The second problem is - that simple glass lens you made earlier acts like a prism. It breaks up white light into bands of color (chroma - see Newton, rainbows, etc.) In pictures this shows up as Chromatic aberration - things have fuzzy rainbow-tinted edges, especially at the corners of the image.

Generally Asph lenses reduce the total number of elements needed to correct distortions that are inevitable in lenses. This makes the overall lens shorter and lighter. As the nember of elements is reduced, it also allows more correction and the overall distortion can be less.

2) a very high degree of correction for all non-chromatic aberrations. This point is critical. Leica insists that spherical aberrations, distortion, etc. also be corrected to a high degree before calling a lens "Apo".

Throw in an a-spheric surface, and it will help correct the 'spherical' and other aberrations by getting the light back on track. One rule of thumb is that one aspheric surface does the correcting of two spherical surfaces - so if you use two aspheric surfaces, you can eliminate 2 complete spherical elements (4 surfaces) from the design - or get better correction with the same number of elements.

The first attempt to fix chromatic aberration was the development of the 'mirror' lens - first for telescopes and then for cameras. Since a mirror reflects light to a focus instead of refracting it through the glass, the mirror does not break up the light into colors.

Aspheric lenses advantages disadvantages

I am also assuming that the level of quality and craftsmanship of the Leitz aspherical lens is obviously and considerably better than that of my Sigma 28-105 "aspherical" lens, which I do enjoy, operates faithfully and consistently, and I register no serious complaints for everyday use.

Aspherical means a lens element with a compound curve instead of a spherical curve for its surface shape. This allows lens designers to correct for abberations without resorting to using multiple spherical elements with various refractive properties to correct for the same set of abberations. Hence, it allows for smaller, lighter and more compact lens designs.

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I also noticed in the lens schematic that both the front and rear elements on this lens areconcave on the front surfaces. I presume this has something to do with the aspherical characteristics you described. This surprized me, since, in my experience, the front element of a lens was always convex. Interesting.

Apochromatic does describe the performance of the lens, and generally means a lens that's capable of bringing light of 3 different wavelengths into focus at once, though different manufacturers have different standards for what they call apochromatic.

Adapter rings can allow you to connect C-mount lenses to CS-mount cameras, broadening the application reach of the equipment. Note that CS-mount lenses cannot be connected to C-mount cameras due to the shorter flange focal distance.

APO = apochromatic, meaning bringing the three primary colors of light (Red, Blue & Green) all into focus at the same point. This eliminates the "color-fringing" along point-sources of light commonly seen in cheap optics at higher magnifications.

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There seems to be some debate here on the forum regarding which is "better" if such a designation even exists at all or if we can agree what "better" means.

I am also assuming that a 90mm Elmarit APO is one designed for a telephoto to control chromatic abberasions (which will drive us all crazy when stargazing with a cheap telescope) and does not require aspherical elements to accomplish this task?

Asph lens have been around quite a while-Cinamascope and other widescreen "anamorphic" movie lenses have different focal lengths horizontally and vertically, thus they are able to squeeze a wide panorama into a standard 35mm movie frame. They were very expensive to grind and thus out of the range of consumer optics.

Aspherical just means the lens contains one or more elements with a surface that's not a section of a sphere. For example, it might be a hyperbola or parabola instead. It doesn't necessarily say anything about the performance of the lens.

Instead of taking a classical lens design and using the computer to tweak the performance, they just started out with a chunk of (simulated) glass and began playing with it - adding elements and curves and ASPH surfaces and tossing around refractive indices and then tracing the light rays and seeing how well the aberrations were corrected.

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re: ASPH and size - well, ASPHs can reduce the number of elements and size while retaining the the same performance - OR - they can improve performance while retaining the same size or limiting the amount of increase needed.

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First, a spherical surface is NOT the ideal shape for projecting or reflecting a sharp image. It really should be a parabolic surface (which is why parabolic microphones USE parabolas to focus sound waves, instead of a half-sphere). If you want to have a single-element lens that throws a good image it should have parabolic, not spherical, surfaces. And any surface that is not spherical is - a-spheric. Including parabolas and cylindrical lenses.

But in creating the 35 f/1.4 ASPH (the f/2 ASPH is essentially a modification/recomputing of the 1993 1.4 ASPH design) Leica threw out their old (double-gauss) designs and started from scratch with a clean sheet of paper (actually a clean computer screen). And both the ASPH surfaces and the concave surfaces result from this 'from the ground up' redesign.

Note that the Voigtlander 35 f/1.7 also has a concave front element (and ASPHs) - obviously optical engineers from around the world have looked at what Leica is doing and started applied the same ideas themselves

In other words: an apochromatic lens actually does something with regard to the point of focus of different colored wavelengths of light and produces an image that is qualitatively different (better?) than a nonapochromatic lens.

OK, so mirror lenses (in the 60s) produced better color correction than the straight-through glass teles of the era. And were also more compact. But they had bugs - doughnut-ring bokeh and no controllable aperture.

1) There is no international std for apochromatic correction. Manufacturers use this term as they see fit. Sigma uses it very liberally (it seems just about all of their longer lenses are "APO"). Canon never uses the term. Leica uses it sometimes for some very highly corrected lenses. Its use is at the discretion of the manufacturer.

The next advance was the development of extra-low dispersion (ED, in Nikon's terms) glass, which bent the light just as much but reduced the amount of 'dispersion' into separate colors. Again, this is important for long lenses or zooms that include some long focal lengths, but less so for normal/wide lenses. You see ED 200s, 300s, and 600s from Nikon, and an ED 75-300, but no ED 24s or 35s.

In order to connect a lens to a camera, we need a threaded connection—the mount. Mount sizes are standardized and are indicated by letters, with the C-mount being the most common type used for machine vision cameras. It is used for sensor diagonals measuring up to 20 mm, or 1.5 inches. The thread has a nominal diameter of 1 inch (24.5 mm) and a pitch of 1/32 inches. The flange focal distance of this lens mount is 17.526 mm (0.69 inch).

Examples include the famous 100/2.8 Apo-Macro-Elmarit-R and the 90/2.0 Apo-Summicron-ASPH-M (as well as other longer lenses. mostly R). I think there is little doubt that the lenses Leica calls Apo are "as good as it gets" (unfortunately also as expensive as it gets).

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Richard: are you sure anamorphic lenses need to be aspherical? the differential focal length horizontally and vertically can be achieved by introducing a cylindrical element, no? Anamorphics have also been achieved with prisms, as is the case with the variable anamorphic lens.

C-mount lenses can be used in a CS-type lens mount with a 5mm thick adapter ring to secure them, but CS-mount lenses cannot be used in a C-type lens mount ...

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But until the 1990s it was always cheaper to make spherical surfaces and then combine several of them to 'tweak' the light paths until the final image was mostly sharp over most of the frame. This is why lenses have so many elements/surfaces inside - they're all busy straightening out the aberrant ("wandering") light rays caused by using spherical surfaces in the first place.

When referring to polarization states, the p-polarization refers to the polarization plane parallel to the polarization axis of the polarizer being used ("p" is ...

Chromatic aberrations get magnified (along with everything else) by long-focus lenses. Tha's why you see lots of APO telephotos, but no one bothers selling APO wide-angles.

An Asph lens may contribute to an APO (which if I recall corrrectly is the Greeek prefix for OK, or normal)chromatic lens. real APO lenmses need no correction for infra-red focussing, as all colours align at the same point. Focussing all wavelengths of light means the image is sharper as there are no overlapping fringes of light caused by rays arriving at different places.

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You just created a spherical lens, with a 'plano' (flat) surface on one side and a convex 'spherical' surface on the other. 99.999% of lenses ever made have spherical surfaces, because they're easy to make.

In that case, what is the advantage of an ASPH/aspherical lens over a non-aspherical lens. I got a bit bashed on this issue a few days ago and I'd kinda like to know the benefits of such.

Some mention was made that using one or more aspherical elements reduces the number of elements needed in the lens and provides for a more compact unit; however, the 35mm Summicron ASPH I received today is physically larger than the pre Asph version. I like the larger size...it makes focusing much easier for me.

In that case, what does an aspherical lens actually DO that is any better/different than a nonaspherical lens in terms of image quality?

2) Aspherical. The use of an aspherical surface does not imply better correction than a lens without such a surface, since other factors contribute to lens performance. Many P & S lenses have one or more (up to 4!) aspherical elements. Don't bet that they are better than a 50/2 Summicron M which does not have an aspherical surface. These terms are more for advertising than to describe lens performance.

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By the time they'd finished Leica's designers had essentially invented an entirely new design formula for small fast wide-angles, which is why they've been able to crank out the 35 ASPH f/2 and the 21/24 and the 28 'crons so relatively quickly. This new formula pays no homage to the older 20th century (and 19th century) designs - if the computer simulations demonstrated that concave front/back surfaces made for better image quality, that's what they built, even if the lenses 'look' strange.

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I am looking forward to using the 35mm Summicron ASPH, having bought it on the strong recommentations of many here on the forum, hoping it will stimulate a long relationship with Leica, and, having to place my son in indentured service to afford the expense! ;>)

... objective and ocular lenses (see figure below). The magnification of the ocular lenses on your scope is 10X. Objective lens X Ocular lens = Total ...

re: ASPHs and concave glass - the front/rear elements are not concave BECAUSE of the ASPH surfaces per se (i.e. there are lots of ASPH lenses that still have convex outer surfaces).

How do you find the right lens for a camera? What roles do resolution, sensor size, image circle, focal length, and lens size play in creating optimal image quality?

The Leica manual that comes with the lens makes the point that this lens is far superior to the previous non-aspherical model...almost to the point of overkill. It indicates that lens apertures wider than F/5.6 particularly benefit from these aspherical properties, and will equal the quality of the Pre unit at f/5.6 or smaller.

I should add to my poste that Leica Apo lenses are particularly good performers, because Leica (Leitz in past) has had fairly stringent standards as to what should be called an Apo lens including:

They are the ones, after all, who ACTUALLY make lenses, and are pretty good at it. And, I DO remember reading in a Canon lens brochure about an aspherical element for example having the periphery flatten out a bit. I don't have that brochure anymore, nor could I find it online.

To get a sharp image, you not only need the right camera, but also the right lens to go with it. Our beginner's guide to lens selection will help.

Previous posters pretty much covered the 'chroma' terminology: a chromatic lens has no correction, an achromatic lens gets two of the colors to focus back together, an apochromatic lens gets all three pretty close, and a superchromat (Zeiss makes a couple) does even better with RGB and even corrects the UV and/or IR wavelengths to close to the visible light focus.