Lens specs lingo.

Innocence · Oct 8, 2006

Hello world!

Yet another question! I am very curious heehee.

When I read in a lens specification and it says ...

" X elements arranged in Y groups ... with Z aspherical elements..."

Ok.

I am hoping someone can enlighten me as to:

1/ What information am I supposed to attain from the above statement? (at the moment, it is gibberish to me..haha) Please, technical explanations are very much appreciated (even welcomed!)

2/ I was just about to ask what 'aspherical' is and does, but then I thought Rob would tell me to google it, so i did, and it tells me that it means... "non spherical" haha.

Ok, what does it mean? And more importantly, what does it do?

Thank you~

Don Simon · Oct 8, 2006

The benefit to you, in theory (other than a cheaper and maybe smaller/lighter lens), should be that it helps reduce distortion and other abberations in the image. But a lot of zoom lenses use aspherical elements; it doesn't necessarily mean they're especially good, or even that they're necessarily better than non-aspherical lenses.

ladyphotog · Oct 8, 2006

Maybe this will give you a better idea of what we are talking about. Hope this helps.

I have noticed it more on wide angle lenses, 28mm and below than on others.

DocFrankenstein · Oct 8, 2006

Theoretically, it allows more possibility for the lens designers to make better lenses which perform better and nicer than others.

Practially, it's just marketing. They're used by the manufacturers to reduce the development costs and introduce cheap designs with performances that are barely "good enough" for a consumer not to notice.

So they mean nothing. You have to find out how good the lens design and quality control is yourself.

The more expensive - the better. haha

fightheheathens · Oct 8, 2006

x elements signifies the number of lenses in a lens. so a lens with two pieces of glass would have 2 elements.
Basically, the less elements you have, the less times light has to pass through a glass/air barrier. The less times is does that, the sharper the image.
This is why prime lenses are often sharper then zoom lenses. (zoom lenses by nature will have more elements)

Alex_B · Oct 8, 2006

actually it is interesting to know about how many elements / groups and aspherical elements are in your lens, it will most likely never influence your decision to buy a lens. To decide simply try it out, read test reports, sample images.

I agree on the last post anyway, the more elements, the more loss you will have. but in zoom lenses you need many. not going into tech talk now

Torus34 · Oct 8, 2006

Let's finally answer your question.

X elements arranged in Y groups with Z aspheric elements.

'X elements' is the number of individual lenses -- individual disks of glass.

'. . .arranged in y groups.' If you have two lenses placed together to effectively make a single lens, you would have x=2 lenses arranged in y=1 group. In other words, the difference between the x number and the y number is due to lenses held together to form doublets, triplets, etc. In earlier lens designs, individual lenses were often cemented together. Today they may be either cemented or air-spaced.

Aspheric has been discusses above. Lensmaking is easiest if the surface of the lens is a section of a sphere. The process of grinding and polishing a lens tends to produce a spherical surface almost automatically. When making a telescope mirror, as an example, the surface is first brought to a spherical surface and then slightly changed to a parabolic surface. A parabolic surface [actually a surface of revolution of a parabolic curve] is an aspheric surface -- a surface other than spherical. Another geometric surface other than a spherical surface would be a hyperbolic surface formed as the surface of revolution of an hyperbolic curve. It is more difficult to make a specific non-spherical surface and therefore it is avoided if possible. It is also more difficult to test an aspheric surface for accuracy than a comparable spherical surface.

In designing a lens, one problem is, of course, to focus the incoming wavelengths [colors] of light correctly. With a single lens, light of different wavelengths [red through violet] will come to a focus at different distances from the lens. For b&w film, the result is an unsharp image. For color film, the result is an unsharp image with a colored halo. This is known as chromatic aberration. There are different types of glass [crown and flint, eg] which have different effects on the focal distance of the various wavelengths of light. By choosing lenses of specific strengths [curvature] and types of glass, it is possible to bring all the wavelengths of visible light to a focus at almost the same distance from the lens. Such a lens is called an achromat -- a lens with little chromatic aberration as compared to a single lens.

This is a pretty problem with a lens with a single focal length, but it was solved rather neatly many years ago through such classic designs as the Zeiss Tessar, Schneider Xenon, etc. These lens designs are still excellent even by today's standards.

When you go to zoom lenses, however, you introduce an additional complication. The lens must still bring the various wavelengths to essentially the same focal point even when the lens focal length changes [achieved by changing the distance between some of the elements or groups.] This results in lens formulas with a much higher individual lens count [x]. Add in the requirement that the lens shall not require refocussing as the focal length changes [parfocal] and you have added yet another level of complexity. In some cases, the design problem is best solved through the use of one or more aspheric lens surfaces, even though the cost of production of that surface is higher than a spherical surface.

Finally, there is not only the problem of focussing the various wavelengths of light correctly, but you must also eliminate or reduce to an acceptable level five other types of lens aberrations!

Now, aren't you glad you asked? [heehee -- and also haha!!]

As an aside; when computing a lens formula for a single focal length lens, the fact that the lens can be stopped down eases the problem of equation solutions. Stopping down increases the depth of focus. You don't have to worry about the smaller lens stops quite so much. You can set up your design to maximize the correction at a large lens stop. Since you can assume that the lens will not be used wide open very often, it makes sense to maximize your solution for a stop 2 to 3 stops down from the maximum aperture. From thence comes the old rule of thumb on maximum sharpness.

fmw · Oct 8, 2006

Looks like you got a good bunch of responses. Get a basic photography book and read about lens aberrations. They affect sharpness, color - all kinds of things. These groups of lenses are necessary to partially or completely correct some or all of these aberrations.

What the complex design should tell you is that the lens designer has done what he can to correct the aberrations.

However, complexity itself can be a problem with lens design. As an example, the more lens elements the light has to travel through, the lower the contrast of the lens because some of the light scatters where the elements are joined. That's why zoom lenses are never as contrasty as lenses with a fixed focal length.

So there is some merit in simplicity as well. In my experience the sharpest lenses in the world are those designed for view cameras. They don't have to focus. They don't have to zoom. They correct the aberrations with more simplicity than complex lenses. the images these lenses can make are truly stunning. Take a look at a 4X5 transparency made with a view camera. It will take your breath away.

So, in a nutshell, there is both good and bad in complex lens designs.

Innocence · Oct 9, 2006

thank you torus, for the education haha. :lol:

you are a good teacher.

so, all in all, there is no intuitive understanding of the numbers X and Y ?

I do understand that to gague how good a lens is i need to try it myself (or read up on it!) but surely the manufacturers don't release this [useless?] information purely for the sake of letting us know how many pieces of glass are in our lenses? (or do they?

)

Do pros like you guys use the X and Y numbers at all?

Torus34 · Oct 9, 2006

Thank you, Innocence.

The x, y and z numbers are interesting to lens wonks but as a practical matter you can pretty much ignore them. They allow the manufacturers to tout minor differences as if they really matter. To a large degree, they are the equivalent of the endless performance statistics once quoted on 'hi-fi' gear.

Whenever you wish to buy a lens, you'll do well to search the web for reports on those which fall within your price range. If there is a major difference in the quality of construction or the quality of the resultant image, use the information in making your purchase choice. The search for the 'perfect' lens can result in paying a huge premium for a lens which will have only a marginal difference in image quality.

Always remember that the majority of photographs which we now regard as great were made using lenses which we would consider unacceptably primitive. The real problems the photographer faces today are the same as those faced by the pioneers in the art -- subject, lighting and composition.