Mirror lenses have long since fallen out of fashion, with refractive optics offering smaller and more functional lenses with vastly superior image quality. However, that could all change soon with new patents from Canon suggesting it is actively working on new mirrored lenses for its Canon cameras.
The latest patents, revealed by asobinet show Canon might be preparing two mirrored lenses, the first is a fast Canon 300mm f/2.4 lens, and the second is a slightly slower Canon 400mm f/5 lens. There's an exceptional range of Canon lenses, but the company has not offered a mirrored lens for decades. Given Canon's recent moves to focus on their mirrorless cameras, these would most likely be for the RF mount.
What are mirror lenses?
Mirror lenses work by bouncing incoming light back and forth inside the lens, before directing the light onto the sensor. This light can be passed through convex mirrors, which multiply the magnification several times, resulting in small lenses with big focal lengths.
When mirror lenses were first released, they were a way to make smaller and more compact lenses, as well as make lenses more cheaply as they have fewer complex parts. However as refractive lenses have matured, we are increasingly seeing lenses become more compact, and manufacturers offering quality budget options.
The main drawback of mirrored lenses is they have a fixed focal length and a fixed aperture, limiting their creative use. And without the complex arrays of glass elements like aspherical and dispersion glass in refractive lenses, mirrored lenses lag behind in areas such as bokeh, vignetting, and color fringing.
With the superiority of reflective optics, and all these issues with mirrored lenses, why would Canon be working on new ones? The answer probably lies in the cost of mirrored lenses. Traditionally mirrored lenses have been very cheap to make, so this would be a fantastic budget option for Canon shooters who want big telephoto focal lengths but don't have the budget for Canon's current lineup. These lenses could also be significantly smaller than what is currently available, allowing photographers to carry more kit while traveling.
Canon may also believe it can improve on mirrored lenses by taking developments from its existing lens lineup, such as various optical glass elements, as well as developments from its latest mirrorless cameras such as in-body image stabilization to offer a brand new experience worthy of it best RF mount lenses.