The mirrorless camera revolution was intended to create smaller, lighter cameras, but in reality, camera manufacturers simply took the opportunity to make larger, better lenses. Why is reduced to the physics of lenses.

Focus control is difficult

The focal length of a lens, which we studied in detail earlier, is the distance between the rear nodal point and the focal point. In a simple convex lens, this is the distance between the center of the lens and the focus. However, no camera lens is a simple convex lens; they are all «composite lenses», which are lenses made from a combination of individual lenses called «lens elements».

Cameras have a «flange focal length», which is the distance between the lens mount and the sensor. For example, on Canon SLR cameras it is 44mm. The problem for camera manufacturers is that manipulating focal length is complex and typically involves adding more lenses that add more and more weight. The Canon EF 40mm lens is the smallest due to the fact that it matches the flange focal length so closely and therefore requires very few lens elements.

The farther you get from the focal length of the flange in any direction, the larger the lens will be. A 600mm lens doesn’t have 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 tricky. The same goes for the 11mm fisheye lens.

A small sweet spot, from about 24mm to 50mm, where you can make lenses that are not so large, but for everything else, the optics of focal length manipulation is a significant obstacle to miniaturization.

Aperture — hard limit

Aperture is a function of focal length. When we talk about f/5.6, we are talking about the aperture of the lens being open for the focal length divided by 5.6. For example, 50mm at f/2 has a 25mm lens aperture hole; at f/8 the iris is open to 6.25mm.

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While this does not apply to wide angle lenses, it very quickly becomes a problem with fast telephoto lenses. Take the incredibly popular Canon 70-200 f/2.8: at 70mm the lens aperture is 25mm and at 200mm it’s 71.5mm. This means that with infinitely thin materials, the smallest possible front element size is around 72mm — it’s actually 88.8mm — and there’s simply no way to make it smaller.

It doesn’t matter what Canon wants — or Nikon or Sony — they can’t physically make a 200mm f/2.8 lens with a front element smaller than 80mm or so. The laws of physics won’t budge.

Technical developments are a problem

Many old lenses were not very good. They had charm, but autofocus was off, there was regularly heavy vignetting or distortion, and the image wasn’t sharp across the frame. Modern lenses have solved many of these problems by adding more lenses, which of course also increase size and weight.

Similarly, modern developments such as powerful image stabilization add weight to already heavy lenses.

And let’s not forget zoom lenses. The primary lens will (almost) always be smaller and lighter than a zoom lens that covers the same focal length because they are much simpler. Zoom lenses take on, you guessed it, more lens elements and moving parts.

Indeed, physics is a problem

The problem boils down to the fact that the laws of physics are a pain in the ass.

Optics is a well-studied and complex field. Manipulating light so that distant objects appear closer or closer to objects, being even further away, while blurring the background or keeping everything in focus and maintaining a high level of image quality simply requires large, heavy lenses.

The dream of making professional cameras smaller is a dream at the moment.

Image credit: li gh tp o et / Shuterstock, LeonRW

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