Archived posting to the Leica Users Group, 1999/05/30
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>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 |[ ]| http://www.archiphoto.com