Color Aberration

CP950 Chromatic Aberration

The CP950 does suffer from mild chromatic aberration. This has been called "color fringing" in some discussions and reviews of the camera.  See my evidence below that this is really chromatic aberration.. There is another separate problem with the CP950 that people have called "Red Color Fringing", involving blooming or bleeding of red colors from red objects. That is a separate problem, and does not appear to be caused by chromatic aberration. I don't know what causes the red color fringing,

Chromatic aberration is the inability of a lens to focus all colors to the same point. This is because red light is bent less by it's passage through glass than blue light is (the same effect causes a prism to separate white light into colors). Lens designers attempt to minimize chromatic aberration, but there are always tradeoffs, and all lens systems have some.  The effect is always worse the more curved the surfaces are and is usually worse towards the edge of the field on a photo. (Here is a link to a site about it)

To test the CP950, I made a paper target. I took black paper, and placed some white dots on it at measured distances from the center.

Below is a shot of my home made target..

 

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Snapshot of target with CP950, wide angle setting. I purposely over-exposed the white dots to maximize the abberation effect. Reduced from the original, you can't see the aberration at this scale.

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Blowup of  the dot 15 cm from the center of the target, lower left (third dot from the center on the photo of the target)

15 cm upper right dot.

Observations:

All of the dots near the edge showed the effect. The dots all had their green aberration towards the center of the target, blue/red away from the center. The central dots showed no aberration.  The pattern was radial, the amount of aberration just depended on the distance from the center of the target.

Below is a graph showing a plot of distance from the center (X-axis) against the size of the blue aberration (measured in arbitrary units, I held a ruler up to the screen and took a good guess) on the Y-axis.

As you can see, the amount of aberration depends on the distance from the center, as expected for classic chromatic aberration. The line is not a perfect curve because my measurement of where the color started and stopped was subjective.

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Hmmm. Where is my copy of Excel?

Conclusion:

Some of the "color fringing" observed by many people on some CP950 photos is classical chromatic aberration, a common lens attribute.

Discussion:

I'm going to expand on this, I just wanted to get something up.  A couple of points... the CP950 aberration is worse at the wide angle setting, and is worse at wider lens opening, as is typical for the effect. It is worse than that seen in the CP900. Don't get the impression though that I don't like the camera! I just like to take things to the limit. If you want to test for aberration yourself, use some overexposed white objects or lights. Use a multi-spectral light. Mercury and Sodium vapor lamps will not work because they are monochromatic.

Red Color Fringing

There is another issue involving bleeding of red colors around red objects and objects with a lot of red in the color. I don't think that that is caused by chromatic aberration because it does not appear to exhibit the radial from center pattern. It appears to have something to do with how the camera interpolates color or compresses images. 

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Real Life Example:

To the left is a real life example showing chromatic aberration sent in by an anonymous reader. This was taken by the 950 at the wide angle setting.  There are several interesting things about this picture, first, you can see (better on the original, see the full size crop below.) how the aberration on the bright sky showing through the holes is worse near the edges. You can also see that the aberration is only visible on those bright spots that have a dark background; the ones on the roof show it, but bright spots on the beams right next to those do not because they have a bright background. You can also see that the purple fringe is always away from the center of the picture. ( didn't take this shot, it was sent to me)

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(Above) full size crop of the worst area near the top.

Q&A:

Why don't I see this on all of my shots; what makes it worse? You ask...

* The effect is worse at wider angle.

* The effect is worse at lower f numbers. (wider opening for the lens). You need to examine the particular shot.

* The effect is worse near the edges of the field. You need a bright object on a dark background to see it.

* The effect is only noticable when you have a sharp lighting contrast; a white object on a black background, for example or objects that contain opposite colors that are involved.. Abberation does not involve ALL of the light in question, just some of it. This means that you usually need a dark background to see the effect. On two brighter colored objects, a little of the colors involved will be mixed, but the tendencey is just to blur the image and your eye can't see the actual colors. This is the most important condition for seeing the effect.

* The colors of objects can make a difference. Objects that don't have the colors in them that are subject to aberration don't show the effect. Likewise, you can't see the violet fringe against a violet object.

* The color of the light can make a difference. Some lights that have pure spectral colors (a laser pointer or mercury vapor light are examples) will not show the effect because they don't contain the violet and green colors that are affected. Or, if they do, then the entire image is shifted, and you can't notice it.

Isn't this just Infra Red light or something?

* If it were, the effect would be present on the entire image, not just near the edges. And the zoom level and f stop should not have anything to do with it.. Also, a person who has a hot mirror filter has informed me that the effect is still present, even though the filter blocks IR light.. Also, the near IR light that the camera is sensitive to looks blue-white (take a picture of your remote control), not green and purple

No, I think that it is actually Ultra Violet light.

* Again, the same arguement. I don't think that the camera is very sensitive in the UV either. Plus you can see the aberration effect just fine using incandecent lights which have very little UV component.

I heard this was really some kind of CCD effect, bleeding of the colors due to the way color was interpolated, or bleeding of charge beteen cells on the CCD or.... (pick your own theory here).

* Well, you never know, but your theory has to account for the facts.  The main fact that argues against this is the way that the effect is worse near the edges. If it were a color interpolation effect, or something to do with the CCD, wouldn't it be the same all over? And how would you account for the radially symmertic pattern? And why would the degree change with the zoom level? How does the CCD "know" what zoom level you are using, the light hits the CCD at the same angle. There IS color bleeding due to interpolation, but it is a different effect.

No, I heard that this  is really a firmware bug.

I think that the same arguement applies. Plus, since so many other cameras have this effect too, you would have to say that they all share the same firmware.

Why doesn't Nikon fix this with a firmware update?

* They can't change the lens with firmware. It is unlikely that Nikon will ever admit that there is anything wrong, since nothing can really be done about it, and the aberration is no worse that the typical camera in it's class. Some cameras show this effect even worse than the Nikon, some are better.

 

 

 

Perspective/Opinion:

   It is very difficult to design a lens system that has perfect color correction. There is always a tradeoff between field flatness and color correction; you could have excellent color correction, but then can't have a flat field. Having made several telescopes, I know a little bit about it. The more lens elements that are involved, and the more moving lens elements there are just make the problem worse. Just ask any "pro" photographer what his sharpest lens is.S/he might not know much about lens design, but they will tell you that it will be a single focal length lens (no zoom) and he will also tell you that it is sharpest when stopped down a little (smaller aperture). He will also tell you that to get the "sharpest" pictures you should shoot in B&W with a color filter over the lens (that way there aren't more colors to show the aberration effect) They test for these effects when they review lenses in the big photo magazines. The worst lenses are the combination zoom lenses, and the lenses are worst when wide open.

   The Nikon 950 has a lot of lens elements. Customers wanted wide angle, zoom, as well as a super close macro that is closer than any other camera, this takes a lot of lens elements, several of which must move. Some color aberration can't be helped without giving up something in this situation. If the camera would still sell without the extra lens elements needed for macro, then a better corrected lens could have been designed. The other factor is cost. By using different and more expensive types of glass, and aspherical surfaces, you can have better correction, but at greater price. The fact that customers want a 3X zoom lens and a small camera at the same time means that the lens system has to be physically small to get the focal length needed for that much zoom, and this increases the cost also.

   If you really care about good color correction, then a different camera might be in order. The Canon Pro 70 has good correction, for example, but at the price of smaller zoom and poor macro ablity. A digital camera that would let you use interchangable lenses would be a good solution, but you will have to spend a lot more money. Using the good lens, the new Nikon D1 can produce shots with virtually no color aberration, for example, but that will run you at least $6000.