Hello and welcome back. Today’s post is the end of a trilogy about lens distortions. I hope you have found the previous posts about barrel and pincushion distortion helpful.
Notice the purple fringe along the edges of the leaves
Chromatic Aberrations, what nice big words they are? You may have seen this topic referred to as CA or purple fringe. It haunts our images by appearing on our subject’s edges in high contrast areas. Sometimes, you can barely see it, and other times it just screams at you, distracting you from the subject matter. Look for it, not exclusively, on reflective objects (chrome bumpers or hood ornaments) , the outline of building tops, or the edges of leaves and branches. See example.
CA occurs when the colors of light fail to meet at the same point on the focal plane and/or by magnifying certain colors differently. The result is basically a softness of the object’s edge where a particular color did not focus properly.
Now there are two types of CA, Axial (Longitudinal) and Transverse (Lateral). If I didn’t just lose your attention. I’m not going into too much detail. I’m just going to inform you on how to spot the differences and how to minimize or eliminate.
Axial Chromatic Aberration: See example below. Basically, the colors of light meet at different points on the optical axis (dotted line). Imagine a perpendicular line (focal plane) crossing where the two green lines (light rays) meet. The red and blue rays do not meet at the plane and become out of focus.
How do I identify Axial CA ?
Due to modern advances in lens manufacturing, axial CA is rare; but if it happens, it occurs anywhere throughout the image frame and it’s usually a purple or magenta color .
How do I fix this?
In camera, you can minimize axial CA by increasing your depth of field by closing down your aperture the appropriate numbers of stops until it’s gone. This allows those out of focus colors to fall into the acceptable focus range. Also, a number of camera brands can automatically process out the CA, just check your manual . Last but not least, there is always a photoshop way. Just google it for a how-to.
Transverse Chromatic Aberration: See example below. This is the most common type of CA. This happens when some colors get magnified and do not line up perfectly.
How do I identify Transverse CA?
See example on the right. Within the image, objects with transverse CA will usually have purple or magenta/green fringing and the shape of the object may be slightly off. Transverse CA never occurs in the center of the frame and it gets worse towards the edges of the photo.
How do I fix this?
Adjusting your aperture will have no affect, though slightly defocusing can help. Of course, there is also software like photoshop, that can repair this issue.
And, as always what some may find as a negative, you may see CA differently and use it creatively in your favor.
I hope you got something out of this Techie Tuesday, join me every Tuesday and please feel free to make a comment or ask any questions
Thank You,
Peter
Peter Pechacek is a photographer and filmmaker in Orlando, Florida. He is new to the blog and will be contributing weekly on Techie Tuesday
