The mirrorless camera revolution was meant to bring about smaller, lighter camera gear but in reality, camera manufacturers have just taken the opportunity to make bigger, better lenses. The why comes down to the physics of lenses.
Manipulating Focal Length is Complicated
The focal length of a lens—which we’ve looked at in depth before—is the distance between the rear nodal point and the focal point. In a simple convex lens, it’s the distance between the center of the lens and the focal point. However, no camera lens is a simple convex lens; they’re all “compound lenses” which are lenses made from a combination of individual lenses called “lens elements.”
Cameras have a “flange focal distance” that is the distance between the lens mount and the sensor. On Canon’s DSLRs, for example, it’s 44mm. The problem for camera manufacturers is that manipulating focal length is complicated and generally involves adding more lens elements that make things bigger and heavier. The reason Canon’s EF 40mm lens is their smallest is that it so closely matches the flange focal distance and thus requires very few lens elements.
The further you move away from the flange focal distance, in either direction, the bigger a lens is going to be. A 600 mm lens doesn’t need to be 60cm long, but for it not to be 60cm long—which it would be if it were a simple convex lens—the optical design is complicated. It’s the same with an 11mm fisheye lens.
There is a small sweet spot, between about 24mm and 50mm where it’s possible to make lenses that aren’t as big but, for everything else, the optics of manipulating focal length are a significant barrier to miniaturization.
Aperture is a Hard Limit
Aperture is a function of focal length. When we talk about f/5.6, what we’re saying is that the lens iris is open to the focal length divided by 5.6. For example, a 50mm at f/2 has a lens iris opening of 25mm; at f/8, the iris is open to 6.25mm.
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