Originally posted by chals I recently bought a Pentax K5 with some lenses, including tha famous 31mm ltd. I bought this lens for its wide-open capablities to go withe the smooth bokeh. I understand that it cant be as sharp at 1.8 as on 2.8. But, to me it looks like something is wrong with my copy of this lens. Look at the transition from white to darker areas, it is kind of leaking white into the darker areas. And the sharpness is not great either. The picture is taken on 15-20 meter distance. If taking pictures at a closer range, the result is the same, ie, the number "3d" engraved in my Pentax 15mm leaks onto the black lensbarrel in the same way.Can someone who knows this lens help me with this? You can fin a picture here:
Zenfolio | Carlo Fals | div 2 The f1.8 image appears a bit out of focus, but that is just an aside. The "bleeding" that you are seeing on the f2.8 image is CA and can be easily fixed in post processing. A lot of lenses have CA, and can be taken out when the lens is designed, however at the expense of something else within the design. So the lens designers tend to leave some of the CA in since it is so easily corrected for, and thus do not have to compromise in some other area of the lens' optics.
CA is Chromatic Aberrations (CAs). Here are some links, explaining what it is, why it occurs and how to correct (or fix) it on your images.
You can also, look up in your post processing tool - Chromatic Aberrations (CAs), and it will provide specific directions on how to repair it. Or you can google you tool with Chromatic Aberrations (CAs) and probably find additional instructions.
Also, if you scroll down in the first link you will see the MTF (resolution) section. f4 is the aperture for the highest resolution for the lens in the center and f5.6 for across the full lens (center and boarders). Scrolling down to the Chromatic Aberrations (CAs) section you will see that f5.6 provides the best aperture in terms of minimizing Chromatic Aberrations.
hope that helps ....