Axial (longitudinal) aberration?

[Abitconfused]

Do extra low-Dispersion glass elements correct for axial chromatic aberration or spherical (structural?) aberration?

[Dave Humphries]

Hi Ed,

Probably both - if used optimally within the lens barrel - so much depends upon the design of the lens.

Unless you're looking for an answer for an optics exam paper, I'd say; why worry?

If you're being guided by advertising copy, I wouldn't trust that, likely not going to be comparable between models of lenses, let alone brands.

If you have a lens with a problem (be that axial or spherical CA), look up a review for it, then find a lens to replace it that offers better results in those regards.

Or avoid subjects and lighting that highlights the problem with your current lens.

An example: I have a 105mm macro lens that's great in many respects, but showed (I thought) awful axial CA when I shot a band with musical instruments outdoors at wide apertures (e.g. narrow DoF) - and all those specular sun reflections on the shiny knobs and keys that were in front of, or behind, the plane of focus developed colourful magenta or green halos - that took some cleaning up. However, for most situations, it's not a problem.

Dave

[Abitconfused]

Thanks DAVE, I have been trying to sort out terminology and distinguish color aberrations (which result in color shifts or the traditional "fringing") from structural flaws that result in distortions such as pincushion or a general lack of more precise focusing. Coma, for instance, seems to be more related to the overall lens design than the use of any one substandard lens element. But I "need" to sort all this out just because.

[xpatUSA]

Hi Ed,

Probably both - if used optimally within the lens barrel - so much depends upon the design of the lens.

Unless you're looking for an answer for an optics exam paper, I'd say; why worry?

If you're being guided by advertising copy, I wouldn't trust that, likely not going to be comparable between models of lenses, let alone brands.

If you have a lens with a problem (be that axial or spherical CA), look up a review for it, then find a lens to replace it that offers better results in those regards.

Dave, is "spherical CA" a normal term especially when the OP is asking about spherical aberration, not spherical CA.

I understand the other kind of CA (not axial or longitudinal) to be called "transverse", or "lateral":

https://en.wikipedia.org/wiki/Chroma...erration#Types

[Manfred M]

Ed - from an academic standpoint, I find the different approaches that lens designers use to create a specific lens design quite interesting, but as an end user, the simple answer tends to be that all lenses are designed to hit a certain price point that the marketing groups hit sales and profit margins.

By nature, any designer has to work with a set of tradeoffs, and this goes for anything not just lens design. This also includes things like the manufacturing techniques used to make a lens, as the tooling costs have to be amortized over a reasonable time period, so if I know the market for a specific lens is 100,000 units, that will give the designer different options than a lens that will only have sales of 1,000 units.

Low dispersion glass has been used for a long time in lenses to correct chromatic aberration; a high dispersion glass element would be bonded to a low dispersion glass; the classic doublet design. The first element (high dispersion) would bend the light appropriately, but not all wavelengths would focus on the same point. Add the convex low dispersion element to it to correct that issue.

When we see these terms in lens literature, they are little more than marketing documents. I personally only look for two things when I buy a lens; the features (including focal length (range) and maximum aperture) and the price I have to pay. The details, I leave to the lens design team.

[Dave Humphries]

But I "need" to sort all this out just because.

Yes, I'm like that about some things.

Not that I'm an expert, but just because one manufacturer chooses to make an identifiable (say the front or rear) element of a certain type of glass, doesn't mean that another manufacturer, or even another lens design from the same manufacturer, is necessarily fitting 'one substandard lens element' - well, except perhaps the lenses 'built down to a price'.

Which elements within a lens that need to be ELD glass (or have an aspheric curvature on one or both sides) depends upon all the other elements, their sizes and positions, how well they mechanically track when zoomed and/or focused, and how much of them are in the light path, aperture used, etc. Lens design, perhaps more than many things in life, is a massive exercise in compromises, due to their complexity and tolerances of construction/wear, et al.

Dave

[Dave Humphries]

Ted,

Dave, is "spherical CA" a normal term especially when the OP is asking about spherical aberration, not spherical CA.

You may have a point that my answer spoke to two types of chromatic aberration, which happen to be called "axial" (aka longitudinal) and "spherical" (aka lateral), so since the two words were in same sentence, I assumed CA was what he meant.

However, since Ed mentions "(structural?)" alongside spherical and subsequently mentioned geometric issues such as pin cushion distortion, you may have a point, but I think the thread is resolving these terminological issues as it progresses

Added later:
I agree "lateral" is more normally used word for the type of CA that's worse towards the edges of frame, but since it must be worse in corners than anywhere else by dint of distance from centre of the lens, 'spherical' seemed to suit quite well.

Cheers, Dave

[Abitconfused]

I am interested in lens elements as, for one thing, I think the Sigma 50mm Art was done very right, and I would like to understand all aspects of it. So I drew the sketch below to try to understand all aspects of the camera (D810) and lens but am uncertain about all my descriptions.

[xpatUSA]

Ted,



You may have a point that my answer spoke to two types of chromatic aberration, which happen to be called "axial" (aka longitudinal) and "spherical" (aka lateral), so since the two words were in same sentence, I assumed CA was what he meant.

However, since Ed mentions "(structural?)" alongside spherical and subsequently mentioned geometric issues such as pin cushion distortion, you may have a point, but I think the thread is resolving these terminological issues as it progresses

Added later:
I agree "lateral" is more normally used word for the type of CA that's worse towards the edges of frame, but since it must be worse in corners than anywhere else by dint of distance from centre of the lens, 'spherical' seemed to suit quite well.

Cheers, Dave

And so "spherical CA" thereby enters the lexicon of confusing photographic terminology . . .

[Manfred M]

I am interested in lens elements as, for one thing, I think the Sigma 50mm Art was done very right, and I would like to understand all aspects of it. So I drew the sketch below to try to understand all aspects of the camera (D810) and lens but am uncertain about all my descriptions.

13 elements in 8 groups tells me that the lens designer had a "mandate" to create the sharpest lens possible. The highly regarded 55mm f/1.4 Zeiss Otus is 12 elements in 10 groups.

My lowly f/1.8 50mm Nikkor D has a mere 6 elements in 5 groups.

Throw aspheric elements, special refractive glass, etc. and you can get some amazingly sharp imagery.

[Abitconfused]

I got "Spherical" from: https://www.lensrentals.com/blog/201...y-aberrations/ From that site: "Spherical aberration occurs because a spherical lens refracts light that enters near the edge more than light that enters near the center (Figure 7). As a result, the image cannot be focused to a sharp point. A point of light seen through a lens with spherical aberration will have a fairly uniform halo, and the effect is seen in the center as well as the edges of the image."

My belief is that there is no standardization of terminology for discussing lens aberrations nor consistency in describing their cause. Not so much for the simple stuff such as axial (also known as longitudinal) aberration but for the others. Additionally, writers often use less than crystal clear English in their discourse causing confusion. For example, I may have written above "axial or longitudinal aberration" leaving you wondering if I was referring to one or two different lens phenomena.

[dje]

Ed

You might find this article useful.

In summary,

• Low dispersion glass is used (usually in a doublet with "normal" glass) to make an Achromatic lens element whose primary function is to reduce Chromatic Abberation.
• An Aspheric lens element is not purely spherical in shape but has "counter curves" towards it's edges. It's primary function is to reduce out of focus blur at the edges but apparently it does also help with Chromatic abberation.

In your diagram above, you've correctly identified the Achromatic elements (shown as dark grey). The light grey element is an Aspheric element. See this diagram from Sigma (go to tech specs tab).

One more comment on your diagram :-

Your reference to "reducing diffusion in the lens" is not the correct terminology. Diffusion is something quite different to dispersion. Your reference to "light ray scattering" is not really correct either.

Hope this helps
Dave

[tao2]

"My belief is that there is no standardization of terminology for discussing lens aberrations nor consistency in describing their cause. Not so much for the simple stuff such as axial (also known as longitudinal) aberration but for the others. Additionally, writers often use less than crystal clear English in their discourse causing confusion. For example, I may have written above "axial or longitudinal aberration" leaving you wondering if I was referring to one or two different lens phenomena."

Hi Ed,

There are "standard" definitions and 6 main forms of aberration.
Chromatic Aberration - both Axial and Transverse - no Spherical CA. It doesn't exist and is confusing and inaccurate tae use that term.
Spherical Aberration - aye, only spherical...
Astigmatism
Curvature of field
Coma
Distortion
Refraction plays a major part in the first two. e.g. Light rays are affected when they pass fae the air into the glass in a lens (A denser medium). Normal rays, at right angles tae the glass, are slowed down but pass straight through. Oblique rays are also slowed but they are altered in course due tae the angle they enter the lens (refracted). The refractive ability of the lens depends on the materials quality used and the wavelength of light.

CA causes light of different colours tae form FOCAL points at different distances fae the lens. The IMAGE point forms a series of differently coloured POINTS at different distances fae the lens (Dispersion). This colour-separating effect of dispersion is known as CA (colour fringing).The shorter focal lengths tend tae exhibit CA more than others.

Spherical Aberration : A spherical lens is not ideal for sharp photos. The amount of refraction of a ray incident on the lens surface depends,partially, on the angle of incidence (the angle the light ray strikes the lens ). So parallel rays fae a far point on the lens axis meet the lens almost at right angles and pass straight through. Other rays, further fae the axis, hit that curved surface of the lens at an angle that increases, the further they are fae that axis. BUT, since that spherical surface is too curved, the edge rays are refracted too strongly and cut the lens nearer tae the lens surface.Only the central rays (Paraxial) form a point image. The edge rays, and the ones between the edge and the centre, form discs, causing sharpness reduction. Sorry Dave, nae CA...nowhere.
Many lens designers, in their pursuit of ever lower, super, stunning, astonishingly low low low dispersion glass., have been accused of allowing other aberrations tae reappear such as Spherical Aberration.

Ah would've gone on tae the others but ah'll leave them for you tae study at yer leisure....and anyway, ASTIGMATISM ...is reeally difficult tae explain...

[Abitconfused]

Thank you. It will take some time for my small brain to assimilate all of this but you have given me some good leads.