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05-21-2019, 11:01 AM   #1
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Some questions about lens design diagrams and a type of element

Alright, so I've noticed a few things in some of the lens design diagrams in the lens reviews, and I'd like to know what they are.

I'm using the diagram of the Pentax-M 28mm F3.5 as the example because it has the things I'm unsure of. I'll attach a photo with colored circles around what I'm talking about.

On the line that runs down the center of the elements, there are several 'breaks' with small dashes in them. What are these for, if anything? They are circled in RED.

There's an oddly-shaped element in some of these wider-angled lenses. does this type of element have a name? I assume it's what allows the lens to have a focal length lesser than the flange distance, but correct me if I'm wrong. The element is circled in GREEN. I've also seen arrangements of multiple elements in other wide angle lenses that appear to have a similar purpose. Does this type of grouping have a name?


On the element circled in green, there are small indents on the top and bottom. Do these provide any optical function, or is it more of a groove to hold the element by? The indents are circled in ORANGE.


Thank you!

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05-21-2019, 11:38 AM - 1 Like   #2
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QuoteOriginally posted by Ropuchy Quote
Alright, so I've noticed a few things in some of the lens design diagrams in the lens reviews, and I'd like to know what they are.

I'm using the diagram of the Pentax-M 28mm F3.5 as the example because it has the things I'm unsure of. I'll attach a photo with colored circles around what I'm talking about.

On the line that runs down the center of the elements, there are several 'breaks' with small dashes in them. What are these for, if anything? They are circled in RED.

There's an oddly-shaped element in some of these wider-angled lenses. does this type of element have a name? I assume it's what allows the lens to have a focal length lesser than the flange distance, but correct me if I'm wrong. The element is circled in GREEN. I've also seen arrangements of multiple elements in other wide angle lenses that appear to have a similar purpose. Does this type of grouping have a name?


On the element circled in green, there are small indents on the top and bottom. Do these provide any optical function, or is it more of a groove to hold the element by? The indents are circled in ORANGE.


Thank you!
I can't really speak to the design of that central element, but a dashed line like that is the conventional technical drawing notation for a center line. It just represents the central axis of the lens.
05-21-2019, 11:43 AM   #3
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QuoteOriginally posted by Blamking Quote
I can't really speak to the design of that central element, but a dashed line like that is the conventional technical drawing notation for a center line. It just represents the central axis of the lens.
So the locations of the dashes themselves don't really mean anything?

The dashes are all spaced uniquely, so I thought there would be a specific purpose for their spacing and location.
05-21-2019, 12:53 PM - 1 Like   #4
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As Blamking said, that dashed line pattern denotes the centerline. If they are unevenly spaced, it's probably because they are hand-drawn.

The indentations on that thick central element are likely for stopping stray light that could scatter off the frosted edges of that element and reduce the contrast of the image.

05-21-2019, 01:00 PM   #5
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QuoteOriginally posted by photoptimist Quote
As Blamking said, that dashed line pattern denotes the centerline. If they are unevenly spaced, it's probably because they are hand-drawn.

The indentations on that thick central element are likely for stopping stray light that could scatter off the frosted edges of that element and reduce the contrast of the image.
Ah ok, that'd make sense.
Thank you for the information on why those indents may be there! Any idea what sort of element it is?
05-21-2019, 02:20 PM - 1 Like   #6
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That circled central element is most likely a group of cemented or air-spaced pieces. It could be a single thick element but that's impossible to tell from the diagram. Actually, that element (or group) is the main objective which focuses on a distorted shrunken (virtual) image created by the first two to the left of it. That virtual image appears to be very close so this objective along with the elements to its right is much like a macro lens focused on an object very near the front of the lens. That makes the probability that it is an element cluster (corrected for color) more likely. If the two elements to the left were removed, you would have a maco (or nearly micro) lens (but it wouldn't focus anywhere near infinity).
05-21-2019, 02:55 PM   #7
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QuoteOriginally posted by Bob 256 Quote
That circled central element is most likely a group of cemented or air-spaced pieces. It could be a single thick element but that's impossible to tell from the diagram. Actually, that element (or group) is the main objective which focuses on a distorted shrunken (virtual) image created by the first two to the left of it. That virtual image appears to be very close so this objective along with the elements to its right is much like a macro lens focused on an object very near the front of the lens. That makes the probability that it is an element cluster (corrected for color) more likely. If the two elements to the left were removed, you would have a maco (or nearly micro) lens (but it wouldn't focus anywhere near infinity).
So the first two elements are creating a shrunken wide-angled image that's focused very near behind them, and the funky element along with the elements behind it act as a macro lens focused on that image?

What do you mean by "corrected for color"?

Also, how are you able to figure all of this from the diagram?


Thank you for the input!
05-21-2019, 06:18 PM - 1 Like   #8
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Note the different center line styles...

05-22-2019, 09:31 AM - 1 Like   #9
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QuoteOriginally posted by Ropuchy Quote
So the first two elements are creating a shrunken wide-angled image that's focused very near behind them, and the funky element along with the elements behind it act as a macro lens focused on that image?

What do you mean by "corrected for color"?

Also, how are you able to figure all of this from the diagram?


Thank you for the input!
Corrected for color pertains to lenses that are made up of several elements (of different glass formulae) which bend light so all colors focus to one point (ideally). These are called achromatic or apochromatic lenses (the latter are better in that three colors are corrected for whereas achromats correct for two). Hence that "big" element may be a collection of several elements (called a "group") put together to better focus different colors of light to the same final focal plane. This reduces colored halos around objects.

I can't figure that from the diagram - only guessing, but usually the color correction lies in that element rather than elsewhere since it is the main imaging element. Other glass elements can be low dispersion glass (which bends all colors more or less the same amount), but that's only known to the manufacture, who does the lens design.
05-22-2019, 09:45 AM   #10
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QuoteOriginally posted by UncleVanya Quote

Note the different center line styles...
For some reason your image isn't loading for me. What is it?

---------- Post added 05-22-19 at 09:50 AM ----------

QuoteOriginally posted by Bob 256 Quote
Corrected for color pertains to lenses that are made up of several elements (of different glass formulae) which bend light so all colors focus to one point (ideally). These are called achromatic or apochromatic lenses (the latter are better in that three colors are corrected for whereas achromats correct for two). Hence that "big" element may be a collection of several elements (called a "group") put together to better focus different colors of light to the same final focal plane. This reduces colored halos around objects.

I can't figure that from the diagram - only guessing, but usually the color correction lies in that element rather than elsewhere since it is the main imaging element. Other glass elements can be low dispersion glass (which bends all colors more or less the same amount), but that's only known to the manufacture, who does the lens design.
Ah, alright. Thank you for all the information!

You seem to know a bit about this stuff. Do you know of any good resources on this subject that I could use to learn more about it?
05-22-2019, 05:16 PM - 2 Likes   #11
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QuoteOriginally posted by Ropuchy Quote
For some reason your image isn't loading for me. What is it?

---------- Post added 05-22-19 at 09:50 AM ----------


Ah, alright. Thank you for all the information!

You seem to know a bit about this stuff. Do you know of any good resources on this subject that I could use to learn more about it?
The lens is designed with 6 elements, in 6 groups. That is stated in the lens database.That means there are no doublets or triplets, and that the Orange-marked lens is a single element.
I think the peripheral cut-out may be for lens assembly and mounting purposes but don't really know. On further reflection,it is possible that it serves as the internal limiting optical aperture for the lens, as suggested in part by one of the other respondents.

---------- Post added 05-22-19 at 05:33 PM ----------

As you have expressed an interest in further reading on lens design I think you may enjoy reading this link:
Optical Lens Design Forms: An Ultimate Guide to the types of lens design

Last edited by Pentaxis; 05-22-2019 at 05:37 PM. Reason: Added information.
05-22-2019, 06:44 PM   #12
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QuoteOriginally posted by Pentaxis Quote
The lens is designed with 6 elements, in 6 groups. That is stated in the lens database.That means there are no doublets or triplets, and that the Orange-marked lens is a single element.
I think the peripheral cut-out may be for lens assembly and mounting purposes but don't really know. On further reflection,it is possible that it serves as the internal limiting optical aperture for the lens, as suggested in part by one of the other respondents.

---------- Post added 05-22-19 at 05:33 PM ----------

As you have expressed an interest in further reading on lens design I think you may enjoy reading this link:
Optical Lens Design Forms: An Ultimate Guide to the types of lens design
Thank you for the added input!

Maybe the cut-out is for both mounting and optical purposes? Looking inside my copy of the lens, there's a small ring with notches meant for a lens wrench, and it's around where I'd imagine that odd element to be.


And thank you for the link, I'll be sure to look at it a bit!
05-23-2019, 05:42 AM - 1 Like   #13
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Hello,

I just saw your post this morning... Being an optical designer, I can shed some light or confirm a few things.

The horizontal line is the optical axis, indeed the dashed lines do not mean anything. The optical axis typically passes through the center of a lens, and represents the axis along which rays coming from infinity will travel.

The orange circle shows an optical element that's oddly-shaped, but not special from an optical standpoint. I agree with others, it's quite possibly a two-parts element, cemented together. I say this because of the funnel shape on one side, it would be annoying to have to grind-polish this shape. It could very well be a single element, however, the drawing doesn't show this. Common practice is to separate elements with different optical properties (different glasses, refractive indexes, etc) so I would guess that this oddly-shaped element is a single type of glass. In essence, disregarding its shape, it's a simple, if thick, loupe (biconvex glass element). The funnel is probably there either to minimize mass, glass volume (it could be an expensive type of glass) or to control stray light.

The notches are probably simply positioning helps. They might be there to control stray light also, but that's not the way I would do it, so I doubt it's anything special.
05-23-2019, 06:15 AM - 1 Like   #14
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QuoteOriginally posted by Ropuchy Quote
For some reason your image isn't loading for me. What is it?
http://www.informatik.uni-bremen.de/foto-ag/oldpage/F-FA28/Linsenschnitte-28.jpg

Several diagrams of lenses of 28mm.
05-23-2019, 07:44 AM   #15
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QuoteOriginally posted by bdery Quote
Hello,

I just saw your post this morning... Being an optical designer, I can shed some light or confirm a few things.

The horizontal line is the optical axis, indeed the dashed lines do not mean anything. The optical axis typically passes through the center of a lens, and represents the axis along which rays coming from infinity will travel.

The orange circle shows an optical element that's oddly-shaped, but not special from an optical standpoint. I agree with others, it's quite possibly a two-parts element, cemented together. I say this because of the funnel shape on one side, it would be annoying to have to grind-polish this shape. It could very well be a single element, however, the drawing doesn't show this. Common practice is to separate elements with different optical properties (different glasses, refractive indexes, etc) so I would guess that this oddly-shaped element is a single type of glass. In essence, disregarding its shape, it's a simple, if thick, loupe (biconvex glass element). The funnel is probably there either to minimize mass, glass volume (it could be an expensive type of glass) or to control stray light.

The notches are probably simply positioning helps. They might be there to control stray light also, but that's not the way I would do it, so I doubt it's anything special.
Ah, thank you very much for the info!

If the element is two parts, is it only counted as a single element because it forms a biconvex element? In other diagrams, I see multiple elements put together, but they're counted distinctly. (Like the 5th and 6th elements in the M28/2) SMC Pentax-M 28mm F2 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database

---------- Post added 05-23-19 at 07:45 AM ----------

QuoteOriginally posted by UncleVanya Quote
Thanks for the diagrams!
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