On Thu, Jan 31, 2002 at 01:34:36PM -0700, Daniel J. Mitchell wrote:
> > Cons are that it almost always has a fixed apeture
> > (usually at f8 or so).
>
> Why is this the case? I've only ever seen mirror lenses at f8, but what is
> it about the design that stops them putting aperture blades in there?
What would be the gain? - The depth of field increase would be maringal.
A apperture in a mirror lens would only make the doughnut-shaped
circle of confusion to become lighter, but the diameter would be
about the same, so there is not more DOF. To control the light a ND-filter
just does the same.
>
> Also, how awkward _is_ this? f8's a reasonable aperture, I guess, but do
> you end up needing a lot of ND filters or something in practise?
What is the speed of the film? Remember the sunny F-16 rule. This is the
usual brightness of sunlit scenes. There are not too much cases, when you`ve to
deal with a lot more light. (double amount = a lot more, but just one stop!)
100 ASA -> ~ F16+1/125s -> F8+1/500s
400 ASA -> ~ F16+1/500s -> F8+1/2000s
I´ve no experience with mirror lenses. But I´d guess one ND-2 filter would help
in most cases.
Frieder Faig
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