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Re: [OM] Heliar (Was: 50mm 1.8 (Re: Hoarding & Varimagni))

Subject: Re: [OM] Heliar (Was: 50mm 1.8 (Re: Hoarding & Varimagni))
From: Moose <olymoose@xxxxxxxxx>
Date: Sun, 23 Jan 2011 20:54:39 -0800
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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