LENSESSo if extra magnification is beneficial in most cases, why not use full-frame lenses on sub-full-frame bodies? For one thing, there’s the extra cost of producing full-frame lenses, which are more expensive not only because of specialized production, but also because of a lower market demand. More importantly, magnification is actually a crop factor, which reduces image quality when compared to a lens that has been designed to match a particular sensor.
With smaller sensors, the image circle (the image projected to the sensor plane) also is smaller so optics can be designed to be more compact. Less material is used to manufacture the lenses, so they’re less expensive to make. Lenses designed specifically to match the precise dimensions of a smaller sensor also provide higher resolution by matching the dimensions of a sub-full-frame image field.
Conversely, lenses designed for sub-full-frame sensors can’t be used with full-frame sensors because the image circle is too small, which would cause extreme vignetting—so extreme, in fact, that the image won’t fit the whole sensor, which produces a perspective similar to the circle of a telescope.
MORE THAN JUST OPTICSWith analog lenses, flaws in design would have required an entirely new lens to be produced. Cameras have become computers themselves, though, and lenses now feature digital technology that has been incorporated into the body. Digital communication between the lens and the camera provides autofocusing that’s precise and fast, and often lenses can be updated through firmware fixes that optimize performance even after the lens is in the hands of photographers.
Digitally designed lenses also utilize special coatings for reducing optical aberration, ghosting and flare. This is especially important with the highly reflective surfaces of D-SLR sensors. These coatings are able to shape the light as much as the glass does, and thanks to their introduction, lenses are now much lighter and more precise. All of this should give you an idea of just how complex lens design is. However, while lens construction has certainly become more advanced, the same goal remains: a sharp image without sacrificing speedy shooting.