What's so special about Groups and Elements

4

480sparky

Senior Member
One thing that I've never figured out is what's so special about a lens' Groups and Elements? Why do manufacturers even bother to list them? What purpose does such a specification serve?

Suppose I'm shopping for a new 50mm f/0.45 lens. I've narrowed down my choices to the FlareMaster and the PurpleFringer. Both are 50/0.45s, they're the same size & weight, take the same size filters, etc. But the FlareMaster has 9 elements in 5 groups, and the PurpleFringer has 8 elements in 6 groups.

What would those numbers tell me, and why should I even care? I gotta know, or else I won't sleep tonight!
 
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Mike D90

Senior Member
I probably have this wrong but I recently wondered the same thing while researching some lens information.

I think that the fewer the elements the better the image quality can be. The way they are grouped has to do with lens function and how it disperses the light which I think adds to image quality. Special glass, such as aspherical and ED elements, are sometimes bonded to another element. I do not know what to think about the number of groups though.

Maybe that will get some others to chime in. I doubt I know what I am talking about.
 
Scott Murray

Scott Murray

Senior Member
From Wiki:

Camera lens - Wikipedia, the free encyclopedia

[h=2]Number of elements[edit][/h]Main article: Photographic lens design

Distinct reflections are visible from the surfaces of different lens elements in this 45mm f/2 MD-Rokkor lens. The lens contains 6 elements in 5 groups.​

The complexity of a lens — the number of elements and their degree of asphericity — depends upon the angle of view, the maximum aperture, and intended price point, among other variables. An extreme wideangle lens of large aperture must be of very complex construction to correct for optical aberrations, which are worse at the edge of the field and when the edge of a large lens is used for image-forming. A long-focus lens of small aperture can be of very simple construction to attain comparable image quality: a doublet (two elements) will often suffice. Some older cameras were fitted with "convertible" lenses of normal focal length. The front element could be unscrewed, leaving a lens of twice the focal length, and half the angle of view and half the aperture. The simpler half-lens was of adequate quality for the narrow angle of view and small relative aperture. Obviously the bellows had to extend to twice the normal length.
Good-quality lenses with maximum aperture no greater than f/2.8 and fixed, normal, focal length need at least three (triplet) or four elements (the trade name "Tessar" derives from the Greek tessera, meaning "four"). The widest-range zooms often have fifteen or more. The reflection of light at each of the many interfaces between different optical media (air, glass, plastic) seriously degraded thecontrast and color saturation of early lenses, particularly zoom lenses, especially where the lens was directly illuminated by a light source. The introduction many years ago of optical coatings, and advances in coating technology over the years, have resulted in major improvements, and modern high-quality zoom lenses give images of quite acceptable contrast, although zoom lenses with many elements will transmit less light than lenses made with fewer elements (all other factors such as aperture, focal length, and coatings being equal).[SUP][16][/SUP]
 
Scott Murray

Scott Murray

Senior Member
480sparky said:
I'm not looking for the definition of Groups and Elements. I want to know WHY the manufacturers want my to know how many of each are in a particular lens.
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I do not think it means anything and the reason why Manufacturers include it in the specs page is to make the lens sound better than what it may be.
 
A

aroy

Senior Member
Here is what I have understood of the lense design.

The simplest lense design is the Gaussian doublet. Two lenses separated by space will magnify a distant object and bring them to a focus at a plane. This is a design of two Groups. More Groups are introduced when you want to reduce the length or distortion.

Lenses are cemented together to reduce aberration. Thus the simplest group is one convex and one concave lense cemented together. The reason is that a single element will have different focal length for each wavelength, which is called chromatic aberration. When two lenses, one convex and the other concave, of different refractive index are cemented, then the difference in focal length across the visible spectrum of one is counteracted by the other - they tend to cancel out and (at least theoretically) all the wavelengths focus at the same point. You have now got a chromatic aberration corrected lense - the achromatic lense. Planar design was one of the earliest successful design to improve upon the Gaussian doublet.

Though the Gaussian doublet can be designed to be perfectly achromatic its length depends on the focal length and so does the rear focusing distance. For longer focal lengths you will get very long lenses, and for wide angles the lense will not focus on the sensor as the distance it too short, because the point where the image is formed is at the focal length distance behind the lens's optical centre.

So now you bring in more lenses into the design.
. To shorten the length of Super Telephotos
. To create enough space for a wide angle to focus on the film/sensor. This is called the retro focus design. The extra elements ensure that the image plane is well behind the rear element, enough to clear the space taken up by the mirror.

Extra lenses mean more groups thus larger and heavier construction.

Here are some more cases for the designers to introduce more groups in the lense design.
. Internal Focusing. If you do not want the lense barrel to change the length, you go for another group of lenses, which will move one particular group, while the rest of the groups remain static.
. Faster Lense. For a larger aperture you use a larger front element and a larger opening for the aperture blades. This means that many elements will be bigger (not just the front element) and residual distortion which was insignificant at smaller apertures of the design, will now begin asserting it self. So you add another group to reduce this distortion.
. Correction for sharpness fall off at edges. This is usually a problem with faster lenses. The older F4 or lower had pretty uniform sharpness. The reason is that, say for 50mm design, the internal elements were quite big compared to the aperture, so only the central portion of the glass was used. With faster aperture, without increasing the element diameters more of the lense diameter is used increasing both non linearity and decreasing sharpness. You have two methods of dealing with this. A) Introduce more groups or B) increase the element diameters. Both are used in the current high end fast lenses - Zeiss Otus and the Sigma Art series. That is why their lenses are larger in diameter, bigger over all and containing those extra elements/groups. With so much glass the lense is bound to be expensive.
 
4

480sparky

Senior Member
OK, maybe it's time to rephrase the question.


I'm shopping for a new 50mm f/0.45 lens. I've narrowed down my choices to the FlareMaster and the PurpleFringer. Both are 50/0.45s, they're the same size & weight, take the same size filters, etc. Setting them side-by-side, they appear identical. But the FlareMaster has 9 elements in 5 groups, and the PurpleFringer has 8 elements in 6 groups.

Which one should I choose, and why?
 
Horoscope Fish

Horoscope Fish

Senior Member
480sparky said:
OK, maybe it's time to rephrase the question.


I'm shopping for a new 50mm f/0.45 lens. I've narrowed down my choices to the FlareMaster and the PurpleFringer. Both are 50/0.45s, they're the same size & weight, take the same size filters, etc. Setting them side-by-side, they appear identical. But the FlareMaster has 9 elements in 5 groups, and the PurpleFringer has 8 elements in 6 groups.

Which one should I choose, and why?
Click to expand...
Manufacturers list the groups and elements because that information is part of the standard technical specification for a camera lens. It also gives fanboys, nitpickers and technophile-photographers something to sink their teeth into and argue about. The number of groups and/or elements, by itself, has no bearing on the overall quality of the lens, or the image quality the lens will produce, so it should not be a deciding factor in which lens you should buy.

...
 
4

480sparky

Senior Member
Horoscope Fish said:
Manufacturers list the groups and elements because that information is part of the standard technical specification for a camera lens. It also gives fanboys, nitpickers and technophile-photographers something to sink their teeth into and argue about. The number of groups and/or elements, by itself, has no bearing on the overall quality of the lens, or the image quality the lens will produce, so it should not be a deciding factor in which lens you should buy.

...
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So what do they argue and nitpick about?
 
Horoscope Fish

Horoscope Fish

Senior Member
480sparky said:
So what do they argue and nitpick about?
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Whether X number of groups/elements gives better light transmission, defeats or creates aberration, enhances or degrades contrast, the nature of the epoxy used to glue said elements together and whether said epoxy gives better light transmission, defeats or creates aberration, enhances or degrades contrast and all in relation to Y number of groups/elements.

...
 
4

480sparky

Senior Member
Horoscope Fish said:
Whether X number of groups/elements gives better light transmission, defeats or creates aberration, enhances or degrades contrast, the nature of the epoxy used to glue said elements together and whether said epoxy gives better light transmission, defeats or creates aberration, enhances or degrades contrast and all in relation to Y number of groups/elements.

...
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So an argument between bomb disposal experts as to whether cutting the green wire or the blue wire would be about the same?
 
W

WayneF

Senior Member
480sparky said:
OK, maybe it's time to rephrase the question.


I'm shopping for a new 50mm f/0.45 lens. I've narrowed down my choices to the FlareMaster and the PurpleFringer. Both are 50/0.45s, they're the same size & weight, take the same size filters, etc. Setting them side-by-side, they appear identical. But the FlareMaster has 9 elements in 5 groups, and the PurpleFringer has 8 elements in 6 groups.

Which one should I choose, and why?
Click to expand...



You may be kidding yourself. :) Any f/0.45 lens worthy of purchase probably has more than 9 elements.

Adding more elements is the hardest way, of design, of manufacture, of assembly, of justifying the purchase cost. It is done only because it is necessary for decent performance. Many buy the one with the highest performance. And zooms tremendously complicate everything, additional corrections have to be present for all focal lengths too.

In the early days (speaking of 1960s, when the earliest computer ray tracing was just beginning), Nikon used to publish detailed specs of every lens, including the cross sectional view of all the elements. Number of elements and groups was the FIRST line on the spec page, aperture was second. FWIW, there were mighty few zoom lenses at that time.

Longer telephoto lenses likely were four elements. And also not wider than f/4, which allowed four elements. A 400mm FX lens has a diagonal field of view of six degrees. Tiny angles, so light coming in at a greater angle was of no concern.

Wider lenses, wider than say 50mm, and wider than f/2.8, used many more elements (even back then).

50mm f/1.4 was 7 elements.
35mm f/2.8 was 7 elements.
24mm f/2.8 was 9 elements. (all typically a couple fewer groups)
20mm f/3.5 was 11 elements. (this was in the early 1960s - there is nothing new here).

These were NOT zooms.

Fnumber = focal length / aperture diameter.... so f/2.8 was a large diameter lens at 24mm. Large means that all those light rays from the outer extremes (far from center line) came in at a larger angle than those in the center. These angles require much more correction. This is what the extra elements address. Modern coatings reduce the downside, but groups also minimize internal reflections at the air-glass surfaces.

A 24 mm lens has a diagonal field of view of 84 degrees (FX), and 20 mm (FX) is 94 degrees. Compared to six degrees, these angles are an extreme concern about the rays not tracking right (not being sharp), requiring the additional correction and expense of more elements. A zoom, esp a wide zoom, will require more.

So instead, count your blessings. :)
 
4

480sparky

Senior Member
OK, apparently I need to rephrase the question YET AGAIN.


WHY do I need to know how many Groups and Elements there are in a lens? Does a lens with 9 elements in 5 groups perform better or worse than a similar lens with 8 elements in 6 groups? What does this information mean to me?

I merely used a mythical 50mm f/.45 AS AN EXAMPLE. I was under the assumption that the fictional trade names of FlareMaster and PurpleFinger would be a huge tip-off that the lens doesn't exist.
 
W

WayneF

Senior Member
480sparky said:
WHY do I need to know how many Groups and Elements there are in a lens? Does a lens with 9 elements in 5 groups perform better or worse than a similar lens with 8 elements in 6 groups? What does this information mean to me?
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Maybe nothing, you may be more comfortable to ignore it. The design details are largely beyond us all, and I think it only helps me to justify the cost, and to indicate concern of the design.

I see it as being like bore and stroke in car specs... and trunk cubic feet and rear seat room. Specs, but not concerns for many drivers, nothing we can quote from memory.

I always heard (for decades) that f/0.5 was the absolute conceivable limit, due to the conflicting corrections needed for the wide angles (a lens twice wider than it is long). Of course wider construction is surely possible, but not with acceptable performance. It seems f/1.2 or f/1.4 has pretty much been a practical limit (20% or 40% longer than wide).

Zeiss recently made a f/0.7, but only ten copies were ever made.
 
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