Yes. The effective maximum aperture of a system is a function of the diameter of the objective. The formula is D=F/f where D = the objective diameter F = focal length f= the maximum aperture Change any one variable in the formula and another must change.
So if you make the objective larger the maximum aperture will be greater. For the F* 300mm f4.5, 300/4.5= 66.666... so it should not surprise us that the filter size is 67mm. The formula above is for telescopes, which normally only have an objective lens grouping, while telephoto lenses have multiple lens groupings, but that formula will get you close. The 300mm f4 will give you an answer of 75mm for the objective diameter (77mm filter size) while a 300mm f2.8 will result in a 107mm diameter objective (meaning that any filters are ususally put at the rear). On Tue, Oct 1, 2013 at 11:43 PM, David Mann <dmann...@gmail.com> wrote: > On Oct 2, 2013, at 3:18 AM, Darren Addy <pixelsmi...@gmail.com> wrote: > >> The introduction of IF allowed Pentax to use a smaller >> diameter front lens group (the F* 300mm f4.5 is 67mm filter size >> instead of the M* & A* 77mm size) > > It also helps that the F* is half a stop slower than the M*/A* lenses :) > > Cheers, > Dave > > > -- > PDML Pentax-Discuss Mail List > PDML@pdml.net > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. -- Nothing is sure but death and Pentaxes. -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
