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Old Feb 21, 2004, 5:41 PM   #1
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Default Cause of purple fringeing

I was under the inpression that purple fringeing (blooming) was caused by digicams' sensors. I have had a couple of people say it's lenses. I did a lot of film-based photos, with various lenses and zooms, and never noticed the problems I am getting with digital photos, or for that matter scanning slides with a (not very expensive) slide scanner which has caused the same problem.

So is it sensors or lenses?

Thanks for any advice.
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Old Feb 21, 2004, 5:45 PM   #2
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Sorry. I should have searched first. It's the sensors.

I had started to get a bit ratty, with another poster elsewhere, and was a little hasty to enquire.

Again. Sorry
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Old Feb 21, 2004, 9:52 PM   #3
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Default I thought it was the lense

When reading the reviews, it always talks about the lense being sharp and not producing the purple fringing. I have always thought it was an optic quality issue. Could it be that both the sensor and the lense have something to do with it. I thought however the Sensor was responsible for the noise in the images.
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Old Feb 21, 2004, 11:58 PM   #4
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Here's an interesting URL.

It's Sony's forum Watch the wrap.


http://www.sonycams.com/forum/showth...=8359#post8359

I do wonder if it's both. The general feeling is that it a digicam issue, not a lens issue.

The fact the film cameras never had it seems to indicate that it has to be the snsor or perhaps the electronics. But if we take two lenses of pretty much equal quality, and then ask them to service 35mmm slide and then a 6mm sensor with the same conditions and expectations, any lens problems are just a _little_ strained by the smaller sensor.
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Old Feb 22, 2004, 1:28 AM   #5
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It's the lens.

I'm taking an astronomy class this semester, and in learning about telescopes, I've learned about chromatic aberrations.

Quote from my textbook:

"Refracting telescopes suffer from a serious optical distortion that limits their usefulness. When light is refracted through glass, shorter wavelengths bend more than longer wavelengths, and blue light comes to a focus closer to the lens than does red light. If we focus the eyepiece on the blue image, the red light is out of focus, and we see a red blur around the image. If we focus on the red image, the blue light blurs. The color separation is called chromatic aberration. Telescope designers can grind a telescope lens of two components made of different kinds of glass and so bring two wavelengths to the same focus. This does improve the image, but these achromatic lenses are not totally free of chromatic aberration."

And a refracting telescope works on the same principles as a camera lens.

Good telescopes use mirrors instead of lenses to focus light without chromatic aberration. Why aren't there mirror lenses for cameras? Is it because they'd be too large?
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Old Feb 22, 2004, 2:06 AM   #6
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Default Re: Cause of purple fringeing

Quote:
Originally Posted by Nickagain
I was under the inpression that purple fringeing (blooming) was caused by digicams' sensors. I have had a couple of people say it's lenses. I did a lot of film-based photos, with various lenses and zooms, and never noticed the problems I am getting with digital photos, or for that matter scanning slides with a (not very expensive) slide scanner which has caused the same problem. So is it sensors or lenses?
It is the LENS! When a white light ray passes through a glass, it will be dispersed into all kind of colors based on the wavelength of each color. Newton knew it with his well-known prism experiment. In lens design, designers can make green and red colors to focus at a point fairly easily and leaving the blue color not focused. When these not-focused rays hit sensor or film, a purple fringe may appear along high contrast areas. Expensive lenses may use special glasses so that the red, green and blue light rays are not so dispersed. Some special lenses can make correction so that red, green and blue light rays can focus so close that one many think they focus at a single point. In optic terms, the problem that a lens cannot make all prime colors to focus at a single point is referred to as chromatic aberration, which is a common problem in cheap lenses. That you did not see it on your film/print is perhaps because you did not do large print or use high magnification loupe to view your slides. Or, it could be that you have a collection of very high quality lenses.

Blooming and fringing (i.e., chromatic aberration) are two different things. The blooming refers to the over-charged electrons propagate to the neighboring photosites, causing the over-expose effect to spread to neighbor area and making the image (in that area) blurred. Well, hope this is a good intuitive explanation.

See the "Lens Overview" section of my 5700 user guide for more about chromatic aberration. Images shown there were taken with a very popular lenses on slides, and scanned to digital format. This shows that chromatic aberration is a universal problem of lenses.

CK
http://www.cs.mtu.edu/~shene/DigiCam
Nikon Coolpix 950/990/995/2500/4500/5700 User Guide
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Old Feb 22, 2004, 5:06 AM   #7
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Default Re: Cause of purple fringeing

[quote="shene"][quote="Nickagain"][color=#AAAAFF][b]I was under the inpression that purple fringeing (blooming) was caused by digicams' sensors. I have had a couple of people say it's lenses.
It is the LENS!

snip of science....

Blooming and fringing (i.e., chromatic aberration) are two different things. The blooming refers to the over-charged electrons propagate to the neighboring photosites, causing the over-expose effect to spread to neighbor area and making the image (in that area) blurred. Well, hope this is a good intuitive explanation.

Well you seem to be going against all wisdom. All of thes things are chromatic aberration IMO, but I am talking about the purple of blue egde to a dark object next to a white overexposed object. The strong impression I am getting is that its the electronics, not the lens.

I did make poster prints and slide projections from 35mm stuff. I never noticed anything _like_ the problem I am getting with a digicam. If I scan those same slides, I get purple edging. Maybe digicams all have crappy lenses? I had Pentax, Chinon, Tokina. They were all OK lenses, but I never bought the top.
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Old Feb 22, 2004, 6:51 AM   #8
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It's both, one leads to the other: it starts with the optic but is made worse by the electronics... http://www.dpreview.com/learn/?/Glos...rations_01.htm

Chromatic Aberration is not a universal problem: some cameras such as the D7's/A1... has an excellent apochromatic G series zoom which can achieve relatively free CA at all focal lenghts!
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Old Feb 22, 2004, 9:33 AM   #9
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There is some evidence of what NHL has written.


Quote:
Chromatic Aberration is not a universal problem: some cameras such as the D7's/A1... has an excellent apochromatic G series zoom which can achieve relatively free CA at all focal lenghts!
The A1 (5 mpxl CCD sensor) has no purple fringing, but the A2 which uses exactly the same lens, but has a different CCD (8 mpxl) does exhibit purple fringing under some conditions. Photos showing the A2's PF have been posted on another forum.

BTW, the A2 CCD is the same one that is used in the Sony 828. The 828 has severe purple fringing issues.

-------------edit-----

A link to the PF shot that I referred to above has been posted in the Minolta forum here at Steves. http://www.stevesforums.com/phpBB2/v...4c32d2f88a220f

It is in Kent Ollson's gallery.
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Old Feb 22, 2004, 9:41 AM   #10
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The purple fringe issue spans multiple cameras and multiple lenses. Actually, no camera or lens is completely free of this issue including the very best film and digital cameras and lenses made.

That there are multiple causes is also quite apparent. The problem is that we sometimes confuse classic chromatic (lateral) aberration with other color fringing issues and in the case of the F828, it's more difficult to know exactly which issues are chromatic aberration (in the classic sense) and which are the result of sensor blooming and a combination of the two. Even infrared light entering the lens and striking the sensor can contribute to color fringe.

When the photosites (sensor points on the chip itself - also called sensor "pixels") are overwhelmed with excessive light at points of high contrast they often spill off electrons to adjacent photosites. This condition is referred to as sensor "blooming" and is most frequently noticed in places such as where leaves or dark branches from trees contrast with an intense bright sky or around white lights in night shots.

Typically, there is a spread of white light which makes a rather fuzzy appearance or "halo" in the vicinity, but a blue/purple color is also frequently present. The larger the photosite "wells," the less of a problem this generally is so that professional level cameras such as the Canon EOS-1DS, or dSLR's in general are less prone to have this issue, but indeed it does sometimes happen.

On the other hand, even in film cameras, there is sometimes a similar or identical issues so that it's possible that lenses contribute to the problem and that it's not completely a sensor issue but exacerbated by the sensor which is more sensitive to highlights in general than most film. One generally is not so aware of the presence with film unless it's scanned and viewed on a monitor at 100 percent or larger.

The confusion comes because we typically think of chromatic aberration (CA) in the classic lateral red/green form which is most frequently found in the periphery of the frame and can be manipulated with software in several ways. But there is also another type of chromatic aberration which is sometimes referred to as "transverse" chromatic aberration and it can occur not only at the periphery but in the center of the frame and it affects the blue spectrum in much the same way as blooming appears to.

So when we see the blue/purple fringe, we really don't know for certain whether it's transverse chromatic aberration, blooming, infrared interference or a combination of one or more.

For whatever reason, the Sony F828 produces more of this anomaly than most digicams and since the same sensor is used by other manufacturers whose cameras are less prone to this issue (some like Minolta seem to be almost immune) we tend to immediately point to the lens and say it's a lens issue. But perhaps we are jumping to this conclusion a bit too quickly.

Zeiss says it's absolutely NOT a lens issue and I tend to believe that they are correct. If you look at resolution chart testing you find that Minolta's new digicams which show very little or no CA tend to render lower resolution than Sony counterparts which have decidedly more of this. Does this then mean that Minolta's lenses are better designed and remove chromatic aberration and blooming but render less resolution than the Zeiss counterparts on the Sony? I don't think so.

I suspect that Minolta and perhaps other companies (Canon, Olympus, etc.) are using firmware correction to do similar processing to what is easily done in software to remove or at least ameliorate CA and/or blooming. But in doing this, they loose a little of the resolution and crispness. Remember, the new Sony F828 has additional color information supporting their "Real Color" feature. The associated electronics and filtering may exacerbate this blue/purple fringe and without firmware correction which would affect resolution a bit.

There have been many experienced and knowledgeable people dedicated to finding answers to this question, and currently there has been no definitive answer. If you overexpose and shoot wide open aperture in high contrasting and backlit environments with the F828 you will get purple/blue fringing. There are numerous and excellent PhotoShop actions available which solve the problem very nicely, but stay tuned because the true cause or "causes" are yet to be disclosed. To date, the following link has what I consider the best information available on this issue.

Lin

http://www.sd3.info/pf828/Sony_828_P..._Analysis.html
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