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
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