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On many photo gear review sites, cutaway diagrams of lenses are featured which show the layout of the elements in each lens. Here's the style of diagram I'm talking about, taken from Nikon's website for the 800mm f/5.6 lens:

A cross-section of the Nikkor 800mm FL f/5.6 lens with FL and ED elements highlighted

I'm curious what information can be scraped from this kind of diagram; right now, I see it as:

  • A handy, but superficial, proxy for how much the lens will weigh and where the CG will be (although this does also depend on the density of the glass)

  • An okay resource for understanding which elements may cause glare or internal reflection issues (especially when ED or similar elements are highlighted, as above)

  • An okay resource for understanding where light loss occurs between the front element and the sensor or film reel (although, when this would be useful isn't clear to me; I see no benefit to treating lenses as anything more than a black box in terms of light loss)

It also seems to serve as a source of non-information about:

  • Colour fringing (since this depends very heavily on the design, process, and QA during glass manufacturing)

  • Bokeh

  • Clarity

... Since any sensibly-designed mass-market lens would most likely only have those attributes affected by factors that can't easily be represented in a cutaway diagram like this.

Is there any additional information that can be reliably gained by looking at this kind of diagram? Are any of my assumptions about non-information offered by this kind of diagram wrong?

  • Other diagrams from other sources often show information not included in your example. For a good example of this, please see this question/answers – Michael C Jun 18 at 0:27
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    P.S. If photo gear review sites were sports sanctioning bodies (i.e NFL, MLB, NBA, etc), Ken Rockwell would be the WWF/WWE. – Michael C Jun 18 at 0:29
  • Light loss and flare are largely about the anti-reflective coatings applied to the lenses ... which these diagrams don't show. The presence of ED and Flourite are indicators that the lens was designed to control CA issues. ED = Extra low Dispersion glass. Flourite is an excellent ultra-low dispersion "glass" but this is a crystal that must be grown in a kiln ... very slowly (growing too fast will create imperfections in the glass) -- excellent "glass" but it is an expensive process. – Tim Campbell Jun 18 at 1:10
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    @MichaelC and mattdm -- good point, I'll reword the question. – 0xdd Jun 18 at 13:22
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    "Fluorites" — presumably flourites are used for making bread rather than lenses. :) – mattdm Jun 18 at 13:40
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That all depends upon what information the lens diagram in question offers.

Other diagrams from other sources often show information not included in your example. They may show things such as the location of IS elements, aperture and secondary aperture locations, floating elements, focusing elements, etc.

Consider this Canon published block diagram for the EF 24-105mm f/4 L IS. It shows the location of the primary and secondary aperture diaphragms (the vertical lines on either side of the cyan color-coded Super Ultra-low Dispersion lens element) as well as the location of aspherical (green) and UD (cyan) lens elements.

enter image description here

Or consider this listing for the Canon MP-E 65mm f/2.8 1-5X Macro, which includes block diagrams for when the lens is fully compacted at 1X (1:1 magnification ratio) and fully extended at 5X (5:1 magnification ratio):

enter image description here

This lens diagram of the Tokina 17mm f/3.5 AT-X in this article shows floating elements as the lens is focused:

enter image description here

The same article also offers information on how the arrangement of elements in a block diagram can inform lens characteristics.

As Roger Cicala demonstrates in this lensrentals.com blog post, lens block diagrams allow one to identify lenses with similarities to "classic" lens designs - in this case variations on the Double Gauss design - that can also provide hints at how the lens will perform. This blog post, also from Roger, compares block diagrams of retrofocus designs and Double Gauss designs.

  • The biggest eye-opener here is that element and group layouts have patterns that can lead to predictable behaviours. Thanks for the answer, I never considered that! – 0xdd Jun 18 at 13:27
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A handy, but superficial, proxy for how much the lens will weigh and where the CG will be (although this does also depend on the density of the glass)

Also depends on the size, so even if you get an idea of the scale from the size of the mount, I don't think you get enough accuracy (the weight is going to be the third power...). The CG is expected to be roughly in the middle of the handle/stand.

An okay resource for understanding which elements may cause glare or internal reflection issues (especially when ED or similar elements are highlighted, as above)

If you have any criteria for this please share. The diagram doesn't even show where the diaphragm is.

An okay resource for understanding where light loss occurs between the front element and the sensor or film reel (although, when this would be useful isn't clear to me; I see no benefit to treating lenses as anything more than a black box in terms of light loss)

You don't need a diagram for this, just the accumulated thickness of the lenses.

It also seems to serve as a source of non-information about:

Which lenses are moving, and if it's an internal focusing lens or not.

The only information I get from the diagram is that it seems there is a drawer for an internal filter.

  • If you have any criteria for this please share this may have been a misconception. I assumed that seasoned photographers could glean things like how bad glare might be based on the positioning of e.g. N or ED glass (assuming Nikon trade names). – 0xdd Jun 18 at 13:29

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