Thomas Chen Posted November 11, 2014 Share #41 Posted November 11, 2014 Advertisement (gone after registration) ......and contrast-detection focusing are methods that focus for sharpness; those won't suffer from focus shift when focusing at working aperture. . Yes, this is the way I focus MANUALLY when use 35 Summilux ASPH (pre-FLE) on A7R. Thanks! Link to post Share on other sites More sharing options...
Advertisement Posted November 11, 2014 Posted November 11, 2014 Hi Thomas Chen, Take a look here 35mm 1.4 floating vs non floating element. I'm sure you'll find what you were looking for!
colonel Posted November 11, 2014 Share #42 Posted November 11, 2014 Focus shift does not exist if you manually focus a RF lens on a CSC like the A7. (It can exist if you focus at a different aperture to that which you eventually take) You are free to try any RF lens on a CSC and see the maximum potential it has at any aperture Link to post Share on other sites More sharing options...
01af Posted November 11, 2014 Share #43 Posted November 11, 2014 Spherical aberration is only corrected with aspherical lenses (obviously). Oh ... so we're dealing with an expert. A floating group does not correct spherical aberrations per se, it just optimizes the performance at close focus. And what do you think is the reason why close-focus performance will drop in the first place? Link to post Share on other sites More sharing options...
pgk Posted November 11, 2014 Share #44 Posted November 11, 2014 And what do you think is the reason why close-focus performance will drop in the first place? Looking at the 're-designed' lens, I would have thought that as it is focused closer and focal length marginally increases, the image formed by the rear group moves (backwards I assume) and the FLE system adjusts to optimise its position relative to the shifted image. So does the marginal (non-adjustable) displacement of the rear section of the lens on the pre-FLE version mean that the effects of spherical aberration are accentuated at close focus? I'm intrigued by the positioning of aspheric elements/surfaces and their effect - I assume that the effect of the aspheric surface in the 35mm Summiluxis greatest at wide apertures since it is very close to the diaphragm and as the lens is stopped down it increasingly behaves like a non-aspheric lens. My observations suggest (I have the pre-FLE) that the aspheric element is in the design to maximise full aperture performance (at infinity) so anyone not needing a fast aperture general use 35mm should probably use the Summicron or Summarit, both of which have very even prformance (from mmy experience) rather than a faster lens. Link to post Share on other sites More sharing options...
jaapv Posted November 11, 2014 Share #45 Posted November 11, 2014 Spherical aberration is only corrected with aspherical lenses (obviously). Errr... How do you think it was corrected before the introduction of aspherical lenses? One of the basic methods is overcorrecting another lens, leading to constructions like an Orthoscopic Doublet, for instance. Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/233400-35mm-14-floating-vs-non-floating-element/?do=findComment&comment=2704419'>More sharing options...
otto.f Posted November 11, 2014 Share #46 Posted November 11, 2014 Focus shift does not exist if you manually focus a RF lens on a CSC like the A7. (It can exist if you focus at a different aperture to that which you eventually take) You are free to try any RF lens on a CSC and see the maximum potential it has at any aperture Exist, exist... Of course it exists. It exists in the lens. It does not show because you correct for it when you see through the lens by way of EVF. Please read Adams' The Camera and/or The Print (again). Link to post Share on other sites More sharing options...
Rick Posted November 11, 2014 Share #47 Posted November 11, 2014 Advertisement (gone after registration) And what do you think is the reason why close-focus performance will drop in the first place? The design of the non-FLE 35 Summilux was optimized for spherical aberration at distance for a wide beam of rays incident on the lens, parallel to the axis, so as to bring all rays to a single point of focus. At nearer points of focus, the rays incident to the lens are no longer parallel to the axis and are therefore not corrected as well for spherical aberration to a single point of focus, causing a decrease in contrast at near. Further, when the lens is stopped down rays that are further from the central axis of the lens are illuminated, and the point of focus moves as the bundle of rays condense to a smaller point of focus, and contrast increases. The new FLE design allows the rear element to adjust to a more optimum position in the path of the more divergent near point rays, and allows for more correction of spherical aberration (and probably helps optimize S2-S5) and results in a tighter bundle of rays at the focal point. Contrast is increased and the effect of stopping the lens down has much less effect on changing the already narrower bundle of rays at the focus point. The primary effect of the FLE is to increase contrast (tighten the ray bundle at the focal point). The secondary advantage is less shift of the focal point is noted when stopping the lens down. This is why the new FLE has much improved image quality at near as compared to the non-FLE. Sometimes I think this is overlooked in the backwards discussion of focus shift. Link to post Share on other sites More sharing options...
CheshireCat Posted November 11, 2014 Share #48 Posted November 11, 2014 Errr... How do you think it was corrected before the introduction of aspherical lenses? Fair enough, there were/are other methods. But the point here is that a floating group does not correct spherical aberrations per se. It just optimizes the overall performance of the lens at focusing distances different than the one the lens was computed for. The actual correction is still performed by the aspherical element, not by the floating group. By optimizing the lens performance, the floating group makes the lens work better also at close focusing distances, and better means all kinds of aberrations. This is why it is not correct to say that the FLE has been added to the SX35 to reduce focus shift. It is correct to say that the FLE improves the overall lens performance at close distances (also including, but not limited to, focus shift). 01af is usually very picky with definitions, I am surprised he is not in this case. Link to post Share on other sites More sharing options...
Rick Posted November 11, 2014 Share #49 Posted November 11, 2014 Fair enough, there were/are other methods.But the point here is that a floating group does not correct spherical aberrations per se. It just optimizes the overall performance of the lens at focusing distances different than the one the lens was computed for. The actual correction is still performed by the aspherical element, not by the floating group. By optimizing the lens performance, the floating group makes the lens work better also at close focusing distances, and better means all kinds of aberrations. This is why it is not correct to say that the FLE has been added to the SX35 to reduce focus shift. It is correct to say that the FLE improves the overall lens performance at close distances (also including, but not limited to, focus shift). 01af is usually very picky with definitions, I am surprised he is not in this case. I am not sure if this is completely true. It is certainly simplistic. The spherical aberrations are corrected by the shape factor of the positive and negative lens pairs that are glued together with the weaker lens having more spherical aberration by design, but in the opposite power. Next, the entire lens design has several lens pairs that can be used to reduce spherical aberration (and other aberrations) in this way, as well as the aspherical lens surface in the FLE group. The aspherical surface is not the only lens correcting spherical aberration. By moving the other lens elements to compensate for near focal distances, the lens can be optimized for spherical aberration for more than just one (infinity) focal distance. This is the primary reason for the FLE. Link to post Share on other sites More sharing options...
Jeff S Posted November 11, 2014 Share #50 Posted November 11, 2014 In the link I posted (#25), Karbe also states the importance of new glass characteristics (see narrative starting at top…and later pics) for several different lens elements to correct both primary and residual aberrations. Jeff Link to post Share on other sites More sharing options...
pgk Posted November 12, 2014 Share #51 Posted November 12, 2014 The design of the non-FLE 35 Summilux was optimized for spherical aberration at distance for a wide beam of rays incident on the lens, parallel to the axis, so as to bring all rays to a single point of focus. The primary effect of the FLE is to increase contrast (tighten the ray bundle at the focal point). The secondary advantage is less shift of the focal point is noted when stopping the lens down. This is why the new FLE has much improved image quality at near as compared to the non-FLE. Sometimes I think this is overlooked in the backwards discussion of focus shift. Can I add that your first paragraph needs the caveat of being true (ie optimised) at FULL aperture (f/1.4) and at infinity (distance). The use of aspheric elements/surfaces appears to be a double edged sword as they clearly can be used to optimise performance at specified apertures and distance but introduce other less desirable effects such as focus shift which then requires mechanical complexity to deal with. Is this essentially because an aspheric surface has a 'focal length' which is, in effect, somewhat 'variable' and so its effect depends on its positioning relative to the diaphragm and the aperture in use (at small apertures an aspheric surface close to the diaphragm has less effect so the lens behave more like a spherical design)? FWIW my pre-FLE does not seem to suffered from a great deal of focus shift so I assume that the rear group is positioned marginally rearwards of the infinity optimised position. Link to post Share on other sites More sharing options...
01af Posted November 12, 2014 Share #52 Posted November 12, 2014 The use of aspheric elements/surfaces appears to be a double-edged sword as they clearly can be used to optimise performance at specified apertures and distance but introduce other less desirable effects ... Yes. ... such as focus shift which then requires mechanical complexity to deal with. No. Is this essentially because an aspheric surface has a 'focal length' which is, in effect, somewhat 'variable' ... No. It's just the other way around: A spheric element has a kind of variable focal length, and aspheric surfaces are one method (out of many) to deal with this effect. Link to post Share on other sites More sharing options...
pgk Posted November 12, 2014 Share #53 Posted November 12, 2014 Thanks 01af. So what exactly does the aspheric surface do in this design and is its effect aperture dependent as its close to the diaphragm? What is the route cause of the focus shift? Link to post Share on other sites More sharing options...
01af Posted November 12, 2014 Share #54 Posted November 12, 2014 So what exactly does the aspheric surface do in this design ...? If you want to know exactly then better ask Peter Karbe. My guess is, it primarily helps getting residual spheric aberrations under control, and it possibly secondarily possibly indirectly also helps reducing chromatic aberrations and/or field curvature. Ultimately, it improves the lens' performance while keeping the lens' size small. ... and is its effect aperture-dependent as it's close to the diaphragm? I think so ... which makes sense, as spheric aberrations also are aperture-dependent. What is the route cause of the focus shift? Residual spheric aberrations. Link to post Share on other sites More sharing options...
CheshireCat Posted November 12, 2014 Share #55 Posted November 12, 2014 What is the route cause of the focus shift? Check this video: It clearly shows how the focus point (better called "the circle of least confusion", due to spherical aberration) shifts by stopping down. Link to post Share on other sites More sharing options...
semi-ambivalent Posted November 12, 2014 Share #56 Posted November 12, 2014 ...while keeping the lens' size small. To me, this more than anything sets Leica lenses apart. s-a Link to post Share on other sites More sharing options...
CheshireCat Posted November 12, 2014 Share #57 Posted November 12, 2014 If you want to know exactly then better ask Peter Karbe. My guess is, it primarily helps getting residual spheric aberrations under control, and it possibly secondarily possibly indirectly also helps reducing chromatic aberrations and/or field curvature. Ultimately, it improves the lens' performance while keeping the lens' size small. No need to ask, quoting PK from the already mentioned interview: PK: The optical design of the Summilux 35mm f/1.4 ASPH showed that placing an aspherical element behind the aperture (iris diaphragm) enables an effective reduction of oblique spherical aberration. However, that means that you have to “flatten out” the resulting curvature of field by using high refractive index glass in the positive (convex) lenses. So Peter Karbe clearly says that the aspherical lens is used only to reduce spherical aberration (as expected). He also says that the aspherical lens has the drawback of increasing the field curvature, and - indirectly - it also increases chromatic aberrations making the SX35 design a big challenge. Link to post Share on other sites More sharing options...
Jeff S Posted November 12, 2014 Share #58 Posted November 12, 2014 My favorite part of the Karbe interview that I linked above regards his comments on glass within the 50 Summilux ASPH… Establishing a balance between factors, including performance, size, choice of the appropriate glasses, and production tolerances, required lots of patient development work and numerous experiments and trials. For example, the glass I chose for lens element 3 is of crucial importance in minimizing the secondary color aberration. This glass, formerly made at the Leitz glass laboratory, was for a long time offered by another supplier who had taken over its production. But they had stopped making it, so I had to “encourage” another German glassmaker to literally reinvent this glass type. Today this glass is extremely expensive. Indeed, the material for this lens element alone costs as much as the glass used in all the other lens elements of the Summilux 50mm f/1.4 ASPH! In short, optical design software, as useful as it can be, will not help in choosing the appropriate glass types, especially those used to minimize secondary chromatic aberration. Jeff Link to post Share on other sites More sharing options...
Rick Posted November 16, 2014 Share #59 Posted November 16, 2014 If you want to know exactly then better ask Peter Karbe. My guess is, it primarily helps getting residual spheric aberrations under control, and it possibly secondarily possibly indirectly also helps reducing chromatic aberrations and/or field curvature. Ultimately, it improves the lens' performance while keeping the lens' size small. Yes. Link to post Share on other sites More sharing options...
Twilight Posted December 4, 2014 Share #60 Posted December 4, 2014 Hi Since more people tend to use their Leica lenses on the new FF sony alpha 7 series and since I am thinking of adding a A7 (probably a7s for the silent shutter and high iso IQ), could users of the preFLE (a lens which signature I adore) especially on the a7s (but off course FLE and other 7 series experiences could be interesting) give their opinion on IQ Greatly appreciated Link to post Share on other sites More sharing options...
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