Why is chromatic aberration more evident for some positions of aperture?

Question

for Lens and Mirror Optics

Answer 1

The angle at which the light is refracted is more acute at the front element near the outer edge(big aperture) than near the center(small aperture). This angle of refraction affects the ability of the lens to focus all colors to the same point at the same time. Color fringing is another matter... Below is a good discussion of lens design.

http://www.usa.canon.com/consumer/controller?act=GlossaryAct&fcategoryid=216

Answer 2

Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (the dispersion of the lens). The term "purple fringing" is commonly used in photography, although not all purple fringing can be attributed to chromatic aberration.
Longitudinal and lateral chromatic aberration of a lens is seen as "fringes" of color around the image, because each color in the optical spectrum cannot be focused at a single common point on the optical axis.

Answer 3

All lenses have a sweet spot in terms of aperture. Most of the time its between f5 to f11. Outside this range, you are more prone to taking pictures of less quality (or in your case, chromatic aberration becomes more evdient).

But this is a lens construction problem and more of a problem of older lenses. The new lenses have elements which minimize (if not eliminate) chromatic aberrations. Again, there is a sweetspot for apertures, but if you have newer lenses, this should be no problem.