Just What is a "Digital Lens"?
OK. So you've sold your old film body and bought a new digital SLR. What about your lenses? If you've looked into the subject at all, you've undoubtedly noticed that more and more lenses are being marketed as being "designed for digital." This week we'll take a look at just what this means and whether you should care.
Image Circle versus Sensor Size
With few exceptions, the sensors used in digital cameras are smaller than a standard frame of 35mm film. A lens always projects a round image, but the image created by any given lens will be cropped more on digital than on film due to the smaller sensor. As such, this "crop factor" makes lenses on digital create images that appear to have been shot with a longer focal length than they actually were. When I was shooting film, my widest angle lens was Nikon's 17-35mm f/2.8 AFS. When I converted to digital though, this ultra-wide lens became equivalent to 25-52mm due to the 1.5x crop factor all Nikon SLRs have.
It wasn't until Nikon released their excellent 12-24mm f/4 DX lens that I got my wide angles back. Such a lens would have been unheard of back in the days when film ruled but was made possible because Nikon engineers used the crop factor to their advantage. By designing the lens to create a smaller image circle, they could produce a lens that itself was smaller and lighter (and less expensive) than would be needed for film. Since the sensor in the camera was smaller, the lens no longer needed to produce an image as big as more traditional SLR lenses did. It's still big enough to cover the size of the camera sensor, but if used on a film body the 12-24mm DX lens won't cover the entire frame at all focal lengths.
Nikon has released a number of DX lenses since then, all taking advantage of this same idea. If a Nikon lens is marked as DX it will still focus and expose correctly on film, but may result in severe vignetting on a film body. By only needing to produce a DX sized image, Nikon has produced a growing list of innovative lenses.
Canon has done a somewhat similar thing in the design of their new EF-S lens mount. These lenses produce an image circle sized to cover the smaller sensor used in compatible Canon digital SLRs but not necessarily large enough to cover their full frame sensors or 35mm film.
But EF-S lenses also have an additional design difference from standard Canon EF lenses. The "S" stands for "short back focus" and indicates that the distance from the back of the lens to the camera's sensor is less than that of an EF lens. Cameras such as the 20D, 30D, 350D and 400D that accept EF-S lenses can also use regular EF lenses, but EF-S lenses won't even mount on Canon film bodies or EF mount digital cameras. The mirror in an EF Canon body would actually hit the back of an EF-S lens if Canon let you mount it. So while Nikon DX digital lenses will still work on film bodies albeit with some limitations, Canon EF-S lenses are digital only. And not even all Canon digital since EF-S lenses won't work on bodies such as the 1Ds and 5D that have larger sensors.
Wide Angles, Purple Fringing and Light Falloff
Each individual photosite (pixel) in a digital sensor literally sits at the bottom of a well with a tiny micro-lens on top of it. As light passes through a lens, it crosses over and begins to spread out again in order to focus on the sensor plane. In the center of the image frame, the light therefore strikes the sensor head on, but as we get further out from the center of the sensor the angle of incidence becomes more oblique. This means that light doesn't have a straight shot at a photosite near the edge of the frame, being partly blocked by the walls surrounding it. This results in diffraction at the edges of the photosite, which in turn produces light falloff and a form of chromatic aberration that appears as "purple fringing" along high contrast edges. Light strikes film at an angle near the edges too, but since film is a flat medium rather than a three-dimensional structure as is a digital sensor, the effect is rarely noticeable.
Fringing and light falloff are worse with full frame sensors since the edges are naturally further from the center. They are also much more common with wide angle lenses due to their large angle of view. Some lenses marked as digital have optical designs that help to minimize this problem by adding lens elements to straighten the light path out to a degree as it emerges from the rear element.
Another difference between film and digital is the degree to which the medium itself reflects light. The emulsion side of film stock tends to be somewhat dull and non-reflective, but the surface of the typical digital sensor (or rather the filter that sits directly above it) is highly so. As such, any internal reflections in the lens can create ghosting and other artifacts if not sufficiently controlled. To help do so, lenses designed for digital tend to have improved coatings on their elements. Owing to the increased popularity of digital though, such coatings are becoming common on all new lens designs.
To help control internal reflections, the Nikon 200-400mm AFS VR lens goes one step further. The front element of most "big glass" telephoto lenses is a glass flat but that of the 200-400mm is a curved meniscus.
Digital has in fact created a resurgence of innovation in lens design in order to optimize optical performance. As megapixel counts continue to increase, so too does the resolving power possible from digital. Imperfections tend to be much more visible in digital and as lens manufacturers rework their designs they want to sure consumers know about it. Many of these improvements help of course with both with film and digital capture, so labeling lenses as "designed for digital" in some cases amounts to nothing more than "new and improved."
So, the question remains as to whether you need to worry about replacing your lenses with ones designated as being for digital. In many cases, the answer is no. If you have good quality lenses to begin with, I'd suggest not rushing out to buy new ones just because you buy a digital body. You may well need to get a wider angle lens to offset the crop factor as I did with the Nikon 12-24mm DX, but then your longest telephoto now will have added reach also due to the crop factor so this isn't too bad of a tradeoff. You may decide to get a new lens or two later on of course, but you may have been tempted to even without a new camera. New lenses are always tempting, now aren't they?.