06-27-2012, 07:36 PM
[quote name='photonius' timestamp='1340796787' post='19194']
In principle yes, but as you say, there is a small offset. Could this offset affect bokeh with aspherical lens elements? I don't know.[/quote] No. The glass is mounted perpendicular to the optical axis, and therefore has no effect. Besides, the whole idea of multi-coating is, amongst others, to prevent this from happening. If you take the B&W MRC filters, you'll find that it is hard to see the glass at all on a dark velvet background, and with the new nano version you have to know the glass is there in order to see it.
However, aspherical lenses themselves do have an effect on bokeh, and this may well be what you are seeing. It is often the reason of a halo around the subject under certain conditions. The 85L is a typical example of this, actually.
Also, what a lot of people don't realize, is that an aspherical lens is really
1) designed to be best, or optimal, at a certain object distance
2) to be best at a certain aperture
When you close down a lens with an aspherical element below a certain aperture, the lens becomes a normal, spherical lens, although some lenses overcome this at least partly by the use of several aspherical elements, with aspherical properties in different places relative to the optical axis.
The reason for aspherics to be optimal at certain distances is caused by the fact that spherical aberrations are strongest at shorter distances, generally speaking, and disappear at longer focusing distances, due to the smaller refraction angle of incident light. And it is very well possible that because of this an aspherical lens may not be as good at longer distances as a result, as it is still present and applying spherical corrections while there is nothing to correct.
I just realized while writing this out that this also explains why the 50L is so great for macro - it renders beatifully at macro distances with extension tubes, despite it being such a large aperture lens.This is likely due to the aspherical ellement.
See above posts <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />.
Kind regards, Wim
In principle yes, but as you say, there is a small offset. Could this offset affect bokeh with aspherical lens elements? I don't know.[/quote] No. The glass is mounted perpendicular to the optical axis, and therefore has no effect. Besides, the whole idea of multi-coating is, amongst others, to prevent this from happening. If you take the B&W MRC filters, you'll find that it is hard to see the glass at all on a dark velvet background, and with the new nano version you have to know the glass is there in order to see it.
However, aspherical lenses themselves do have an effect on bokeh, and this may well be what you are seeing. It is often the reason of a halo around the subject under certain conditions. The 85L is a typical example of this, actually.
Also, what a lot of people don't realize, is that an aspherical lens is really
1) designed to be best, or optimal, at a certain object distance
2) to be best at a certain aperture
When you close down a lens with an aspherical element below a certain aperture, the lens becomes a normal, spherical lens, although some lenses overcome this at least partly by the use of several aspherical elements, with aspherical properties in different places relative to the optical axis.
The reason for aspherics to be optimal at certain distances is caused by the fact that spherical aberrations are strongest at shorter distances, generally speaking, and disappear at longer focusing distances, due to the smaller refraction angle of incident light. And it is very well possible that because of this an aspherical lens may not be as good at longer distances as a result, as it is still present and applying spherical corrections while there is nothing to correct.
I just realized while writing this out that this also explains why the 50L is so great for macro - it renders beatifully at macro distances with extension tubes, despite it being such a large aperture lens.This is likely due to the aspherical ellement.
Quote:Also, some Canon filters (to complete sealing) for (older) longer lenses are apparently curved rather than flat. Why, if not important?
See above posts <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />.Quote:That's certainly true :-)
best regards
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....