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Author Topic: "First Impressions" Review of the Leica S2  (Read 6170 times)
tho_mas
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« Reply #20 on: December 22, 2010, 05:03:10 PM »
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I find I use the 35mm more for landscape shots, the 70mm is less useful for 'front to back sharp' images.
the question was if it is useful for left to right sharp images at wide distances :-)
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Nick Rains
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« Reply #21 on: December 22, 2010, 07:29:18 PM »
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the question was if it is useful for left to right sharp images at wide distances :-)

OK, then the answer is yes, no problem.

No fall-off of sharpness at edges at middle to far distance. Slight curvature of field wide open but for the sorts of landscapes we are talking about you'd be shooting for a bit of DOF anyway. This is a shot from the Barrier Reef, 100% crop from right side of frame. The old pontoon is about 200m away. f8/500th, handheld since the platform I was on was not rock solid. This image is 1100px wide out of 7500px for the full frame.

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Nick Rains
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John R Smith
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Still crazy, after all these years


« Reply #22 on: December 23, 2010, 03:15:42 AM »
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This business of CA is all a bit odd. As Doug says, it really does matter even if you are working in B/W, because it smudges the edges of things like tree branches against the sky. When I was working with film on  the 'Blad, I never really saw any CA at all, even when shooting colour. Then, when I got the 39MP back, to my horror I had loads of CA with the 50mm and to a lesser extent the 60mm. Really bad, in certain circumstances. The 80mm and upwards were fine. I just thought that this was inevitable with old W/A lenses and digital sensors, and resigned myself to dealing with it in software.

Then, I discovered I had a defect on my brand-new CFV back, a vertical magenta line which showed up at 100% about one third in from the left of the frame (a "column fault", they called it). To cut a very long story short, the DB went back to Denmark under warranty, they tried to fix it by re-calibrating but could not, and in the end they fitted a brand-new Kodak sensor, checked it through and returned it.

All was good, and then I realised after a few weeks of use that the CA problem with my W/A lenses had gone. Well, just a tiny bit of fringing if you went mad with TV aerials against the light, but nothing like what it had been. In fact I now have to really look for CA to find it at all. Now, I don't understand the tech stuff behind all of this, but it seems to me that CA is not necessarily just a lens problem, but can also be affected by sensor alignment and calibration too, somehow. Which might explain why we are getting conflicting reports on this with regard to the Leica S2.

John

Edit - having just re-read the above, I am struck by the thought that it all sounds pretty improbable, because how could the sensor calibration affect CA? And yet, that's what seemed to happen.
« Last Edit: December 23, 2010, 04:41:19 AM by John R Smith » Logged

Hasselblad 500 C/M, SWC and CFV-39 DB
and a case full of (very old) lenses and other bits
tho_mas
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« Reply #23 on: December 23, 2010, 04:09:47 AM »
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Nick, thanks for posting the crop.
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eronald
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« Reply #24 on: December 23, 2010, 07:51:16 AM »
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This business of CA is all a bit odd. As Doug says, it really does matter even if you are working in B/W, because it smudges the edges of things like tree branches against the sky. When I was working with film on  the 'Blad, I never really saw any CA at all, even when shooting colour. Then, when I got the 39MP back, to my horror I had loads of CA with the 50mm and to a lesser extent the 60mm. Really bad, in certain circumstances. The 80mm and upwards were fine. I just thought that this was inevitable with old W/A lenses and digital sensors, and resigned myself to dealing with it in software.

Then, I discovered I had a defect on my brand-new CFV back, a vertical magenta line which showed up at 100% about one third in from the left of the frame (a "column fault", they called it). To cut a very long story short, the DB went back to Denmark under warranty, they tried to fix it by re-calibrating but could not, and in the end they fitted a brand-new Kodak sensor, checked it through and returned it.

All was good, and then I realised after a few weeks of use that the CA problem with my W/A lenses had gone. Well, just a tiny bit of fringing if you went mad with TV aerials against the light, but nothing like what it had been. In fact I now have to really look for CA to find it at all. Now, I don't understand the tech stuff behind all of this, but it seems to me that CA is not necessarily just a lens problem, but can also be affected by sensor alignment and calibration too, somehow. Which might explain why we are getting conflicting reports on this with regard to the Leica S2.

John

Edit - having just re-read the above, I am struck by the thought that it all sounds pretty improbable, because how could the sensor calibration affect CA? And yet, that's what seemed to happen.

Maybe the new sensor had cells with a different design from the old one?
Maybe the cover glass creates prismatic effects?

Edmund
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Edmund Ronald, Ph.D. 
ondebanks
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« Reply #25 on: December 23, 2010, 08:57:12 AM »
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my expert friend (and he won a MacArthur Genius award for this kind of thing) says that only symmetrical designs are easily fixed with RAW CA tool.  Asymmetric not so good.


Hmm. I would have thought it was the other way round.

Lateral chromatic aberration causes the sort of colour fringing where each spot is imaged as a stubby spectrum, always orientated radially from the centre of the image. There is basically a slight magnification difference as a function of wavelength, and it can be corrected to first order by re-dimensioning the Red and Blue channels to coincide spatially with the Green. This is easily done in digital post processing and there is a proper optical basis for doing it: each of the RGB channels contained a pretty sharp image; they were just out of register with each other.

Axial chromatic aberration, or secondary spectrum, is a lot messier. It does not vary with field position, only with f-number - it's the type that is worst at wide apertures - and axial distance from the plane of focus. Only one (usually) of the RGB channels contains a sharp image; the others are out of focus. There is no rigorous optical basis to post-processing algorithms for correcting this - from what I've seen, they seem to work by cosmetic fudges like desaturating the fringe colour, rather than by physics - although, from Doug's attachments, it's clear that some of the fudges are more successful than others. At the plane of focus for one colour, you cannot push the other defocused light/colours back into focus, post-capture. The closest you can come is with some sort of adaptive, channel-selective deconvolution process. But that is non-linear, unpredictable, locally variable, and stuff like the noise statistics then go out the window. Outside the plane of focus, where the axial chromatic is manifested as colour fringing on the bokeh 'blobs', even deconvolution alone won't work, because there is also a spatial offset between the defocused fringes of difference colours. That sounds somewhat like lateral chromatic, but it doesn't have its global invariance; rather, this effect varies locally with each bokeh point. To truly correct it would require knowledge of the degree of defocus of all points in the image. Talk about messy!

So what about the lens design? Symmetrical lenses exhibit minimal lateral chromatic aberration, by virtue of their symmetry. Since they lack the easily correctable type of fringing, I don't understand why they would be more "easily fixed with RAW CA tool" than asymmetric lenses, which can show both types of aberration, including dollops of the more easily correctable type.

Ray

PS. Do I now qualify for a MacArthur Genius award?   Grin
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degrub
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« Reply #26 on: December 23, 2010, 10:08:18 AM »
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How about more ELD glass ?

Frank
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madmanchan
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« Reply #27 on: December 30, 2010, 12:11:23 PM »
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The manifestation of longitudinal/axial CA (i.e., how much it shows up in the image) is not only a property of the glass, but also of the spectral sensitivity of the sensor. In other words, the entire optical system must be considered.
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eronald
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« Reply #28 on: December 30, 2010, 03:05:59 PM »
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The manifestation of longitudinal/axial CA (i.e., how much it shows up in the image) is not only a property of the glass, but also of the spectral sensitivity of the sensor. In other words, the entire optical system must be considered.

One needs to consider assymetries in the sensels, I would think.

Edmund
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Edmund Ronald, Ph.D. 
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