The thread about viewfinder image brightness brought up something I
realized I'm unclear on: that is prime lens vs constant aperture zoom
vs variable aperture zoom. Since aperture is an expression of
fraction of focal length, "constant" aperture zooms are really
"constant ratio" zooms, aren't they? The actual aperture size must
change with changing focal length, mustn't it? Could that in any way
explain the difference in viewfinder brightness asked about between a
prime and a zoom?
In a variable aperture zoom, if you dial in max aperture, and then
zoom out, the aperture increases in steps, i.e 4, 4.5 5.6., and each
step remains constant for a certain percentage of the zoom range.
This obviously means that you aren't getting precise f-stops
throughout the range, which is probably part of the lesser cost of
such lenses. In a "constant aperture" zoom how is this avoided? Or
is it? Is there actually a continuous range of aperture available?
I'm sure someone will straighten me out if this is really
misconceived!
Ken Durling
Website http://home.earthlink.net/~kdurling/
Alternate e-mail: [EMAIL PROTECTED]
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