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Thanks jrista, that was clarifying. So that means that it has nothing to do with the design of the specific lens, since the diffracted wavefront already exists when hitting the lens. Criticizing a lens for onion-ring bokeh is therefore incorrect. Right?
There are only two things I don't like about the Otus 1.4/55:
1) The price... because I would have to sacrifice too much other stuff to buy it
2) The onion bokeh... but this is only really noticeable with specular highlights that are OOF. In normal shots its not really noticeable.
Price is price and I agree that you should really want it to buy it. I also agree on the bokeh issue. But there is something I don´t really understand about boked. Because it varies, depending on light source.
I have attached two examples. The first is of five candle lights and the second is a chandelier with electric light. The bokeh from the electric light has a clear onion bokeh, whereas the candle lights are clean. If someone could explain why this happens, it would be most appreciated.
According to Huygens' principle, every wavefront point is a source of secondary wavelets, through which spreads in the direction of propagation. This constitutes a micro-structure of energy field propagation, with the energy advancing in the direction of the wavefront, but also spreading out in other directions. Principal waves, or wavefronts, form in the direction determined by extending straight lines from the point source. Waves moving in other directions generate phase difference, preventing them from forming another effective wavefront (FIG. 1, top right). However, these diffracted waves do interfere with both, principal waves and among themselves.
As a consequence of the existence of diffracted wave energy, placing obstruction of some form in the light path will result in the "emergence" of this energy in the space behind obstruction. But the obstruction did not change anything in the way the light propagates - it merely took out energy of the blocked out principal waves, with the remaining diffracted field creating some form of intensity distribution in the space behind obstruction - the diffraction pattern.
Similarly, by limiting energy field to an aperture, the portion passing through it is separated from the rest of the field, and its energy - this time consisting from both, aperture-shaped principal waves and diffracted waves within - will create a pattern of energy distribution behind the aperture. Again, there is no actual change in propagation for the light passing the aperture, including those close to the edge of obstruction (light does not "bend around the edge"); whatever the form of energy distribution behind the aperture, it is caused by the interference of primary and diffracted waves inherent to the energy field (FIG. 1, middle and bottom).
Excluding telescopes, I've used the EF 600/4 L II with both 2x and 1.4x TCs (a Kenko in this case, as they stack directly without the need for an extension tube...IQ suffers a little bit). That gets you to 1680mm.
With telescopes, I've poked around with focal lengths up to around 8000mm to 10,000mm using SCT and RC type OTAs with barlow lenses. The only real reason you would use such focal lengths is for planetary (to get any real kind of sharp detail on planets, you need at least 8000mm), solar (sunspot closeups) and lunar (individual craters and finer surface detail).
I haven't purchased my own OTA yet, once I do, I really can't wait to do planetary imaging at over 8000mm.
How do telescope focal lengths relate to sensor size? Like, is 8000mm an equivalent in full frame lens terms?
I have the kenko pro 1.4x and the canon 1.4xiii. They both have about the same IQ. The kenko is about half the price and works on just about everything, the canon has weather sealing.