Symmetrical vs Asymmetrical lens design

[goorackerelite]

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I've always been under the impression that symmetrical lens designs are sharper. Someone please school me on the differences of the optic designs.

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[adan]

Generally, the distinguishing feature of symmetrical wide-angles is that they have less rectilinear distortion. Photograph the proverbial brick wall straight on and level, and the lines will remain straight and square. Whereas an asymmetrical lens will usually put some waviness into the lines of bricks.

 

But with 50 years experience improving asymmetrical designs, that difference has gotten smaller.

 

Classic symmetrical wide designs were the Zeiss Biogon and Schneider Super-Angulon lenses (the Biogon name is a bit debased these days, as used for the Zeiss ZM lenses. Not that they are bad - just no longer specifically symmetrical).

 

At the other extreme - "telephoto" long lenses, where the asymmetry made the lens more compact than its focal length, tended to have pincushion distortion. But again, computerized designs mean the difference is no longer as great as it was when the concept was first attempted.

[pgk]

I've always been under the impression that symmetrical lens designs are sharper. Someone please school me on the differences of the optic designs.

If you compare two 21mm f/3.4 lenses, the Super-Angulon (SA - somewhat symmetrical) and the Super Elmar (SE - not symmetrical and aspheric), there is no doubt that the SE is 'sharper' across the whole image. The SA is very sharp centrally but not towards the edges, so your impression is now incorrect I would suggest.

 

I know that lens design is about balancing compromises. Advances in glass types and computer design has changed and no doubt will continue to change traditional views on how to design lenses and, in the case of digital sensor cameras, there are different parameters (such as the angle of incidence onto the sensor) which now need to be taken into account, so it can be difficult to even compare lenses objectively. I have no doubt that there are books on small areas of the subject of lens design such as this - I have tried to read papers on specific lenses and even these are complex and require a greater base knowledge than I have to understand. So, inevitably, I doubt that you will get much more than a superficial smattering of information about the differences in optic design here:eek:.

[Susie]

Hi,

 

Just a thought but you're not getting Asymmetrical and Aspherical confused are you?

 

Susie

[01af]

I've always been under the impression that symmetrical lens designs are sharper.

The symmetry of lenses depends on the geometric symmetry of their use. If subject-to-lens distance is more or less equal to the lens-to-image distance then the lens should be symmetrical. If not then asymmetrical lenses are sharper.

 

The former is the case mostly in reproduction use when the image has the same (or similar) size as the subject—i. e. at 1:1 magnification.

 

In regular everyday photography, subject-to-lens distances usually are greater than the lens-to-image distance by one or several orders of magnitude, so asymmetrical designs are called for.

 

Sometimes, the term "symmetric lens" is supposed to be taken with a lump of salt, meaning "a lens that is neither telephoto nor retrofocus". And no, those are not inherently sharper than telephoto or retrofocus designs. Whether a lens is sharp or not depends on the lens designer's skill and the lens manufacturer's care and accuracy.

[StS]

For a perfectly symmetric lens design, coma, distortion and lateral colour are zero.

Unfortunately, other aberrations like astigmatism and field curvature can't be cancelled out this way.

A symmetric lens desing also allows to place the aperture (and a central shutter, if applicable) in the centre between both halves of the lens, which should avoid vignetting and could (debatable) contribute to a pleasant out-of-focus image.

 

Stefan

[farnz]

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Much depends what you mean by "sharper" (there have been a number of robust arguments here over the years about what sharpness really means so I don't propose to stray into that area). However, a Double Gauss (symmetrical) lens design does not by definition produce pictures with a higher appearance of sharpness than a Sonnar (asymmetrical) design.

 

As mentioned earlier, there are a number of other variables such as glass formula, aberration correction and ray computation that will affect the appearance of sharpness produced by either lens design.

 

If you're interested in further research Rudolf Kingslake's publications on lens design are an excellent place to start.

 

Pete.

[tobey bilek]

SA design is known for low distortion. They have high vignetting to f16 or 11. If you use small stops, it is best of all worlds. I sold mine within 3 months.

 

Bought the 21 pre asph. Has a wavy line distortion near edges, but far less vignetting. Had mine for 30 years now.

Sweet spot is 5.6.

 

Also have 5 wall hangars all done with the SA 3.4. You need bright light. It also interferes with meter on camera as it sits so far into camera.

 

I suspect the shallow light rays will cause havoc on digital.

[01af]

For a perfectly symmetric lens design, coma, distortion and lateral colour are zero.

Only when the lens is used at 1:1 magnification.

[StS]

Yes, in reality systems are symmetrical-ish, which means not zero but smaller aberrations. However, when people did optics with pencil and paper only, I guess it helped to have some aberrations already reduced.

 

The position of the aperture is only one parameter, which controls vignetting. Another one is the cosine-forth law, which means that the illumination is reduced, since a smaller area of the lens is exposed, if the light falls under an angle on the lens.

 

Stefan

[Michael Geschlecht]

Hello Everybody,

 

I think many people would considrer the 135mm F4 Tele-Elmar to be a reasonably sharp & distortion free lens. It is a STRONGLY asymetric design which began production in 1965.

 

Best Regards,

 

Michael