Nikon Objective range changes

[ChrisR]

New to me anyway. The CFI (etc) range appears to be split into 3,
CFI60, CFI60-2, CF IC
An "evolution".
http://www.nikon.com/products/instrumen ... /index.htm

[Charles Krebs]

http://nikon.com/news/2012/0925_cfi60_2_01.htm

[rjlittlefield]

Thanks for the links.

I was confused for a while by this illustration in the nikon.com/news article. But then I decided the text is just wrong and it should be "Refracting lens: longer wavelengths mean farther focal point / Phase Fresnel lens: longer wavelengths mean closer focal point". Is that the way you see it also?

--Rik

[ChrisR]

Actually, try as I might, I can't see where it's written other than
"Refracting lens: longer wavelengths mean farther focal point / Phase Fresnel lens: longer wavelengths mean closer focal point".

I spent a little while looking for a "nice" depiction of how diffractive optics (the Fresnel phase part) work. I didn't really find a particular one, though I think I've got it, having looked at all the links on the first couple of pages of a google search.
The picture being used for the Fresnel part in the Nikon description is somewhat misleading, because that's what a familiar refractive Fresnel lens looks like.
http://upload.wikimedia.org/wikipedia/c ... ns.svg.png
A "circular diffraction grating" doesn't quite right convey the right detail either.

For those who have never come across the idea, Canon's description introduces it:
http://www.canon.co.uk/For_Home/Product ... Lenses.asp

(I can feel Rik twitching - all lenses can be described as diffractive!)

[rjlittlefield]

Actually, try as I might, I can't see where it's written other than
"Refracting lens: longer wavelengths mean farther focal point / Phase Fresnel lens: longer wavelengths mean closer focal point".

Yeah, I know. It's kind of like the old faithful frustration to "count the f's in this sentence", and most everybody misses some of the ones that sound like "v".

But look harder. What's written in the image says "Fresnel lens: Shorter wavelengths mean closer focal point", just the opposite of what they have diagrammed and what's required to work as they indicate.



--Rik

[rjlittlefield]

For those who have never come across the idea, Canon's description introduces it:
http://www.canon.co.uk/For_Home/Product ... Lenses.asp

There is a more complete description at http://www.canon.com/camera-museum/tech ... eport.html, with later info at http://www.canon.com/camera-museum/tech ... rt.html#t8.

The diagrams consistently show the diffractive optic (DO) as having a profile like a Fresnel lens, but the DO certainly doesn't act like a conventional Fresnel lens because the effect of wavelength on ray bending is reversed.

I don't really understand yet how these things work.

--Rik

[rjlittlefield]

The concept is becoming more clear to me.

As described in the Encyclopedia of Optical Engineering by Ronald G. Driggers (copyright 2003), page 616:

In fact, the groove widths, besides being the most prominent characteristic in the appearance of a fresnel lens, define the basic optical performance of the lens. Fresnel lenses with groove width of a few microns operate as diffractive lenses. These diffractive fresnel lenses utilize constructive interference between each groove to produce a common focal point. Fresnel lenses with groove widths of tenths of millimeters or larger operate by the same basic optical properties as a refractive lens or prism. Each groove acts as a small annular lens or prism to refract light to a desired focal point.

As used by Canon and Nikon in their camera and microscope lenses, the key characteristic of diffractive fresnel lenses is that they have strong negative dispersion. As a result, a weak positive diffractive fresnel lens can be paired with a strong positive ordinary refractive lens to cancel chromatic aberration. In these course notes, slides 35 and 36, an achromatic doublet with focal length 25.4 mm is constructed from a refractive element with focal length 26.769 mm and a diffractive element with focal length 496.695 mm, over 18 times weaker.

The profiles shown by Canon look like what the class notes call a "blazed grating". An SEM image of one is shown by Nikon HERE, second photo.

--Rik