Sigma 30mm f1.4 ready to explode?

Started Feb 3, 2012 | Discussions thread
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Joseph S Wisniewski
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Sigma 30mm f1.4 ready to explode?
Feb 3, 2012

I lost a lot of respect for Sigma today...

I was testing some fast lenses on a polariscope today. (I'll explain why in a minute).

I was amazed how good the results I was seeing. The Nikon 50mm f1.4 and the Voigtlander 58mm f1.4 were both essentially perfect, the entire lens went cleanly black at a neutral angle. They are what a technical person would call "strain free". you can run polarized light through the lenses and they won't alter the polarization. You do that in microscopy and sometimes in very serious macro photography.

The Nikon and Voigtlander f1.4 primes actually looked better than my Nikon 0.85NA DIC condenser, a precision piece of microscope optics built specifically for polarized observation, that is supposed to be strain free. That has variations in its response to polarized light, some parts of it are darker than others, and the dark parts and light parts swap when you adjust the polariscope (calibrated crossed polarizers) 1.5 degrees.

My Nikon Achr condenser varies much more, 5 degrees, which makes it unacceptable for polarized light use.

But I never saw anything like the Sigma 30mm f1.4 before. A strong cloverleaf pattern, with 12 degrees of rotation to go from one part to another being as dark as possible. At least one of the lens elements is very badly made, and I'm betting on the much ballyhooed "world's largest molded aspheric element".

You just don't see strain like that in lenses. You do see it, however, in art glass. Based on my experience in that area, I now regard the 30mm f1.4 as a "ticking bomb". A decent thermal shock, and it could blow. And I do mean blow. I've seen poorly annealed glass explode before, scattering shards of glass all over a room.

Heck, a crappy old UV Topcor I have, a relic of the late 50s, is strain free.

OK, so why was I doing this? I'm going to try a little polarized light photography on a macro setup where the lighting is going to be "cross polarized", polarizers adjusted to block as much light as possible, and only light that gets its polarization disturbed somehow will show up. That, and I want to do some similar work on the microscope and my strain-free condenser can't cover a 2.5x objective, and doesn't do all that well on 5x.

The polariscope is a thing of beauty, the rotating polarizer is marked off in degrees, and you can easily eyeball 1/2 degrees. It also has another scale that lets you read in sixths of a degree (10 minutes of arc). So, it really can do a polarization measurement to a very high precision. I was hoping that the Sigma, with a shorter focal length, might perform well as a condenser. Unfortunately, it not only fails as a condenser, it fails as an exercise in lens making.

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Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

Ciao! Joseph

http://www.swissarmyfork.com

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