Could We Lose The Hand Ground Aspheric Myth?

[stephen_w.]

How did this ever start?

 

http://www.asphericon.de/index.php/3d9c19a41a56dc8233b9c756fa17bf01/2/5

[blakley]

The story is that the 50/1.2 Noctilux's two aspheric surfaces were hand-ground, because no machinery existed to grind aspherics in 1966 when the lens went into production. The cost of grinding these elements is usually cited as one of the reasons the lens was discontinued.

[x-ray]

I owned one of the 1.2 noctilux's for four or five years back in the 70's. While a good lens at f 1.2-4 it suffered below that. I understand one of the negatives was the variation from sample to sample. Hand ground aspherics was the reason for the variation.

[jerry_pfile3]

I had the opportunity to use my Uncles' 1.2 back in the 80's. I thought it was a fine lens (but really heavy).

 

That said, I probably never used at more than f/8 but that's not the range it was made for. From 1.2 to 5.6 it performed as one would expect, but wasn't as good as my 50DR from 2.0 through 5.6.

 

I wouldn't mind having one today though. Would probably make my heirs happy.

 

Jerry

[raymond_tai]

Sorry what myth is this? I know there was the ground and polish method versus the more recent blank molding method.

[vivek iyer]

Raymond nailed it.

[stephen_w.]

Seriously, how would one hand grind a lens element? Think about it. I imagine that one could mechanically grind one at a time on a jig, resulting in higher cost, rather than the dozens spherical specimens ground simultaneously, typically. Hand inspecting, yes, but all elements are/should be hand inspected at Leitz and Leica.

[vivek iyer]

You could get injection moulded plastic aspherics making it far cheaper.

 

Distinction should be made between precision ground (hand, machine, whatever means) and less precise mass produced goods.

[kent_tolley2]

I'm not sure I understand your question. Your last post wasn't coherent to me. But hand-grinding optical surfaces has been done for centuries. It's what amatuer telescope makers have been doing with front-surface mirrors since Newton. First you grind a spherical surface with decreasing grades of carborundum. At the final stage, using optical rouge, you turn the spheroid into a paraboloid. Grinding a lens is more difficult but uses the same principles. 20th century techniques use a grinding machine to get the spheroid and turn the spheroid into aspheroid by hand in the final "figuring" stage. The hand work is what makes them more expensive than a machine made aspheroid surface.

[terry_rory]

Hand grinding aspheric lenses 900 years ago...

 

http://www.frojel.com/Documents/Document03.html

[karl_keung]

read from Leica LFI that the latest 90 asph Summicron uses CNC sand blast polishing or

something like that to polish the asph lens surface, because the diameter is too large for

bold blank.

[vinay_patel]

"Seriously, how would one hand grind a lens element?"

 

Lenses were "invented" long, long before the machine age so it's obviously been done a great deal. With that said however, the f/1.2 Noctilux was done on a machine as was the type I 35mm ASPHERICAL. The former machinery was more dependent upon human operator input than the latter, insofar as I have been told by guys who ought to know (Leica lens design dept former employees). The current ASPH lenses are produced by the blankenpresse (sp?) method rather than grinding.

[vinay_patel]

I probably should have subsitutued "industrial age" for "machine age" above, my pedantic and obnoxiously well-informed colleague snooping over my shoulder has just informed me :-)

[arthuryeo]

Wasn't there an issue with size (radius) limitation for the molded technique and that's why CNC-driven grinding is still used for the large radius asphericals elements?

[charles_stobbs3]

When I was a kid my father bought a telescope making kit that involved cementing a wooden block on top of a 55 gal. drum, attaching a flat pyrex disc on top of that and spending hours (days) walking around that using various grades of carborundum and a convex shaped tool and some prescribed stroke to arrive at a 6" lens. Unfortunately the almost lens cracked near the final stage.

[35mmdelux]

hand-grinding cannot achieve tolerances required of true aspherics and it is ridiculous to think so.

[roger_michel]

thank you paul!

[kelly_flanigan1]

Re<i>hand-grinding cannot achieve tolerances required of true aspherics and it is ridiculous to think so.</i><BR><BR>Grinding naturally makes a spherical surface; this has been known for over a thousand years. With fast lenses and, and mirrors; a spherical surface is not the answer; this has been known for 500 years. <BR><BR>Diffraction limited aspherical mirrors have been made for telescopes for many hundreds of years by hand.<BR><BR>Your ridiculous statement just means YOU cannot make a fine diffraction limited item. Its a sad day to see such lack of history, and such negativity.<BR><BR>NON molded aspherical surfaces require ways to make them that are not the regular ancient orbital lens grind & polishers. Many times one grinds to the closest sphere, and the outer radii of the surface are locally modified. <BR><BR>A lens can be chucked up; spun and the outer radii locally buffed down. If one has to invest a huge amount of hand labor to do this; is it "hand ground"; or "you are commanding the bastard curve" with hand adjustments to the machine?<BR><BR>One may be checking the lens against a known flat or reference curve, so the the statement about not being as accurate is pure BS and bunk. You are "closing the loop" OF an aspheric around a reference; whether if by a modern apsheric grind machine, or a hand polish, or a combo of the two. <BR><BR>Making a non molded glass aspheric is expensive, its a non natural curve. <BR><BR>

[kent_tolley2]

I am surprised such a technical subject can generate the apparent contempt and hubris of the original post and Paul Angulo's reply. Maybe the reader is supposed to be convinced of their familiarity with the subject by that tone but it's ironic when it turns out their knowledge on this subject is in fact lacking.

[kent_tolley2]

I'm not sure what you mean by <i>true aspheric</i> Paul. I wonder if you know what you mean. Apsheric merely means not spherical.

[kent_tolley2]

<strike>Apsheric</strike> aspheric