fish
Feb-05-2005, 08:59 PM
Many moons ago our favorite whipping boy, wxwax, complained about one of his P&S digicams having the dreaded purple fringing. At the time, I didn't own that same model of camera and didn't really think much of it, as I figured whatever chromatic aberrations (CA) were, they must be an artifact of the specific camera he was using.
As I continue along my journey of photographic enlightenment, and acquire more sophisticated equipment, I find that I'm more curious about the issues that digital cameras have over film cameras -- and there are quite a few. One of which, is CA. Granted, I didn't really worry about it, until a certain well-known and widely hated (:lol3) photog from NYC who shall remain nameless (andy) mentioned that one of my beloved images exhibited signs of CA. EEK! How can this be? Not only is the dSLR nothing like the camera that the waxinator was using, but for gawdsakes...it probably cost five times as much, including lens. So instead of coming off like I was completely misinformed, I simply responded to said NY photog, "yeah, I know...oh well." :uhoh
And so in order to come clean, I did some major research on CA and found a number of very interesting, but highly technical, websites that explain the phenomenon. Now I can say in complete confidence, that I not only know what CA is, but can also recognize it, and maybe even explain it to someone.
I've included some excellent resources at the bottom of this post.
Let's start with a simple definition of chromatic aberration: An optical aberration caused by a lens bringing different colors of light to a focus at different points.
Further, Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light.
Since the focal length f of a lens is dependent on n, it follows that different wavelengths of light will be focused to different positions. Chromatic aberration of a lens is seen as fringes of color around the image, because all colors cannot be focused at a single common position on the optical axis. However, there exists a point, called circle of least confusion, where this effect can be minimized.
http://coyfish.smugmug.com/photos/15408809-L.gif
Here's a classic example of CA (not my photo):
http://coyfish.smugmug.com/photos/15410087-L.jpg
What makes it worse?
* 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.
For further research into CA, I recommend the following resources:
http://www.olympusmicro.com/primer/java/aberrations/chromatic/
http://www.vanwalree.com/optics/chromatic.html
http://members.shaw.ca/quadibloc/science/opt0505.htm
http://www.kineticbooks.com/physics/17372/17403/sp.html
as you were. :1drink
As I continue along my journey of photographic enlightenment, and acquire more sophisticated equipment, I find that I'm more curious about the issues that digital cameras have over film cameras -- and there are quite a few. One of which, is CA. Granted, I didn't really worry about it, until a certain well-known and widely hated (:lol3) photog from NYC who shall remain nameless (andy) mentioned that one of my beloved images exhibited signs of CA. EEK! How can this be? Not only is the dSLR nothing like the camera that the waxinator was using, but for gawdsakes...it probably cost five times as much, including lens. So instead of coming off like I was completely misinformed, I simply responded to said NY photog, "yeah, I know...oh well." :uhoh
And so in order to come clean, I did some major research on CA and found a number of very interesting, but highly technical, websites that explain the phenomenon. Now I can say in complete confidence, that I not only know what CA is, but can also recognize it, and maybe even explain it to someone.
I've included some excellent resources at the bottom of this post.
Let's start with a simple definition of chromatic aberration: An optical aberration caused by a lens bringing different colors of light to a focus at different points.
Further, Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light.
Since the focal length f of a lens is dependent on n, it follows that different wavelengths of light will be focused to different positions. Chromatic aberration of a lens is seen as fringes of color around the image, because all colors cannot be focused at a single common position on the optical axis. However, there exists a point, called circle of least confusion, where this effect can be minimized.
http://coyfish.smugmug.com/photos/15408809-L.gif
Here's a classic example of CA (not my photo):
http://coyfish.smugmug.com/photos/15410087-L.jpg
What makes it worse?
* 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.
For further research into CA, I recommend the following resources:
http://www.olympusmicro.com/primer/java/aberrations/chromatic/
http://www.vanwalree.com/optics/chromatic.html
http://members.shaw.ca/quadibloc/science/opt0505.htm
http://www.kineticbooks.com/physics/17372/17403/sp.html
as you were. :1drink