Re: [Leica] thambar questions

["Dan Post" <dwpost@email.msn.com>]

Henning-
Thanks! I stand corrected, and I blush at responding with 'hearsay'
Now- if I do use that Avery sticker... could I at least put it on a UV
filter?  :o}~

Now- my next question- since I haven't had trig since college 30 years ago,
but is the Sin 4th law the same principle as what I learned as the Cosine
law- that light falls off as a function of the cosine of the angle from the
optical axis- as the angle increases, the light diminishes by a factor of
the cosine of that angle?

Dan
- -----Original Message-----
From: Henning J. Wulff <henningw@archiphoto.com>
To: leica-users@mejac.palo-alto.ca.us <leica-users@mejac.palo-alto.ca.us>
Date: Sunday, May 30, 1999 10:22 PM
Subject: Re: [Leica] thambar questions


>>Hi!
>>I am sure Marc, our LTM Guru, will set me straight if I am wrong, but I
>>thought the central spot was to minimize the light falloff at the edges
when
>>the lens was used wide open- when it was at its softest. The couple of
>>copies I have seen were in pretty miserable condition, too; it was as if
>>they were not appreciated for their rarity before most had been neglected
>>and ill treated.
>>Me, I'll stick with what I can afford!- the Good Ole Uncoated Summar for
>>softness!  :o}~
>>Dan
>
>I won't try to speak for Marc, but the correct answer to this is as posted
>before: the central disk cover up the center of the lens so that the
>central rays (which image with the least spherical aberration) are deleted,
>making the overall effect even softer, especially at apertures slightly
>stopped down from f/2.2. You can try this by sticking a black Avery dot on
>the center of your Summar. Just make sure you compensate by exposing a bit
>longer. The effect will be the same. BTW, the Thambar had a second aperture
>scale to use when the central spot filter was used to compensate for the
>loss of light the spot caused.
>
>As for your point, that this is to minimize light falloff; this method
>wouldn't work, as it would actually increase light falloff. The central
>rays which the disk deletes only have falloff due to the sin 4th law,
>whereas the images produced with the outer rays have falloff due to the sin
>4th law as well as falloff due to the mechanical (mount) vignetting.
>
>   *            Henning J. Wulff
>  /|\      Wulff Photography & Design
> /###\   mailto:henningw@archiphoto.com
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