What exactly is it that causes purple fringing. It is just point and shoot camer

Question: What exactly is it that causes purple fringing!. It is just point and shoot cameras or does it happen with SLRs

I was editing some photos belonging to a friend today and they had purple fringing around the white areas!. He has a point and shoot!.

I find it interesting, if somewhat annoying!.Www@QuestionHome@Com

Best Answer - Chosen by Asker:

It's called 'Chromic Abberation' and is caused because the red, green and blue parts of an image focus at different planes, the lens tries to correct for this, wide angle lenses are more susceptible than telephoto, zooms more than primes (it rarely occurs with a single focal length lens as it can be designed out), but it can occur with any lens!. Usually it happens with high contrast edges such as a dark tree line against a bright sky!.

The Bayer filter used in digital cameras exacerbates the problem!.Www@QuestionHome@Com

In addition to all of the correct answers above me, I believe that the problem is exaggerated in point and shoot cameras with a gazillion pixels packed onto minuscule sensors!. First of all, if there are any stray bands of weird light bent off the normal path by a cheap lens, they are more likely to hit something and be recorded!. That's not very scientific, but it just sounds right!. Second, with the pixels being packed tighter than sardines, the signal from one will spill over into the signal from a neighbor!. This is a source of noise in small sensors and it just seems to add to the problems associated with "optical noise" like chromatic aberations, including fringing or color shifts!.

If I am totally off base here, please someone make a comment in your answer and I'll just admit that I am wrong!. This answer is an intuitive response and not based in science at all!. Optical electronics and amplification are not all that different in sonic amplification, so I tend to "translate" some things in digital cameras based on my knowledge of some things that happen in the sound world!.Www@QuestionHome@Com

I think Screwdriver hit the nail on the head!. Chromatic aberration, and the related sensor contribution would be the Bayer Filter!. Beyond that point, the way the sensor information is interpreted and file compression may, or likely did, add to the problem!.
Dr!., I don't think you're way off base!. Sensor design is always a factor, but that doesn't always relate to sensor size!. I think if you look at this from an audio perspective, sensor noise is the same as signal to noise ratio in audio!. It's a ratio and the manufacturers decide what is acceptable for their product!. Light or image spilling over to another sensor pixel would be like sound spilling over between channels, it's not really noise!. I've seen this explanation for small sensors with large numbers of pixels before and it doesn't make sense!. The more pixels recording light from any part of an image should (there are design limits) be a good thing!. I would guess this explanation came from sensor noise, as in electronic noise, between sensor pixels!. That electromagnetic induced noise would be another animal altogether!.Www@QuestionHome@Com

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)
- Alan?N!.?Marcus

steveWww@QuestionHome@Com

The fringing is most often known as chromatic aberration!. As I understand the phenominon, it occurs as a result of light striking the surface of the image sensor at different angles, instead of being collimated (or straight)!. This results in a sort of optical illusion or shift of one image superimposed onto another and is most often seen at the edge of objects that are set against a bright backround!. The sharp contrast between the bright and dark area makes this more visible than at any other tonal area!. This is an inherent property of smaller image sensors, where there exists a higher pixel density requiring the light that strikes it to be more collimated!. It's often seen at larger apertures!. So yes, I would say it's a problem with both point and shoot cameras as well as DSLR's with APS-C or DX image sensors (my nikon D80 gets them, especially with the 18-135mm kit lens)!. From what I have read, it doesn't seem to be much of a problem with full frame image sensors, but there still exists differences between the surface of a digital image sensor and the surface of film!. So optics will play a major role in how much aberration is inherent to a particular camera!. That is why there has been so much development and research into optics that have been optimized for digital as a way to minimize the various artifacts and aberrations found to be inherent in digital photography, through the use of coatings and more aspherical hybrid lens elements!.Www@QuestionHome@Com

the fringing you are speaking of is the resulkt of the quality of the lens!. regular glass (low quality) as opposed to lead crystal is one culprit, a really good lens is made from quartz, in some cases a lexan lens or plastic can cause the purple fringing and an example of this can be seen if you take a sheet of lexan, hold it in your hands as if you are looking across it, you'll see the purple!. a good lens as in "quartz will have very thin fringing, almost un-noticable around bright objects in the photo and it will be blue, fading to yellow, that's the difference between a good and a cheap lensWww@QuestionHome@Com

That is an artifact of the lens!. The more elements that you try to put into a lens the more problems like this and others that you will get!. The more of a zoom that you try to get the more elements that you will need!. Theres a real science behind it!.

But it's the lens and SLRs are not immune to the problem!.Www@QuestionHome@Com

MixedMojo and others have answered the question admirably!.

I'd just like to add that you can also do it by sewing little pieces of purple wool on your photo vest!.

With matching handbag and shoes, it looks quite fetching!.

VWww@QuestionHome@Com

From the research I have done on this issue I begin to wonder if Mixed Mojo wrote the books I've read!. He nailed all the important points!. I notice it most using full zoom and large apertures on dark objects with light backgrounds e!.g!. branch with sky behind!.Www@QuestionHome@Com

Point and shoot cameras don't have very good lenses!. This is probably chromatic aberration due to poor lens quality!.Www@QuestionHome@Com

It also might be smudging from hand grease on the lens!. Purple fringing, I have not seen before without some form of fogging of the film either through a light leak or out of date/ improperly stored film!.Www@QuestionHome@Com