On 1/21/2011 9:49 AM, Carlos J. Santisteban wrote:
> In fact, the Planar / double Gauss formula is somewhat earlier, from the late
> 1800s... but without anti-reflection coatings, so many air-to-glass surfaces
> made it unfeasible because of flare.
And the cause of the flare, reflections, also reduces the light getting through
the lens. Reflective loss at an
air-glass interface starts at 4% for crown glass and goes up from there. That's
also why designers made compromises to
get matching curvatures to make cemented elements.
Six individual elements means 12 air-glass interfaces. With a mix of crown and
flint glass, that's a loss of say 5% x 12
= 60%, a whole stop. F/4 becomes t/5.6
> This was the reason behind the Tessar and Heliars -- a development of the
> classic triplet instead of a high performance design from the scratch, in
> order to keep the three-group configuration for good flare resistance.
Three elements, two cemented together, only one high index, would mean light
loss less than 1/2 stop., maybe only 1/3.
Elemental Moose
PS: I just ran spel chuck, and it suggested I replace Santisteban with
Obscurantist.
--
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