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Photography Question 
Samantha L. Dean
BetterPhoto Member Since: 12/17/2006
 

Blue or Purple Fringing


I purchased a new Sony 75-300mm lens less than four months ago, along with a new camera. For about the last month, the photos taken with the Sony lens have blue or purple fringing and seemed to have a soft focus instead of clear, crisp focus. Is this something I am doing or is it the lens? All advice will be most welcome!


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3/6/2008 7:59:18 AM

 
Alan N. Marcus   Hi Samantha,
In a perfect color-corrected optical system, all the colors come to a focus at precisely at the same location, forming an image at the surface of the digital imaging chip (focal plane). Sorry to report, that dream has never been achieved. What actually happens is: the image is composed of multiple images superimposed on top of one another. Each different color of the vista comes to a focus at differing distances from the lens. Blue light forms an image further down stream and thus the blue image is the larger image. Red light focuses closer in thus it is the smaller image. The other colors come to a focus at intermediate distances; each thus is minutely different as to size. The purple fringing you have identified is caused by two types of chromatic aberrations. One is called transverse; a variation of focal lengths by color, the other is longitudinal whereby the actual location of the image is a function of its color.
Lens makers know about aberrations. They design complex multi-element systems to counter each and they succeed to a high degree. Countermeasures for aberrations are more difficult when the lens is very long or very short as to focal length.
Sorry to report that digital cameras introduce another phenomena that piles on top of the chromatic abnormalities. The digital chip is divided into tiny sights (pixels). These are the light sensitive locations. These pixels are then further divided into sub-pixels each covered with a strong red or green or blue filter. This arrangement fashions a matrix with surfaces that act just like tiny biconvex lenses (lenticular array). Thus the purple fringe is a combination of chromatic aberrations and the lenticular contour.
The good news is: Your digital editing software now contains automatic tools to deal with purple fringing.
Alan Marcus (marginal technical gobbledygook)


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3/6/2008 3:28:16 PM

 
Bob Cammarata
BetterPhoto Member Since: 7/17/2003
  To add to Alan's detailed analysis:
Fringing is most prevalent toward the outside portions of the frame and is even more pronounced along lines that are not sharply focused.
(I've noticed this while conducting test with super-telephoto/converter combinations and with extremes in macro.)
In other words, break the "rules" and compose your primary point of interest in the center of the frame AND make sure your point of critical focus is on target.
This will reduce or eliminate the effect of chromatic aberration on your subject.
Any fringing that occurs toward the outside portions of the frame can be cropped out.


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3/6/2008 4:02:55 PM

 
Samantha L. Dean
BetterPhoto Member Since: 12/17/2006
  Thank you both so much for your explanations. From what you've both told me, it would appear that the problem occurs due to both equipment and operator error and / or anomalies. I will try to get better. :-)


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3/7/2008 6:52:00 AM

 
Alan N. Marcus   Hi again Samantha,
Don’t beat yourself up over this! Lens aberrations are a fact of life. John Dolland in 1757 demonstrated the first lens to significantly correct chromatic aberration. He did it by combining two lens shapes. One element positive, shaped like a lentil seed i.e. convex, the other negative, shaped concave like the inside of a bow. Seems the color error of these different designs are nearly exactly opposite. When combined they almost but not quite cancel color error. Today’s lenses are made-up using multiple elements some positive, some negative. Additionally lens makers intermix different types of glass like crown and flint. Some blends contain rare earth elements. In the old days it took months and sometimes years to calculate curve shape and materials (using a slide rule). Today’s computers solve as fast as you can input the data. You benefit, highly corrected accurately made lenses are affordable. Tomorrow’s lenses – mixtures of different glasses and plastics mixed while molten and spinning. Just think, a lens made with denser glass at the edges, this reduces the amount of curve needed – it’s going to be great.

Alan Marcus


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3/7/2008 7:58:32 AM

 
Barefoot Photography by Tina Doane
BetterPhoto Member Since: 1/3/2005
  This is all very confusing, I am all of sudden having these purpleish swirls on my images..driving me crazy!!!! I havent changed my lighting or camera settings, I used fixed lighting set up in studio and have had the same set up for two years. My question is the same time I started having the problems with the swirls I also startrd having problems with my lens. I use a canon 5 D withe 28 to 135 mm lens. Could the two problems be related?? Please help!


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8/7/2008 7:26:56 PM

 
W.   
Maybe you could expand a bit on those lens problems, Tina? What are they? When do they occur?
And why don't you post one of those photos with the purpleish swirls?
A picture paints a thousand words.


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8/7/2008 7:45:10 PM

 
Barefoot Photography by Tina Doane
BetterPhoto Member Since: 1/3/2005
  I am having a purlpe/blue tinted fringe on alomost all of my images, out of the blue. I Have tried everything to eliminate but nothing has worked. I am also having focusing problems with my lens. On my 5D the lens will sometimes do nothing when I press the shutter button, then I power off and on again and then it will work for a few minutes then repeat. This has become more frequent latley. So it finally wouldnt work at all so I took the same lens and put on my back up camera a 30D, now it keeps making a clicking sound like it is trying to focus but will not stop clicking and will not give the "focusing beep" like it usually does. To make a long story short, the purple fringing is becoming worse and the focusing problem that once was a nuscance is now a constant problem. So the only conclusion after all my trouble shooting is the thier has to be a connection between the lens and focusing problems and I come to that because I have a second studio with the exact camera, lenses and light set up and I have never had the purple fringing there or the focusing problems. Both studios are identically set up to the tee!!.


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8/7/2008 7:53:13 PM

 
Barefoot Photography by Tina Doane
BetterPhoto Member Since: 1/3/2005
  I am having a purlpe/blue tinted fringe on alomost all of my images, out of the blue. I Have tried everything to eliminate but nothing has worked. I am also having focusing problems with my lens. On my 5D the lens will sometimes do nothing when I press the shutter button, then I power off and on again and then it will work for a few minutes then repeat. This has become more frequent latley. So it finally wouldnt work at all so I took the same lens and put on my back up camera a 30D, now it keeps making a clicking sound like it is trying to focus but will not stop clicking and will not give the "focusing beep" like it usually does. To make a long story short, the purple fringing is becoming worse and the focusing problem that once was a nuscance is now a constant problem. So the only conclusion after all my trouble shooting is the thier has to be a connection between the lens and focusing problems and I come to that because I have a second studio with the exact camera, lenses and light set up and I have never had the purple fringing there or the focusing problems. Both studios are identically set up to the tee!!.


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8/7/2008 7:53:13 PM

 
W.   
Offhand I would say that lens was bumped so hard that it broke the AF motor and dislodged some lens element.
I would get it in for service and repair.

Have fun!


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8/8/2008 1:59:09 AM

 
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