel:
>
Not bad at all!
Venus will be about the size of the lowest sunspot in your image.
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