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Blue borders.

Discussion in 'Photography Forums' started by stevegreen, Aug 20, 2005.

  1. stevegreen

    stevegreen
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    Can anybody shed any light on this little issue I have.

    Below is a crop from a picture i took yesterday, if you notice, there seems to be some 'blue' hue around the edge of the white.

    Is this to do with the fact it was bright sunshine or is it a fault of the camera?
     

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  2. Pete Delaney

    Pete Delaney
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    What brand of washing powder do you use? :rolleyes:

    err no seriously it's a spot of chromatic aberation isn't it? ;)
     
  3. stevegreen

    stevegreen
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    Whats chromatic aberation? :confused:
     
  4. Johndm

    Johndm
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    Chromatic Aberration - Also known as the "purple fringe effect." It is common in two Megapixel and higher resolution digital cameras (especially those with long telephoto zoom lenses) when a dark area is surrounded by a highlight. Along the edge between dark and light you will see a line or two of purple or violet colored pixels that shouldn't be there.

    The main cause of the problem is the residual chromatic aberration of the focusing group of lenses. It is difficult to solve because of inherent limits in the dispersion (wavelength characteristic of refractive index) of optical glass. The secondary spectrum of Canon L series lenses is corrected by using fluorite crystal, which has different dispersion characteristics from ordinary optical glass, or by using a type of glass with an extraordinary dispersion characteristic.

    Same old story really....quality costs...more..
     
  5. Pete Delaney

    Pete Delaney
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    Steve,

    You will get this to some extent on any digital compact or digital SLR. It occurs in areas of very high contrast. Take a picture of a roof top on a very bright day and you may notice it on the edge of the roof where it meets the sky. i.e. dark against light. In your case it was the white shirt against a dark background...if you look you will see the CA is more pronounced with the darker areas in the background. CA is a pain, you will not always get it and it can be messed with in photoshop to turn it into a neutral colour (grey works well).

    In short there's nothing particularly at fault with your camera. It can happen to anyone although better lenses/cameras can help reduce the effect.


    Oh and I agree with Johndm (but I'll never vote for him ;) )
     
  6. stevegreen

    stevegreen
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    Thanks for the explanation folks :smashin:
     
  7. David Mackenzie

    David Mackenzie
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    You can counter the effect in Photoshop a bit. Press CTRL+U and select Blue. Then turn down the saturation - your purple fringes will now change into less noticeable gray fringes.

    Of course, that will suck some of the legitimate blue colours out of your image too, so it's a good idea to make two layers and pick and cut the parts that have gone wrong out of the top image so you get the best of both of the "grayed" and "blue" versions.
     
  8. ASH1

    ASH1
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    It is indeed chromatic aberration and if you shoot in raw it's quite easy to get rid of.

    When you open a raw image in PS click on the lens tap. Now you will see three sliders the first one is chromatic aberration R/C which stands for red and cyan, the second one is chromatic aberration B/Y for blue, yellow.

    You need to slowly move theses sliders to get rid of or minimise the chromatic aberration. This is another good reason to shoot in raw.

    But watch you don't over do it or you'll cause other parts of the picture to suffer, job done. :thumbsup:
     
  9. mattym

    mattym
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    some lenses come with 'coatings' that combat this problem, the sigma APO for example...
     
  10. bowenjones

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    Steve, I hope you don't mind but I couldn't resist having a little mess around with your pic in paintshop. I tried what Lyris suggested and, to a degree, it did enhance the photo but there was still a grey glow around the shirt. So I used freehand to highlight the areas that needed changing and then cloned the areas out. The freehand selection line was still visible slightly so I just used the clone brush again to smooth them out. I also smoothed the edge of the shirt a bit as it looked a little jagged after I made the changes. It's far from perfect I know but if I spent more time and had a lot more practice I suppose it would look much better.
     

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  11. stevegreen

    stevegreen
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    You did a good job :smashin:

    I'm not accomplished enough in PS to do things like that :(
     
  12. bowenjones

    bowenjones
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    Beleive me, I am a complete novice and i'm only picking things up by messing around.
     
  13. stevegreen

    stevegreen
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    Thats my learning process too :) I bought a book on PS but never seem to get round to reading it :rolleyes:
     
  14. longleyc

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    Steve,
    I cant really add to this as I totally agree with whats been written.

    I do however, have a nice technique for removal of the purple fringing (CA). Its comprehensive so msg for the details. Its adjustable as not to effect the overall colour balance too.

    CA is not a problem unless you intend to either crop or reduce the dpi resolution in order to scale the picture to A4+ print sizes.
    Unless its a very cheap lens and high contrast pictures you wont notice it at 6x4 7x5 prints.
     

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