The front element always needs to be at least as large as the diameter calculated by the formula Darren gave. The mechanical and optical problems in building a zoom lens frequently require a physical front element that is larger; the longer the zoom range, and the shorter the minimum focal length, the more likely you are to need extra light-gathering ability.
On Wed, Oct 02, 2013 at 06:24:31PM +0200, Alan C wrote: > Is it different for zooms? I have an M42 Polaris 55-300 f4.5 & the > filter size is 86mm. > > Alan > > -----Original Message----- From: Darren Addy > Sent: Wednesday, October 02, 2013 1:32 PM > To: Pentax-Discuss Mail List > Subject: Re: Abused lens images (not for the squeamish) > > 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 <[email protected]> wrote: > >On Oct 2, 2013, at 3:18 AM, Darren Addy <[email protected]> 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 > >[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. > > > > -- > Nothing is sure but death and Pentaxes. > > -- > 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. > > > -- > 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. -- 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.
