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Effect of aperture on chromatic aberation?



 
 
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  #1  
Old May 10th 12, 12:54 AM posted to sci.astro.amateur
Paul Ciszek
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Posts: 110
Default Effect of aperture on chromatic aberation?

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/3585314...t-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
  #2  
Old May 10th 12, 01:05 AM posted to sci.astro.amateur
Sam Wormley[_2_]
external usenet poster
 
Posts: 3,966
Default Effect of aperture on chromatic aberation?

On 5/9/12 6:54 PM, Paul Ciszek wrote:
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/3585314...t-721576231290
10923/lightbox/

(view the 1024 pixel version for best results.)


See: http://www.youtube.com/watch?v=yH7rbRu7Av8
  #3  
Old May 10th 12, 01:53 AM posted to sci.astro.amateur
Paul Ciszek
external usenet poster
 
Posts: 110
Default Effect of aperture on chromatic aberation?


In article ,
Sam Wormley 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

  #4  
Old May 10th 12, 09:31 AM posted to sci.astro.amateur
Martin Brown
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Posts: 1,707
Default Effect of aperture on chromatic aberation?

On 10/05/2012 00:54, Paul Ciszek wrote:
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/3585314...t-721576231290
10923/lightbox/

(view the 1024 pixel version for best results.)


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.

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
  #5  
Old May 11th 12, 04:22 AM posted to sci.astro.amateur
jwarner1
external usenet poster
 
Posts: 156
Default Effect of aperture on chromatic aberation?



Paul Ciszek wrote:

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/3585314...t-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


Part of the theory for reducing the outside edge of a lens,
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.





  #6  
Old May 11th 12, 07:03 AM posted to sci.astro.amateur
anorton
external usenet poster
 
Posts: 25
Default Effect of aperture on chromatic aberation?


"jwarner1" wrote in message
...


Paul Ciszek wrote:

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/3585314...t-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


Part of the theory for reducing the outside edge of a lens,
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.


Turned down edges may be a reason that (rarely) someone will mask the outer
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)



  #7  
Old May 11th 12, 07:54 AM posted to sci.astro.amateur
anorton
external usenet poster
 
Posts: 25
Default Effect of aperture on chromatic aberation?


"Paul Ciszek" wrote in message
...
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/3585314...t-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


Reducing the aperture will reduce all the aberrations, but at some point
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.

--
Adam Norton
Norton Engineered Optics
www.nortonoptics.com

(Remove antispam feature before replying)

  #8  
Old May 11th 12, 05:12 PM posted to sci.astro.amateur
Paul Ciszek
external usenet poster
 
Posts: 110
Default Effect of aperture on chromatic aberation?


In article ,
Martin Brown 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
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.

--
Please reply to: | No nation is drunken where wine is cheap.
pciszek at panix dot com | --Thomas Jefferson
  #9  
Old May 11th 12, 05:25 PM posted to sci.astro.amateur
Paul Ciszek
external usenet poster
 
Posts: 110
Default Effect of aperture on chromatic aberation?


In article ,
anorton 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 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,


What you see has been run through the chromatic abberation correction tool
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


I was kinda pleased with it, too. I also used it to "poach" a picture of
some flowers that were about 5m inside an electric fence:

http://www.flickr.com/photos/3585314...7627049229625/

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?

--
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

  #10  
Old May 11th 12, 05:51 PM posted to sci.astro.amateur
Martin Brown
external usenet poster
 
Posts: 1,707
Default Effect of aperture on chromatic aberation?

On 11/05/2012 17:12, Paul Ciszek wrote:
In ,
Martin 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
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.


I have to say that for picture of the moon and sun I would be inclined
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
 




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