Just a few thoughts on Photodo ratings and MTF. Zoom lens vary MTF performance with focal lenght. Lens such as the FA28/70 F4 are quite poor at the 28mm end and good at the 40-70 end. Tamrons 28-200 is good at the 28-85 range and then seems rather poor at the 200 end. The average rating is rather misleading in this aspect. I normally read off the 40 lpmm curve at 12mm as a indication. A wide spread of tangential and radial numbers is a pointer to colour fringing problems. I visualize this as a 1/80mm circle being resolved as a fuzzy elipse with major and minor number inverse propertional to the MTF percentages. I have been looking at the 28mm lenght of various lens via data from photodo as a prelude to checking some of my own lens where I have duplication between prime and zoom lens. At F8 and 12mm from the centre these are the numbers for 40lpmm, I like to print onto 12x16" paper so hence the intrest in the 40lpmm. Here are the numbers from Photodo.com F28/2.8 52/20 FA28/2.8 50/30 FA20-35/4 64/32 FA28-70/4 40/8 FA28-200 38/25 T*rom 28-200 62/45 T*ron 20-40 70/40 FA28-80/3.5-5.6 40/15 C*tax G 28/2.8 72/56 C*tax T* 28/2.8 59/41 L*ca M28/2.8 66/45 I will be intrested to see how the new limited 31mm lens compares. Colour fringing is a bigger problem for digital camera's CCD so presumably it's radial and tangential curves will be more like the 43 limited's. ===== Regards Richard Saunders __________________________________________________ Do You Yahoo!? Yahoo! Auctions - Buy the things you want at great prices. http://auctions.yahoo.com/ - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org.
