Aspherical element: CNC polishing

[arthuryeo]

<p>There are a few ways to manufacture aspherical elements for lenses:</p>

<ol>

<li>blank press: heat a blank piece of glass and press it to the aspherical shape you want using a mold</li>

<li>hybrid method: do it on plastics and cement them to the glass</li>

<li>CNC polishing: using computer-guided polishing</li>

</ol>

<p>[1] and [2] can be done for small aspheres and most manufacturers use [3] when it comes to large diameter aspherical elements. Furthermore, [3] is costly if your tolerances are strict and mass-produced lenses are designed in such as way as to avoid using large aspheres these days.<br>

A fine example of a Nikkor using [3] is the discontinued Nikkor 28mm/1.4 AFD and the most current Leica Noctilux 50mm/0.95 ASPH. The former will cost you an arm and the latter should cost you about 2 kidneys, if you can find one in stock.</p>

<p>Here's a short clip of how CNC looks like ... courtesy of the former Coastal Optics (now, part of Jenoptics AG):<br>

http://www.youtube.com/watch?v=IaNwUmRd214&feature=player_embedded</p>

<p> </p>

[acbeddoe]

<p>Glass elements are often done by #1 followed by #3. I worked on the control system for this MRF CNC (magneto-rheologic fluid/computer-numeric control):<br>

<a href="http://qedmrf.com/orgmain.asp?orgID=62&storyID=48">QED MRF machines</a></p>

 

[walterh]

<p>Looks surprisingly simple to me.<br>

I would have thought they measure the surface shape online e.g. by a laser interferometer. Is this perhaps the first coarse step of a series of steps?</p>

<p>What accuracy is actually needed? Is 1 wavelength enough?</p>

 

[roland_vink]

<p>Blank press or molded glass apheric elemnents used to be restricted to small diameter lenses, but are now used for large lenses such as the Nikon AFS 17-35, 24-70 and 14-24. It seems to be the preferred method for mass producting large, high quality aspheric lenses.</p>

<p>CNC ground aspherics was used for the AF 28/1.4 and 20-35/2.8, but this technique is no longer used in current lenses. The older aspheric lenses found on the orthographic projection 10/5.6 fisheye and 58/1.2 Noct were hand ground (made before CNC was available).</p>

<p>Hybrid aspherics are used for cheaper zooms.</p>

[arthuryeo]

<p>Do you guys see any difference between Blank Press/Molded aspheres and CNC-grounded ones at large aperture in terms of the images they produce ?</p>

[roland_vink]

<p>If the lenses are made to the same specification there will be no difference between a molded and CNC ground aspheric lens.</p>

<p>Having said that, certain types of glass may not be suitable for one or other process. Some glasses may distort when cooling so cannot be molded, and maybe some other glass cannot be ground easily, so glass with desirable characteristics in terms of reflactive index and dispersion may not be usable. Also, it may be easier to mold very complex aspheric surfaces than to grind them. So depending on the lens in question, there may be cases when the molded type is better than a ground lens, and vice versa.</p>

[arthuryeo]

<p>That really made sense, Roland. Thanks for the response.</p>

[majid]

<p>The strain in the glass from molding alters its refractive index and has to be factored in computing the lens formula.<br /> <br /> Interestingly process (3) is <a href="http://www.frojel.com/Documents/Document03.html">over 800 years old</a> (well, apart from the CNC part).</p>