Don Sanderson wrote:
Maybe someone can enlighten me.
Some zoom lenses are physically longest at
their longest focal length, such as the FA24-90.
Others, like the FA28-70/4 AL are physically
shortest at their longest FL.
Does anyone know why this is, and what
advantage or disadvantage each design has?
I get along much better with short physical length
lenses simply because a bit of movement at the
camera isn't amplified so much at the end of the
lens.
Just curious.
Don
I used to own a 28-70 f/4, and it was not shortest at its longest focal
length. It was shortest at approximately 45-50mm. At 28mm it was at
its longest. And at 70mm, it was *almost* as long as at 28mm. I don't
understand the math behind this sort of telephoto lens, but I know on
the 28-70, the rear element moves as you zoom too. Clearly this is an
important part of the optical formula. And truthfully the front element
of the lens doesn't really move *that* much as you go from 28 to 50 to
70... 1/2 of an inch at the most.
The SMC Pentax-DA 16-45mm f/4 AL is another oddity. It is truly at its
longest at 16mm. At 45mm, it's at its shortest. And the difference is
probably greater than 1.5 inches (maybe 2 even, I don't have it handy to
measure right now). I've heard that referred to as a reverse zoom.
Whatever it is, it takes a little getting used to.
Dave
