Thursday, February 5, 2009

Designed For Digital Lenses

From fixed-focal-length prime lenses to wide-angle to telephoto zooms, modern lenses use a complex combination of high-end glass, unique materials and chemical cocktails capable of producing optics with incredible power.
By Dave Willis Published in Lenses
Designed For Digital Lenses
From fixed-focal-length prime lenses to wide-angle to telephoto zooms, modern lenses use a complex combination of high-end glass, unique materials and chemical cocktails capable of producing optics with incredible power. Digital optical design has advanced so much that zooms now can extend up to an incredible 15x range, while wide-angles produce images with almost no distortion.

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.

OPTICAL DESIGN

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.

Digital Sensors

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 22.2x14.8mm and the APS-H at 28.7x19mm. Canon also offers full-frame, as does Nikon, which also has an APS-C sensor at 23.6x15.7mm. 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 20.7x13.8mm, and Sony, who also has entered the full-frame market with the Sony Alpha DSLR-A900, offers a sub-full-frame APS-C at 28.0x22.3mm.

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.

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2 comments

  • Comment Link Barry Young Sunday, March 15, 2009 posted by Barry Young

    With respect to your discussion of the 35mm frame size: The standard 35mm frame was 36 by 24 mm as you state, but the diagonal was about 43.3 mm, not the 35mm you state. Obviously, the diagonal of a rectangle must be longer than either side, and is equal to the square root of the sum of the squares of the sides (check with Pythagoras or Euclid). The "35mm" film designation referred to the width of the film. That allows for a 24mm frame with and margins for sprocket holes where needed.

  • Comment Link Len Straub Tuesday, March 3, 2009 posted by Len Straub

    Although I have a number of both zoom and prime lenses, it's always my preference to use prime when possible. I have both a Nikon 85mm 1:1.8 D and a Nikon AF Micro Nikkor 60mm 1:2.8 in my kit that I have always used for portrature with my D200. Recently I tried to use one of these lenses for a scenic, at infinity, simply because the focal length suited my needs. After a bunch of fooling around I finally realized that neither of these lenses focus to infinity with larger aperatures. I had to switch to manual aperature and stop down. I've always known that all lenses have a minimum focus distance but wasn't aware that many also have a maximum. Would you please comment on this? Users may inadvertently be taking low quality pics without knowing why.

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