Glass is evil. - Ken Wheeler

What does this sentence mean? Lately the photography gear market has been flooded with modern lenses and an industry to vouch for their superiority over older lenses. Little do we know, such lenses provide inferior overall image quality to the older generation lenses. I will attempt to vulgarise this phenomenon simply. This isn't EXACT SCIENCE but it has been right on most accounts so here's my explanation.

How a lens works.

In a lens, glass elements serve the purpose to direct the light towards the sensor or to correct for chromatic aberrations. Light is composed of different colors (wavelengths), each colors move at a different speed towards the sensor. Those who arrive early or late create the colorful edge glows on the outlines of high contrast areas; such edges are known as chromatic aberrations (lets call them CAs). To correct for CAs, an additional glass element is inserted next to the first one to “slow down” the colors going too fast and redirect them so that they meet up with the others hitting the sensor correctly.

Glass becoming evil

Glass is a capacitor. Simply put, it absorbs light energy thus slows it down and feeds it into the following glass element. There is a loss of information happening every time this process happens. Using many to slow down light energy could very well create impedance. As less luminous information can be recorded by the sensor as the light passes through each glass elements, the lens has to have a bigger front element to compensate for the loss of light going through the lens barrel and its many glass elements.

Glass becoming more evil

There are many ways for the colors of light to still be uncorrected after the first set of corrective glass element. Light adopts a spiral behavior that spins into the lens. That’s where manufacturers use special compound glass elements (Nikon ED glass per example) to homogenize the light faster in order to save on size. The ED glass is a fusion of glass and other types of minerals that accelerate the CA correction process. Once the light is homogenized, it will provide the effect similar to one eyed vision. If you look at an object (a cup) with two eyes, you'll be able to distinguish what's in front and what's in the back. Close one eye and your depth perception will be confused.

7 elements Nikkor AF 50mm 1.4D vs 13 elements (with 3 ED glass and 1 Aspherical ) Sigma ART. Notice the lack of subject intertonality as well as the low contrast background on the Sigma ART

What is lost at the end of all this?

Perceptual sharpness and details aren't lost but everything else (perceptual color and depth information) is lost. Sharpness and details stay since the light has been highly corrected for such values. As such, high element count modern lenses score very high on scientific tests yet those are never evaluated for perceptual color and depth rendition.

How to view and compare depth rendition

Perceptual depth can be witnessed by comparing a lens that renders a scene similar to how a human would view it with two eyes. The low-element count “depth" lens photograph keeps enough binocular disparity for the human eyes to create depth by viewing it. The high-element “flatter" count lens photograph reproduces monocular vision as it perfectly directs corrected light into the sensor, the result will then be devoid of depth. In real world practice, a “depth” lens would separate foreground and background better than a “flat” lens. Here are some real world comparisons. For a more neutral and serious comparison, please refer to this article.

7 elements Nikkor AF 50mm 1.4D vs 13 elements (with 3 ED glass and 1 Aspherical ) Sigma ART
On a wide landscape, the distance between foreground and background objects is captured clearly on the depth lens but confused on the flat lens. The depth lens also capture drastically more tonal nuances than the flat one.

The next lenses of the future will be flat lenses for sure

The rise of the megapixels have given the opportunity to the industry to convince users to buy lenses that can “resolve” the resolution of the high-mp sensor. As such, the lenses are more and more corrected (more elements) to provide a satisfying level of perceptual sharpness when the image is viewed up close from 100% to 400% zoom at maximum aperture. Many review sites, including those who use those lenses in a “real world scenario”, will convince people in buying flat lenses for the sake of sharpness and bokeh or character. They will never be evaluated for depth rendition where they all fail.

More observations on low-element count lenses.

They are older lenses: lenses made for film (people like shooting film because it feels “more real", now you know why lol) with less rounded aperture blades and less recent coating.
They don’t perform as good as flat lenses at maximum aperture: One needs to learn how to close the aperture well according to the light. The bokeh won’t be as creamy as a new flat lens.
They mostly won’t provide corner to corner sharpness until f11: Downside of less glass correction. Wide angle lenses are the most to suffer low-element count.
They are very cheap to buy used most of the time: Most of the world has been sold onto upgrading to a flat lens.
The theory mostly applies to prime lenses: I have tried many low-element count zooms and the differences are more subtle.
The theory applies to focal lengths above 35mm: there are many compromises made to design lenses of 24 and wider.
Ideal low-element count number is below 9
Most low-element count lenses aren’t fast lenses
They don't have plastic elements: The effect of 1 plastic element (or called "hybrid aspherical") is quite damaging to the depth.
The higher the element count, the closer it needs to be used only at max aperture.
Not well reviewed: every review site on the internet will shit on them.
Darn cheap: The used market is full of them. Chances are the user is selling them for purchasing expensive flat lenses.

More observations on high-element count lenses.

They are modern lenses: Made with the latest sauce on “rounded aperture blades” and lens coating mostly for better bokeh and wide open sharpness.
Maximum aperture champions: they provide maxed out values of sharpness and bokeh (most of the time).
Nightcrawlers: Their high correction gives them amazing flare resistance at night.
They can only be used at max aperture: High-element count lenses have “3d pop” only when the background is blurred. When the background is visible, it is flattened on the subject.
Easier to use for beginners and casual consumers: They are corrected for any aperture.
Well reviewed: huge promotional campaign on the lenses on every lens review site/blog.
Heavy: lots of glass in there.
More expensive: tons of materials more.

Compromises to be made, to buy both or not?

A high element count lens isn’t necessarily bad and a depth lens isn’t necessarily good. I would buy such a lens for its “bokeh effect” when used on max aperture or its ability to provide sharp corners on extreme occasions of need (a specific landscape picture that needs corner detail) then quickly go back to the depth lens for general purpose. Some people prioritize sharpness more than depth . In my case, I tell stories in my images. The lenses I use must be able to respect the composition I see with my two eyes and depth information is also part of the story. On some occasions, I would require “bokeh quality” to help shape the mood, on most occasions I need context. Photography for me is the reproduction of reality. I lose fidelity by using a higher element count lens.