> On Jul 19, 2006, at 10:19 AM, P. J. Alling wrote: >>> Perhaps another reason that the DA Limiteds are being designed for >>> compactness and quality rather than speed is that the DSLRs they are >>> designed to work with produce so much cleaner results at ISO 400 to >>> 1600 than the film SLRs that the prior series was designed to >>> complement. >> >> That's an excellent point Godfrey. >> > A large part of a wider maximum aperture is easier focusing, whether > autofocus or manual an F2 lens will lock a lot faster than an > F3.5. My > SMC Takumar 35mm f3.5 gives excellent results on film and digital but > it's a PITA to achieve precise focus, under anything other than the > brightest lighting conditions.
While brightness is very helpful for focusing, I don't think I can easily see a 0.3 to 1.0 stop difference in the viewfinder brightness without comparing. Contrast at the focusing aperture is actually more significant ... For instance, I found the A24/2.8 to be easier to focus accurately than the A28/2.8 due to its higher contrast when wide open, and the FA20-35/4 to be better than either for the same reason, despite being 1 stop slower. This is just as true for auto focus as it is for manual focus, btw. > That probably is their reasoning. Still, fast lenses are important > even when you're shooting stopped down (as I almost always do) because > of the brighter viewfinder. Several times while doing pre-dawn > photography I've switched from my 20/2.8 to a faster lens simply > because I couldn't see how my shots were framed with the slow 2.8 > aperture. I'm sorry, but I just can't believe that you cannot see how a shot is framed with an f/2.8 maximum aperture vs an f/2 or f/1.8, Mark. What lens faster than the 20/2.8 have you gone to with similar FoV? My testing of the Sigma 20/1.8 showed it to have poorer wide-open resolution and contrast than the Canon EF20/2.8. Neither were at their best until f/4.5 or smaller (the Sigma caught up to the Canon at about f/5.6). The Pentax FA20/2.8 is better than the Canon wide open and proves sharper until f/5.6 from the short time I had evaluating one. Manually focusing *any* very short focal length lens like the 20mm or 14mm is challenging, and often times I find the AF system does a little better than I do if I'm in a hurry. When I have the time to work more slowly, the Pentax Magnifier FB 2x helps a lot for critical work as does the Olympus VariMagni's 1.5-2.5x magnification. However, the Katz Eye Optics screen I purchased significantly improves the manual focusing 'tooth' over the standard Pentax screens in the DS so that I find less need to use the optical magnification aids. Godfrey -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net
