Define ASPH or Rectilinear

<p>I'm a bit confused about these two terms and would like to know the definition.<br>

Do they both mean the same thing?<br /> I am under the impression they refer to lens design that maintains vertical lines and minimizes edge distortion (ie for architecture shots) even for wide angle lenses.<br>

Is this correct? <br /> Are these lens designs more specific to rangefinder glass than SLR?<br /> I believe that's the case because of the SLR requirement of having the back of the lens farther from the film plane due to the mirror.<br>

Thanks, Bob</p>

 

<p>No, they're very different terms. </p>

<p>Rectilinear means that straight lines in the scene are rendered as straight lines on the film or sensor (as opposed to a fisheye lens where straight lines in the scene are very curved on the film or sensor). Most lenses have some degree of pincushion distortion (lines "bend" toward the center) or barrel distortion (lines "bend" away from the center), but if they're relatively straight, it's still considered a rectilinear lens.</p>

<p>ASPH is an abbreviation for aspheric, which means that at least one of the lens elements does not have a simple, spherical design--its surface is not a constant-radius curve.</p>

<p> </p>

<p>Mike,<br>

Then what is the advantage and image characteristics of the ASPH lens design?<br>

Thanks, Bob</p>

<p>Aspherical elements can provide more flexibility for the lens designer to make a lens that is sharp and/or contrasty from center to corner.<br>

Historically, aspherical elements were extraordinarily expensive to make in glass, as they had to be hand-ground and corrected, automated processes weren't feasible.<br>

However, many aspherical elements are now cast, so that only the mold has to be made to great precision. They can be cast in plastic or in glass. There are also methods by which aspherical elements are machine-turned in glass.<br>

Most of the single-use cameras have a plastic aspherical single-element lens. So they can be used in dirt-cheap products.<br>

Lots of complex cheap zooms use them, often with a plastic element in the inside (where it won't get scratched) to make the lens smaller and cheaper. While plastic doesn't have the greatest of optical characteristics, the ability to make cheap aspheric elements from it can make up for that.<br>

At the higher end, aspheric elements are used to make lenses that outperform lenses not using aspherical elements. Some in lenses in the $500 price bracket (Cosina), and some in lenses in the $2000 to $10000 price bracket (Leica).<br>

Topcon, who used to be a camera maker, has lots of <a href="http://www.topcon.co.jp/eng/corporate/technology.html">aspheric technology</a> , and is a wholesale supplier of aspheric elements in glass and plastic.</p>

<p>Bob: The advantage that RF wideangles had over SLR wideangles was much greater in the 60s/70s than it is today. Back then, the RF wides were "symmetrical" designs, which were usually more rectilinear and generally better in performance, and the SLRs needed to use "retrofocus" designs to clear the mirror - and retrofocus designs were in their infancy and thus ot yet very good.</p>

<p>Today the difference is smaller. RF wideangle lenses tend to be slightly retrofocus to allow metering, and SLR wideangle designs have matured so that their only real drawback is size (although that also depends on the specific lens design).</p>

<p>Definitions (repeating some already posted)<br>

ASPH = contains one or more aspherical lens surfaces. The actual range of things ASPH elements can contribute is very long, depending on the type of lens and how the ASPH is used in the design. May mean better corner sharpness, smaller size, longer zoom range, no focus shift from one aperture to the next, etc etc.<br>

APO = apochromatic = corrects all colors of light to focus in the same plane, eliminating most color fringing. Usually achieved by choice of glass, and usually only important in long lenses<br>

Rectilinear = straight lines appear straight. Note that a rectilinear lens will still distort - e.g. it will still make buildings appear to fall over backwards if tilted up - but the lines will be straight even as they converge in perspective.<br>

Retrofocus = the physical center of the lens is moved moved forwards relative to the optical center, to allow space behind the lens for an SLR mirror or metering system.<br>

Symmetrical = a lens design in which the glass pieces in front of the aperture are very similar to the layout behind the aperture. The Super-Angulon line of view camera lenses are the quintessential symmetrical wideangles - a design also adapted for use on unmetered Leicas.<br>

http://www.schneideroptics.com/ecommerce/CatalogSubCategoryDisplay.aspx?CID=168</p>

<p><em>"</em><em>APO = apochromatic = corrects all colors of light to focus in the same plane, eliminating most color fringing. Usually achieved by choice of glass, and usually only important in long lenses"</em></p>

<p>In Leica-speak, a lens labeled APO uses Leica's proprietary high refractive index / low dispersion glass in one or more elements to help achieve overall apochromatic correction for the lens. This is much like Nikon's use of ED in their lens names, except that Nikon's optical engineers would rather be using Leica's APO glass. Technically, apochromatic correction means that <strong>three</strong> wavelengths meet exactly at the focal plane, not that <strong>all</strong> wavelengths are corrected. Typically when three wavelengths dispersed along the visible spectrum meet, the other visible wavelengths are pretty danged close.</p>

<p>Where Leica's use of APO terminology and Nikon's use of ED terminology now differ is that in Leica's case it means that the APO glass has been used to accomplish apochromatic correction, while in Nikon's case it means that the ED glass was used, whether or not the resulting lens is apochromatically corrected. In Nikon's original use of ED terminology many years ago it also implied that the lens design was apochromatic, or very close to it.</p>

<p>In Leica's case ASPH lenses contain aspherical elements but are not necessarily apochromatic, unless they are APO ASPH lenses.</p>

<p>Normal single element lens can only focus one wave length of light (colour) at a time to correct focusing point. Most lenses are achromatic and can focus two different wave lengths. Apochromatic means that three different wavelengths focus at the same point and since visible light is made up of quite narrow spread of wavelengths, this means that the in-between colours are also very close to optimum focus. Zeiss has some 'Superachromats' that correct four colours to same point, the most famous of them, 250SA, focuses infrared light to same focusing point and does not need to be separately focused for infrared. </p>