What limit do lens manufactures come upon first when making a wide range lens with large aperture (besides economics)?

Question

Economies of selling such a lens would be my first guess to why a lens doesn't exist but I am more interested in the physical characteristics of a lens.

How do the characteristics of a lens affect its design? manufacturing technique, physics, the sheer size of a lens?

What is the complexity in creating a wide range lens with large aperture? For example a 5-600mm f2 lens? Is it the physics of light? manufacturing technique? manufacturing tolerances? Sheer size of the lens when it is built? Lens bulk needed to make a shape?

My question is more on the physics of light and how a lens is designed to meet it. I listed a example with characteristics as a reference in case someone wants to do calculations to show the physical size of a lens compared to its characteristics.

Thanks for helping a noob.

Answer 1

For a simple lens design - since I'm decently new to optics and can't do the math any other way...

A 600mm f/2 lens would need...

f/number = (focal length)/(entrance pupil)

--> 2 = 600/x

--> x = 300mm

So, the front element would need to be at least 300mm in diameter.

Canon released specially made 1200mm f/5.6 lenses which would have, theoretically, had ~214mm front elements.

These lenses were prohibitively expensive and few were made...(I think they released at $90,000 - but are now well north of 6 figures.)

So, even if you pulled one of those elements into your 600mm lens, you'd still only have a ~f/2.8 lens.

So, the reality of the physics of optics has already squashed the idea of a 600mm f/2 - at least as far as creating one that would be marketable (and not just a great science project).

Now, you want to add a zoom to this equation?

Answer 2

The lens acts like a funnel in that it gathers light. The larger it’s diameter, the more the surface area, the more light it gathers. As light rays transverse the lens, the density of the material and the shape (figure) of the glass alters its direction of travel. The shape mimics a lentil seed hence the name “lens”.

The camera lens is a converging lens. Light rays are caused to turn inward. We can trace their revised path. The trace reveals that the light rays take on the shape of a cone of light. This action is called refraction from Latin to bend backwards.

Now a short focal length lens must have a steep figure. We are talking, thin edge, and thick in the center, This shape is like a slice off of a tiny glass marble. A long focal length lens has a gentle figure, while still thin at the edges; these are less fat at the center. What I want you to know, a short focal length lens is a stronger magnifier while a long focal length lens is a weak magnifier. It is by far easier to make a long lens and super difficult to make a short lens.

We can make a lens with most any focal length. Short lenses give a wide-angle view whereas long lenses deliver a telephoto view. Everything is OK when making a fixed focal length lens. Nowadays these are called “prime” lenses. This is opposed to a variable focal length lens, nowadays called a zoom. Think of the complexity of making a variable focus lens. You add a bunch of individual lens elements and stack them in a tube. Now you must construct a mechanism in the tube that moves the individual lens elements, changing their spacing to each other and to the camera body.

As they move, errors creep into the design. These are called aberrations. The lens maker must deal with seven major aberrations that distort and contort the resulting image. Not easy to do when making a “prime” super difficult when making a zoom. In a zoom, the nature of the aberrations change as the focal length change. To counter more lens elements are needed, each with a different figure. Some are cemented together, some are air spaced. Some are dense glass, some are light weight glass. Each added element has two polished surfaces that reflect light. This reflected light is lost as to making an image. Worst, the reflected light mingles with the focused rays and interferes and baths the image with flare.

It’s not impossible to make a broad range zoom but it is not easy and good ones must also be affordable or the design just sits on the shelf. You only make money if the lens sells. It only sells if its good and priced so people can afford to buy it.

Answer 3

Think of light as being the pellets being shot from a sawed off shot gun. As they get further from the source, they spread out. So the further away you are, the less actual light arrives at your location. Now also think of light as being bullets. Each time a traveling bullet hits a surface it is deflected and also looses a little energy. Now lets look at how a lens works. It must be large enough to catch enough light that is traveling at different angles and the lens must redirect the light (reflect) so that it lands in the right spot on the film/sensor to accurately recreate the scene outside the camera.That in itself is a complicated process. This complexity is greatly magnified with each new reflection in the shorter lens. With that out of the way, I can answer the question. The first limit is simply the amount of physical light that enters the lens to start with balanced with the physical weight it would take to overcome that limit. So without some sort of in-lens light amplification, we are stuck with compromising size and weight for efficiency. ISO is our attempt to match the changing sensitivity of the human eye, which collects more light over time. In fact in the dark the human eye starts to amplify the light as it is collected. Electronic ISO adjustments preform this function in a camera to over come the limits of the lens light gathering limits. Think of it this way, If you could attach your camera to a space telescope and be able to carry it around, you would be able to take almost any picture in almost complete darkness provided you could hold it in place long enough.