On Mon, May 24, 1999 at 11:00:37AM -0600, Chuck Castleton wrote:
> Does increasing the diameter of the front element lead to a lower f
> number? Do the laws of optics rule out a lens with an aperture larger
> than f/1.0, or is it at least theoretically possible to build a 50 mm lens
> with a 100 mm filter ring and gather enough light to get to f/0.8 or
> something?
Basic definition: The f/number is the ratio of the apparent diameter of the
lens' light path as seen from the front to the focal length. Thus, a 50/1
lens has an apparent front diameter of 50mm (and a filter ring at least a
little bigger than that). There's been at least one production lens faster
than f/1, the 50/0.95 Canon they developed for their rangefinder cameras in
the late 50s (and, IIRC, made for their SLRs as well).
In practice, the larger diameter optics require tighter tolerances, and more
precise correction for spherical aberration (or, alternatively, a$pherical
element$), and get much heavier due to the increased thickness of the glass
needed, and are generally a pain to manufacture and use. Nikon ran into this
problem particularly badly with their 50/1.1 rangefinder lens: they had to
add an external lens mount to their cameras to handle the load after
discovering that the standard mount just didn't hold up.
Perhaps, with modern materials and CNC manufacturing and CAD technology,
fast glass can be made to perform as well as the 50/1.8s we're all used to
at a cost that won't break the banking system - but I wouldn't hold my
breath waiting for it. The days of fast glass as the sine qua non of optical
design have passed.
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