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nikon g lenses shifting focus
#11
[quote name='fall' timestamp='1306626796' post='8886']

thanks for the info, i always thought that the purpose of cross type points are for faster and more accurate af, i guess i was wrong. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />[/quote]



In situations in which a line sensor struggles to focus a cross type sensor would indeed be much faster (and eventually more accurate too).
#12
[quote name='wim' timestamp='1306660026' post='8890']

Just one comment as to focus shift with 50L: first of all, one of the people who was most vociferous and had the biggest problems with regard to focus shift on the 50L on POTN, switched to Nikon because of the problems he had with 50L and 1D Mk IV. That lasted a little over a year, when he switched back to Canon, about half a year ago. He recently acquired a new 50L - zero focus shift.



Furthermore, focus shift is/was limited to a very specific range, namely about MFD to 1.8 m, only at apertures from F/1.4 to about F/2.8.



However, it looks like Canon has this problem entirely under control now, and anybody who shoots with a 50L for a while, and likes the 50 mm FL, wouldn't want to miss it.



Same here... <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' /> and I tested 5 different specimens over a period of a year and a half, when things were still quite bad. The specimen I still have, which sits as main lens on my 5D II, only had some constant back focus rather than focus shift and was calibrated to perfection by Canon, even if it took Canon a second go to get it perfect.



Other than that, most fast lenses (F/1.4 and faster) show focus shift, unless they have perfect superapochromatic correction. You need to get to know a lens well so you can deal with it, if you are after the rendering such a lens provides.



HTH, kind regards, Wim

[/quote]



i read that the focus shifting problem of the canon 50L was because of how the lens was designed, and to fix this the overall design had to change which would give us a mkII. was that not right?



good news if they found a way to fix this issue, may i know the date code on your 50L, just so that i know which batch doesn't have this issue. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />
#13
[quote name='fall' timestamp='1306671360' post='8897']

i read that the focus shifting problem of the canon 50L was because of how the lens was designed, and to fix this the overall design had to change which would give us a mkII. was that not right?



good news if they found a way to fix this issue, may i know the date code on your 50L, just so that i know which batch doesn't have this issue. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />

[/quote]

Well, let me put this politely: there are too many people out there on the interweb who don't know what they are talking about, and/or parrot each other.



Essentially, a very fast lens by definition suffers from aspherical aberrations - that's Optics 201 (rather than 101). These aberrations are the same ones which generally provide the great, smooth bokeh many people are after. To some degree this can be mitigated by aspherical surfaces. However, aspherical surfaces really only work well specifically for a single aperture.



So, is this a design flaw? IMO, no, it is inherent to fast (large aperture) optics. However, what Canon engineers did here is create a lens that not only has very good bokeh behind the focal plane, but also in front of it, which is optically very, very difficult, if not virtually impossible. In short, this lens, from a bokeh POV and controlling aspherical aberrations, is at the bleeding edge of optical design. Now, in a lab environment, where each lens is hand calibrated to the n-th degree, it is probably possible to adjust each lens so that it is perfect. In an assembly plant, where 1000s of lenses are manufactured at speed, this may be a bit more compicated to obtain. It requires quite small tolerances to get such a design working properly, and once one realises what the tolerances involved are, it is really amazing that it is possible to create high quality lenses consistently in general.



It looks like it took some time before the plant was at a similar quality level as the labs, and it looks it has been there now for a few years with regard to the 50L.



As to the design 'flaws" that people talk about, they often refer to the 'floating element' that the 85L and 85L II have. Well, first of all, the 85L has floating element[size="3"]s[/size], plural IOW, not one, but all but one. The rear element is a static element, all other elements move ('float') when focusing. This is a design that has been used in the past mostly to prevent mechanical vignetting by the lens barrel and rear optics getting to far from the exit point when focusing up close, but according to Canon, it is used in the 85L to correct for coma, and likely also for astigmatism, which are abberrations with fast, (modified or not) double Gaussian designs such as the 50L and 85L are known to suffer from. In short, this, as in, floating elments, has nothing to do with correcting spherical aberrations - that is what the aspheric element(s) are for, and high refracive index glass elements.



Furthermore, after my own 50L was calibrated, I tested it against my own 85L II for residual focus shift (note: it didn't have noticeable focus shift to begin with, only consistent backfocus), as all fast lenses, even well corrected ones, will still have residual spherical aberrations and therefore residual focus shift. What I found is that both lenses had their strongest focus shift at F/2, at close focus distances (where it doesn't disappear in the DoF), where the 50L had about 1/6 stop of shift in the total DoF area, and the 85L II 1/4 stop. Both these values are well within the tolerances of fast AF points and the AF tolerances Canon promises (1/3 f-stop accuracy). But interestingly, the 85L II also has some focus shift, and even more than my 50L... <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />



HTH, 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 ....
#14
And even lenses with quite horrid bokeh can show a focus shift.



On a side note, I'd love to have a 50mm f1.2 L or 50mm f1.0 L.. but on FF, not on APS-C.
#15
[quote name='wim' timestamp='1306673768' post='8899']

Well, let me put this politely: there are too many people out there on the interweb who don't know what they are talking about, and/or parrot each other.



Essentially, a very fast lens by definition suffers from aspherical aberrations - that's Optics 201 (rather than 101). These aberrations are the same ones which generally provide the great, smooth bokeh many people are after. To some degree this can be mitigated by aspherical surfaces. However, aspherical surfaces really only work well specifically for a single aperture.



So, is this a design flaw? IMO, no, it is inherent to fast (large aperture) optics. However, what Canon engineers did here is create a lens that not only has very good bokeh behind the focal plane, but also in front of it, which is optically very, very difficult, if not virtually impossible. In short, this lens, from a bokeh POV and controlling aspherical aberrations, is at the bleeding edge of optical design. Now, in a lab environment, where each lens is hand calibrated to the n-th degree, it is probably possible to adjust each lens so that it is perfect. In an assembly plant, where 1000s of lenses are manufactured at speed, this may be a bit more compicated to obtain. It requires quite small tolerances to get such a design working properly, and once one realises what the tolerances involved are, it is really amazing that it is possible to create high quality lenses consistently in general.



It looks like it took some time before the plant was at a similar quality level as the labs, and it looks it has been there now for a few years with regard to the 50L.



As to the design 'flaws" that people talk about, they often refer to the 'floating element' that the 85L and 85L II have. Well, first of all, the 85L has floating element[size="3"]s[/size], plural IOW, not one, but all but one. The rear element is a static element, all other elements move ('float') when focusing. This is a design that has been used in the past mostly to prevent mechanical vignetting by the lens barrel and rear optics getting to far from the exit point when focusing up close, but according to Canon, it is used in the 85L to correct for coma, and likely also for astigmatism, which are abberrations with fast, (modified or not) double Gaussian designs such as the 50L and 85L are known to suffer from. In short, this, as in, floating elments, has nothing to do with correcting spherical aberrations - that is what the aspheric element(s) are for, and high refracive index glass elements.



Furthermore, after my own 50L was calibrated, I tested it against my own 85L II for residual focus shift (note: it didn't have noticeable focus shift to begin with, only consistent backfocus), as all fast lenses, even well corrected ones, will still have residual spherical aberrations and therefore residual focus shift. What I found is that both lenses had their strongest focus shift at F/2, at close focus distances (where it doesn't disappear in the DoF), where the 50L had about 1/6 stop of shift in the total DoF area, and the 85L II 1/4 stop. Both these values are well within the tolerances of fast AF points and the AF tolerances Canon promises (1/3 f-stop accuracy). But interestingly, the 85L II also has some focus shift, and even more than my 50L... <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />



HTH, kind regards, Wim

[/quote]



Wim you waisting your good time <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' /> and you are too kind!
#16
[quote name='Brightcolours' timestamp='1306581583' post='8870']

The G lenses (all 3) will lose you the aperture control on micro 4/3rds... The lenses I mentioned concerning the full frame option (with much cheaper lenses doing the same) all have an aperture control ring (35mm f2, 85mm f1.8, 135mm f2 DC).[/quote]

A bit off-topic, but just to set things straight – with proper adapter you won’t lose the aperture control (e.g. I used some G lenses on GF1), the only downside is that you can only set maximum and minimum aperture precisely, any stops in between have to be guess-timated.
  


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