What useful information can one gain from a lens element diagram?

Question

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:

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?

Answer 1

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.

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):

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

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.

Answer 2

Such a diagram can be enormously useful, too, if you are attempting to repair a lens - it all to easily happens that you lose track of which way around a certain element needs to go back in. Having a diagram at hand means you can focus more on the repair and less on documentation work. It can also give you good hints when choosing an access strategy - eg if there are fewer elements to dismantle from the front to access a haze problem near the diaphragm, you go in from the front.

Specialty glasses and aspheres are expensive, and their presence can hint at certain design intents - many telephoto lenses have been built from very ordinary glasses, but it is generally assumed that specialty glasses are more or less a necessity if you want an apochromatic design. A very wide range zoom (10x or so) using no aspheres is likely very cheap or old tech. If you require a low-coma prime lens, you would look at asphere-bearing designs first.

The examples given are of high element lenses of modern and complex design, even if these are prime lenses. With simpler prime lenses, the various basic/classic designs are more recognizable, and there are certain properties known about these designs. For example, with 50mm lenses, in many cases you can recognize them to be of biotar-ish (eg Helios 44 - strongly asymmetric 6 element double gauss), ultron-ish (most f1.4 examples - 7-8 element double gauss with more and thin front elements), tessar-ish (eg, well, a tessar - 4 elements in an easily recognized constellation), sonnar-ish (5+ Elements featuring one enormous block of glass), triplet etc design.

Answer 3

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.