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05-18-2020, 08:19 AM   #1
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Lens design question: what make a lens bigger/smaller?

I'm bad at physics so don't laugh at me I just want to understand more my lenses from physical point of view. Here are my 2 fastest lenses: the recently acquired Pentax DA* 55mm 1.4 (on the left), and the Sigma Art 30mm 1.4 (on the right). Spec wise, they're quite similar: f1.4, 9 elements in 8 groups, has an internal motor (SDM/HSM), except for focal length of course.

My question is:

- Why the Pentax is just slightly longer than the Sigma, having almost 2 times the focal length?
- Why the Sigma is even bigger than the Pentax (the 2nd photo)? Since it's 30mm, to maintain the f1.4, its "opening" should be nearly 2 times smaller that the Pentax's, I can see in the photo, but why its overall size is still bigger? The Pentax is better optimized, or the SDM is small than HSM, or there is another reason?

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05-18-2020, 08:53 AM   #2
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In this case, it has to have been a decision by Sigma to make the lens bigger and appear more high end. It is, after all, an "Art" lens.

Looking at the optical formula:

DA*55 SDM (from the Dmitrov page that is now hosted by the european Pentaxians forum)


Sigma 30 1.4 Art (from LensRentals - first is the old non-Art 30 1.4, 2nd is the Art that we are discussing)


From the picture of the front element of both lenses, one can see that the DA*55 simply has a lot more glass inside than the Sigma. In short, I think they could have made the SIgma thinner if they wanted to.
05-18-2020, 08:55 AM   #3
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All components that are in a lens, such as the number of glass elements, their thickness their curvature and their diameter, the aperture mechanism, an AF motor -if present-, the mechanical parts for floating elements -if applicable-, they all require a certain amount of space. Then the dimensioning and materials used for the construction determine the thickness of the barrel and other parts, and so on, and so on. In the picture you see a cut-through image of a 1.8/85 mm lens from the 1960's.
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05-18-2020, 09:38 AM   #4
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Crude, general rules of thumb:

The bigger the diameter of the front element the wider the body (obviously)
The faster the lens the bigger the diameter of the front element
The more elements and groups the longer the lens

Other design factors will affect the length, girth and bulk of the lens such as internal focusing, retro focusing, zoom, motors for AF, aperture, zoom and image stabilization.

05-18-2020, 09:47 AM   #5
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Not the answer you're looking for, but... the amount of pixel peeping for a given focal length and maximum aperture
05-18-2020, 11:17 AM - 1 Like   #6
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For the OP. Bad at physics, ok we’ll work around that.

Lenses have some simple measures,
- focal length. This is the distance from the lens to the focal plane, when focusing on an object at infinity,
-> higher focal lengths are by default longer than short focal lengths
- registration distance, I.e. the distance from the mount to the focal plane/sensor
-> when the focal length is lower than the registration distance, you need extra elements to manage this making the lenses that are very low focal length actually bigger than they would other wise need to be
- aperture . This is the ratio of the focal length divided by the diameter Of the lens .
-> Fast lenses are ones with big apertures ( big diameters) so you can see fast lenses are bigger than slow ones
- Number of elements
-> makers put different numbers of elements together to correct for all sorts of unwanted distortion, fringing etc, The more elements, to an extent the bigger a lens will be
- prime or zoom
-> zoom lenses take more complicated mechanism and will usually be larger than an equivalent speed prime at any focal length

In addition to this is the High use of internal to the lens focusing motors, aperture control and in some lenses optical stabilization all of which take up room.
05-18-2020, 01:23 PM   #7
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Q. What determines size?

A. Design/performance goals and design constraints, and, to a certain extent, materials/manufacturing/cost constraints



Steve

05-18-2020, 01:50 PM - 1 Like   #8
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QuoteOriginally posted by Bui Quote
Why the Sigma is even bigger than the Pentax (the 2nd photo)? Since it's 30mm, to maintain the f1.4, its "opening" should be nearly 2 times smaller that the Pentax's,
What if I were to tell you that the "opening" size, while a design constraint, is not directly related to focal length or maximum aperture? The two 28mm lenses below are contemporary to each other (early 1980s) and offer similar specifications. The Vivitar is a cult classic 28/2.8 Close Focus Wide Angle (K02) and the Tamron an Adaptal2 28/2.5 (02B). The Tamron is very slightly larger and heavier, but both were considered to be compact lenses at the time.



How can the Tamron be f/2.5 with such a small front element? The answer is that the entrance pupil size* is almost the same for both lenses and that front element size does not always correlate to maximum aperture or light gathering ability. FWIW, on the shelf behind me is a Vivitar 28/2.5 with a 67mm filter size and a very impressive front element (photos at LINK). Ironically, it has the same size entrance pupil (11.2mm) as the Tamron.

The maximum entrance pupil size on your Sigma 35mm is 64% that of your DA* 55mm.


Steve

* F-number is the focal length divided by the entrance pupil diameter. The entrance pupil is what you see when you look into the front of the lens with rear cap off and is not the same as the physical iris opening. You can look at it, no physics required.

Last edited by stevebrot; 05-18-2020 at 02:07 PM.
05-18-2020, 02:12 PM   #9
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QuoteOriginally posted by stevebrot Quote
What if I were to tell you that the "opening" size, while a design constraint, is not directly related to focal length or maximum aperture? The two 28mm lenses below are contemporary to each other (early 1980s) and offer similar specifications. The Vivitar is a cult classic 28/2.8 Close Focus Wide Angle (K02) and the Tamron an Adaptal2 28/2.5 (02B). The Tamron is very slightly larger and heavier, but both were considered to be compact lenses at the time.



How can the Tamron be f/2.5 with such a small front element? The answer is that the entrance pupil size* is almost the same for both lenses and that front element size does not always correlate to maximum aperture or light gathering ability. FWIW, on the shelf behind me is a Vivitar 28/2.5 with a 67mm filter size and a very impressive front element (photos at LINK). Ironically, it has the same size entrance pupil (11.2mm) as the Tamron.

The maximum entrance pupil size on your Sigma 35mm is 64% that of your DA* 55mm.


Steve

* F-number is the focal length divided by the entrance pupil diameter. The entrance pupil is what you see when you look into the front of the lens with rear cap off and is not the same as the physical iris opening. You can look at it, no physics required.
Yes but the front element diameter can not be less than focal length / aperture can it ?. A good check for whether a lens' specs are cheating.
05-18-2020, 02:57 PM   #10
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QuoteOriginally posted by GUB Quote
Yes but the front element diameter can not be less than focal length / aperture can it ?. A good check for whether a lens' specs are cheating.
Yes, that is true, if for no other reason that the physical width of the front element constrains the entrance pupil diameter (pupil cannot be wider).


Steve
05-19-2020, 05:29 AM - 1 Like   #11
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QuoteOriginally posted by Bui Quote
I'm bad at physics so don't laugh at me
I never laugh at someone interested in learning.

QuoteOriginally posted by Bui Quote
Why the Pentax is just slightly longer than the Sigma, having almost 2 times the focal length?
Others have given you elements of the answer. Lens designs vary considerably and it is not possible to evaluate a lens or the quality of tis design by looking at the size only. Designers go down different paths when designing, choosing different ways to optimize (or not) various elements.

QuoteOriginally posted by Bui Quote
Why the Sigma is even bigger than the Pentax (the 2nd photo)? Since it's 30mm, to maintain the f1.4, its "opening" should be nearly 2 times smaller that the Pentax's
You cannot look at the front element and determine the lens' light gathering capabilities, as pointed out by others.

QuoteOriginally posted by ChristianRock Quote
In this case, it has to have been a decision by Sigma to make the lens bigger and appear more high end. It is, after all, an "Art" lens.
QuoteOriginally posted by ChristianRock Quote
In short, I think they could have made the SIgma thinner if they wanted to.
That is not true. The company did not decide to cripple their product, make it bigger, pay more for larger glass elements and their machining, pay more for shippinng because of the size-weight, just to be able to boast that their lenses are bigger. They went withthe size they could manage to get the optical performances they were aiming for.
05-19-2020, 08:45 AM   #12
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QuoteOriginally posted by bdery Quote
That is not true. The company did not decide to cripple their product, make it bigger, pay more for larger glass elements and their machining, pay more for shippinng because of the size-weight, just to be able to boast that their lenses are bigger. They went withthe size they could manage to get the optical performances they were aiming for.
Ummm allow me to disagree with your findings... I never said they made larger glass. I'm the one who posted the optical formulas for both lenses. They are not in the same scale, and I implied that in my response - if you consider that the 55mm is the longer lens, and reduce the size of the picture of the 30mm optical formula, you will see that all the glass elements of the Sigma are quite smaller and the lens is quite narrower.

This is a totally non-scientific scaling but it might give a better idea of what these lenses look like compared to each other, internally. Again, considering the Pentax 55 is the slightly longer lens.



If they look like this internally, then why is the Sigma quite wider? Is their motor that much larger? I kind of doubt it.
05-19-2020, 09:05 AM   #13
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QuoteOriginally posted by ChristianRock Quote
Ummm allow me to disagree with your findings... I never said they made larger glass. I'm the one who posted the optical formulas for both lenses. They are not in the same scale, and I implied that in my response - if you consider that the 55mm is the longer lens, and reduce the size of the picture of the 30mm optical formula, you will see that all the glass elements of the Sigma are quite smaller and the lens is quite narrower.

This is a totally non-scientific scaling but it might give a better idea of what these lenses look like compared to each other, internally. Again, considering the Pentax 55 is the slightly longer lens.



If they look like this internally, then why is the Sigma quite wider? Is their motor that much larger? I kind of doubt it.
If you look at the specs for other modern fast lenses in the 28-35mm range, the Sigma Art 30/1.4 is certainly in line in terms of size and weight.
05-19-2020, 09:08 AM   #14
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QuoteOriginally posted by ChristianRock Quote
If they look like this internally, then why is the Sigma quite wider? Is their motor that much larger? I kind of doubt it.
Take a look at the Sigma line and see if there is a smaller prime in the line-up.* The smallest probably reflects the size of the motor module.


Steve

* Clarification: I should have specified HSM lens for SLR.

Last edited by stevebrot; 05-19-2020 at 10:56 AM.
05-19-2020, 09:51 AM   #15
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QuoteOriginally posted by ChristianRock Quote
Ummm allow me to disagree with your findings
I was not aware that I had proposed findings.

QuoteOriginally posted by ChristianRock Quote
if you consider that the 55mm is the longer lens, and reduce the size of the picture of the 30mm optical formula, you will see that all the glass elements of the Sigma are quite smaller and the lens is quite narrower.

This is a totally non-scientific scaling but it might give a better idea of what these lenses look like compared to each other, internally. Again, considering the Pentax 55 is the slightly longer lens.
Indeed, respectfully it's non-scientific. You can't trust these lens drawings to be a perfectly accurate representation of the reality. An approximation, sure, not a reliable benchmark.

QuoteOriginally posted by ChristianRock Quote
If they look like this internally, then why is the Sigma quite wider? Is their motor that much larger? I kind of doubt it.
Let's apply Occam's razor. You see the lens as being larger/heavier. It could be because the company pulled a "Beats audio" and artificially added useless elements to make it bigger / heavier. Or it could be because the designers did need that volume for the lens to deliver what they intended.

I have not opened any Art lens. I have not seen an actual, usable lens design ray tracing file. Or a STEP file rendering. Or whatever. I do know that guys like lensrentals, who regularly open up lenses, would have a field day of reporting dummy elements, wasted volume, or anything else on that line.

If the Art lens is that way, it's because the company didn't find a way to make it smaller.
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