The size of the rear element is not what limits maximum lens speed for a given focal length, It's the size of the the front element. For an F1.0 lens the diameter of the front element Must always be at least the focal length in mm as an absolute minimum, and it's usually larger. jco -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Miserere Sent: Tuesday, May 26, 2009 12:26 PM To: Pentax-Discuss Mail List Subject: Re: Ricehigh is at it again
I will copy'n'paste what I wrote on PF: Regarding an f/1.0 lens from Pentax: The 50mm f/1.4 has a rear element ~29mm in diameter, giving it an area of 660.5 sq.mm. The 50mm f/1.2 has a rear element diameter of ~35.5mm, giving it an area of 989.8 sq.mm. These numbers make sense: f/1.2 is 1/2 stop faster than f/1.4, so the area of its rear element should be 1.5 times larger; and it is. A 50mm f/1.0 lens would have a rear element twice the area of the f/1.4, which would mean its diameter would have to be square root of 2 larger. That gives us ~41mm. Take any K mount lens and measure that width; you'll see that the aperture lever would have to be removed in order to have a rear element that large and the glass would be very near the edge of the mount. End of paste. Anyone here who knows more about optical lens designs care to chime in? Would a smaller rear element do the job if the image circle were for DA? Not that it matters because the "leak" is DFA 50mm. If this roadmap is for real, it means Pentax is releasing a FF camera, which is not something I have faith will happen within the next 3 years. But like I often say: this *is* Pentax we're talking about--not making sense is what they do best. They also build some decent lenses. :-) Oh, and sign me up for a 50mm f/1 if it's under $1,000. DA 135mm F2.8 Limited? It's about frickin time! But how about a DA* 135mm f/2 on top of that? DFA* 20mm F2.8 SDM? What's 'star' about this lens? I have an FA 20mm f/2.8, nice and small, good IQ. What would make it a 'star' is if it were f/2. Anyway, this is all vapourware until Pentax tells us for real what's going on. But I will be VERY impressed if they manage to release the K-7, the 645D + DFA 645 lenses, and a FF + DFA lenses all within the same year. Forget impressed, I'd be bloody scared the end of the World is nigh! Off to buy a few hundred cans of tuna and spam. Just in case. --M. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
