|
Forum: Pentax SLR Lens Discussion
07-16-2010, 07:36 AM
|
| |
Purple fringing is longitudinal chromatic aberration, and would only be absent on a 'normal' (non-apo, reasonable apertures) lens with perfectly corrected coma and spherical aberration, and even then only within the plane of focus - this is well established even in old optics bibles such as Optics by Eugene Hecht. The use of low dispersion glass minimises longitudinal chromatic aberration. Purple fringing only occurs in front of the plane of perfect focus (the area can appear in focus but is in front of the flat plane of perfect chromatic convergence).
Lateral CA is not purple fringing. Lateral CA is almost always yellow/blue or red/cyan in those pairs on opposite sides of high contrast transitions and can occur within the plane of focus with non-APO lenses.
Purple fringing is less visible on film, but is still present, I have observed it even with very good lenses (Leica 50mm Summilux E60) on film at larger apertures.
The Voigtlander 180mm APO-Lanthar and Coast Optics 60/4 do not display purple fringing at the magnifications of current sensors, ever.
|
|
Forum: Pentax SLR Lens Discussion
01-25-2009, 04:32 PM
|
| |
There is also the Tokina 17mm available as the RMC version (manual focus) and ATX-Pro version (auto focus). I've used the auto one and it's great stopped down, but I'd imagine the best you're going to find is the Pentax DA 14mm at this stage.
|
