While digital technology has produced the most exciting images ever, it also has made lens design more difficult to fully understand. With that in mind, we take a look at what exactly it means for a lens to be designed for digital.
The light that produces an image with digital cameras behaves exactly the same as it did with film. It’s refracted by the optics to a focal point where a miniature version of the image forms. D-SLR sensors react differently than film, however. Made of tiny photodiodes that require light to be collected evenly, sensors are perfectly flat, making it even more important for light to hit the sensor at as close to a 90-degree angle as possible. For this reason, especially on the wide-angle end, lenses need to transmit light precisely, and thanks to modern technology, designed-for-digital lenses do.
The optics of a modern digital lens are made of glass element groupings with various sizes and shapes for refracting light at optimum angles to the sensor. One of the most important lens-design developments has been the advent of aspherical lenses. Aspherical optics use glass and other materials with specialized surfaces to converge light rays to exact focal points from many different directions, which reduces aberration significantly. Aspherics are particularly useful for correcting distortion in wide-angle lenses so that lenses can achieve sharpness from edge to edge. And the best thing about them is that they’re also lighter designs than traditional glass, keeping the overall weight of the lens minimal.
Bigger is often better, but when it comes to D-SLRs, that’s not always true. Smaller sensors can have big advantages when it comes to cutting costs. Because of this, most popular D-SLRs have a sub-full-frame sensor, so named because it’s smaller than the 36x24mm dimensions of traditional 35mm film (35mm refers to the diagonal length of the frame). To a manufacturer, the advantage of creating a sub-full-frame sensor is that they‘re easier to design and less expensive to produce. Thanks to the meticulous designs, sub-full-frame sensors still offer excellent quality with images big enough for most users, and the lower production costs pass savings on to the buyer so that more photographers are likely to invest in new D-SLRs.
The only problem with sub-full-frame sensors is that most camera manufacturers have their own unique design. With film, there was a unifying size to plan cameras around, the 35mm frame. With digital, instead of one standard, there are now competing sensor sizes from many manufacturers, and lenses need to be designed accordingly.
Canon, for instance, has two sub-full-frame sensors within its own line of cameras, the APS-C standard at 22.2×14.8mm and the APS-H at 28.7x19mm. Canon also offers full-frame, as does Nikon, which also has an APS-C sensor at 23.6×15.7mm. Other companies were able to build entire digital systems from the ground up. Olympus offers the Four Thirds System at 17.3x13mm. Sigma offers its Foveon design at 20.7×13.8mm, and Sony, who also has entered the full-frame market with the Sony Alpha DSLR-A900, offers a sub-full-frame APS-C at 28.0×22.3mm.
For Canon, Nikon, Pentax and other companies who already had a large base of photographers invested in their lenses, it was important to develop sensors that were compatible with preexisting lenses in order to persuade people to buy new digital cameras. Naturally, since the sensor is smaller than the size of a 35mm piece of film, and the image is being projected from a lens designed for the larger dimensions of a 35mm image circle, sub-full-frame sensors have a magnification factor. Expressed as 35mm equivalence, this standard is still being used, even for digital-specific lenses. The math by each individual sensor differs, but a 50-200mm zoom on a Four Thirds sensor, for instance, which provides a doubled focal length, actually gives you a 100-400mm zoom.