Canon’s new 600mm and 800mm lenses

Started 3 months ago | Discussions thread
AiryDiscus Senior Member • Posts: 2,138
Re: Canon’s new 600mm and 800mm lenses

Trebor1 wrote:

Mark Scott Abeln wrote:

Chris R-UK wrote:

IMHO these two lenses are the most extraordinary to have been introduced for many years. They are extraordinary in specifications (fixed f/11 apertures!), size/weight and price.

Does anybody know anything about the “gapless dual layer diffractive optics” used in these lenses? What advantages and disadvantages does this technology have compared with, say, Canon’s 400mm f/4 DO and Nikon’s 500mm f/5.6 PF lenses, both of which are very much more expensive than the new lenses?

Clearly having an f/11 aperture reduces the size of the front element of the lens, but does making this a fixed aperture have significant design advantages and cost savings?

A fixed aperture limits the performance of the lens, but f/11is pretty tight already. I think that including an iris would be bad for marketing purposes, because diffraction softening would go from marginal to worse, and so the MTF curves often shown by lens reviews would be atrocious. Anyway, with very long lenses, large f/stop values aren't practical, as these are usually used for distant action shots, where high shutter speeds and natural light are required.

But even having an f/11 f/stop is a very good improvement over some superzoom cameras with similar long equivalent focal lengths. The Nikon P1000 has a full frame equivalent of f/31 at 800 mm and maybe f/29 at 600 mm. While I would fully expect having to use elevated ISO values while shooting these lenses, the noise and optical performance should likely be much better over using a long superzoom.

Diffractive optics can make a lens significantly smaller and lighter, but from what I read, they don't improve optical quality over traditional ground glass lenses.

I thought that the claims for hybrid designs, with both diffractive and ground glass lens elements, also included reduced chromatic aberration (apochromats and superapochromats)?

Diffractive optics have negative (signed) dispersion. This makes them extremely useful for color correction.

The way to think about CA is that it's like a percentage of the focal length. The pixel size is fixed, so for longer focal lengths CA is proportionally more of a problem. The first solution is to have no color at all, which is why materials like calcium fluorite are used. Those have very low index, so unless you're going to aspherize them you will have tremendous aberrations (mostly spherical - small field of view). Those materials are hard to polish and the elements are dinner plate sized; you better have 6 figures on the price tag to support that.

So you need to get your power somewhere else.  If use higher index glasses, they have tons of dispersion, and you need positive power not negative, so that will make the CA broadly uncorrectable.

diffractive elements with their negative signed dispersion let you source power from there and effectively get ~0 primary CA without any negative elements, which is a huge boon.

This is independent of being apochromatic; you need anomalous partial dispersion to do that which is a completely separate matter to DO.

If you had a purely diffractive optic system, you would think it was a kaleidoscope for how much CA it has.

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