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Now, I'm not an expert, so if this post makes you laugh, you're welcome. Still, as far as I know there are basically two components that determine the potential quality of a camera's photos:
I know that sensor technology is still improving over the years, but what about lenses? Is there any development in that area (e.g. smoother glass or something), or has lens technology already been perfected before the digital age?
Yes. There is development in four areas: computer design, material science, features, and finally a category I'm going to call "not better just different".
Lens design has always been a mix of art and science. In the first part of the previous century, art was clearly primary (even for scientific lens designers). Now, lens design software shifts the balance towards science. There's certainly still art involved in making a lens with pleasing rendering, but the science sure helps. Every lens is a compromise between different constraints: optical (aberrations, sharpness, telecentricity, zoom, parafocal vs.varifocal), physical (number of elements, size and weight), and cost (type of elements used, build quality, complication). Software helps designers create a lens within predetermined acceptable criteria, and it lets them test that lens using simulation before spending a lot of money to determine if the concepts are sound.
This software follows both general improvements in the design software (as one might see improvements in Photoshop or in any CAD program), and developments in the fields of optics and photonics. The same advances in computational photography that enable the Lytro light-field camera help out here. And these advances in software in turn are reflected in the modern lens designs created this way.
I'm going to lump improvements in manufacturing in with this category; maybe it deserves its own. Modern manufacturing techniques use computerized machinery to reliably produce complicated individual lens elements, making their use less expensive where they might have been prohibitively costly before.
There are three big areas where this is important.
First, the glass. Different compositions of glass have different optical properties, with varying desirability for photographic lenses — for example, low refractive index, low dispersion, and high light transmittance are all good. Many of the old ways of making glass with desirable properties were quite expensive or have other serious drawbacks. Advances in material science have produced glass with similar properties without those downsides. It's likely this will continue to be the case.
Second, the coatings on the lenses have improved. These are used on all good lenses to reduce glare, which is very important because stray light bouncing around reduces image quality. Newer coatings do this better, more cheaply, and have other desirable properties like repelling fingerprints and dust.
And third: plastics! We're not at the point where plastic elements can replace glass in anything but toy lenses, but plastic is used increasingly in lens construction where metal would be before. In some cases, this is just to make them cheaper with no concern for quality, but when good plastics are used well, they can make a lens lighter and smaller with no compromise to build quality.
I'm going to highlight image stabilization, since that's the obvious one. Modern lenses can offer up to five stops of benefit from stabilization — that is, shutter speeds of up 32× longer with the same sharpness. And newer advances in IS correct for more complicated and different types of movement. Since competition here is fierce and there are a lot of ideas still untapped, expect this area to continue to improve rapidly.
As bokeh — the visual quality of the out-of-focus areas — has become an increasingly important factor, a higher number of a aperture blades and blades with rounded edges are more common. This feature has been available for a long time on premium portrait-lens designs, but now it seems to be almost a must-have feature even on a lower-end "nifty fifty", like those from Nikon and Pentax.
Another example is better in-lens motors, using ring-type ultrasonic designs. And yet another example is the clutch mechanism in newer Pentax lenses, which allows full-time manual focus even with body-driven autofocus. Or, some Pentax lenses have a clever built-in/pull-out lens hood. This isn't anything to do with optical design, but is an example of practical innovation which is really beneficial to the photographer.
Weather-sealing is another feature: there's nothing particularly innovative about that (except some of the material science, perhaps), but fitting it into more lens designs is progress.
As there's more convergence between video and still photography, we'll see some more changes related to that: more silent operation, and stepless aperture settings (rather than being limited to the traditional stops or predetermined fractions thereof; this allows smooth changes while filming without causing jumps in exposure). Arguably many of these features fall into the next category when viewed from the non-video perspective, as for example stepless aperture isn't really a feature with a lot of benefit for still photography.
In this category: changes made to benefit digital, and new designs for smaller sensors.
For digital, designs need to take into account the increased reflectance of sensor material over film. This means that there's more stray light bounced back into the lens than there was before. Additionally, most sensors are less forgiving of light that isn't coming from straight-on, making telecentric design more important.
And, smaller sensors simply means that lenses can be designed with a smaller image circle, or at least with those important properties only optimized for the center without worrying so much about what would be the corners on full-frame. This allows smaller, lighter, and cheaper designs which still offer excellent image quality — Pentax's DA Limited series being the poster-child here, with the smc DA 15mm f/4 ED AL Limited being an example of a recent innovative lens design which incorporates many of the things I've listed above.
There's another change which could be put in this category as well. Many cameras now offer automatic software correction of lens defects like chromatic aberration and barrel distortion. In fact, in some point & shoot and compact interchangeable lens cameras, this isn't even optional — it's just on. The camera communicates electronically with the lens and "knows" how to adjust the image in RAW processing in order to compensate for that lens model's particular quirks. This allows the compromise parameters for the lens design to be different: those factors which can easily be corrected for in software can be left to go wild, and other desired characteristics taken beyond what they could be otherwise. Right now, the focus is mainly on size, weight, and cost, but as image processing gets faster and better, it won't be too surprising to see this thinking come into high-end designs as well.
Speaking from the world of amateur astronomy, there's quite a bit of development happening with lenses. Eyepieces and objectives are all using new, exotic glass and computing resources to design well-corrected refractive devices. New glass mixes don't come along very often and the proper effort to better mate the shapes and characteristics still requires technical skill.
A recent newsletter from Stellarvue (a US telescope manufacturer) discusses the way they work with the different types of glass available to give new twists to older designs. Stellarvue Newsletter
Oh, and if you know an amateur astronomer, mention "Ethos" eyepieces to them. They're a recent design that's become all the rage.
From the world of photography, there's been recent offerings to have newer lens designs that give more light at a straight-on angle at the CCD/CMOS vs the older film designs that could accommodate the different angles. These lenses tend to be called "digital" or digital ready as they're built to give better illumination for the CCD wells.
Additionally, recent offerings have included lenses that take advantage of only having to illuminate a smaller image circle to cover smaller sensors. This allows making the overall lenses smaller and less expensive.
For a survey of how things have changed within a specific brand, you can browse Bojidar Dimitrov's Pentax K-Mount Page. I suggest looking at how the design for the primes and zooms have changed over the years. Still, it would take an optical engineer to describe the details of how and why certain changes were made.
Aside from optical design, anti-reflective coatings have also improved over the years. They've become more durable and more efficient.
Hope this helps!
Bringing down the size and weight of lenses seems to be one of the key areas of focus. E.g., the newer Canon EF 600mm lens with IS is actually lighter than the previous non-IS one (5.4 kg vs 6 kg).
At smaller focal lengths, you have diffractive optics being used that make for smaller & lighter lenses like Canon's 400mm f/4 DO & the 70-300mm f/4.5-5.6 DO.
Besides the LYTRO light field sensor already referred here there are other technologies that caught my attention recently, Liquid lenses, an Organic sensor and a Global shutter.
Patents for this lenses have been submitted at least by Olympus, Samsung and Panasonic (Patents page).
They use an electrical impulse to change the format of a bubble of liquid inside the lens case, which in this case is a lens element itself.
There are already some lenses like this out the market, but they are used on small scale devices like webcams (Experimenting with a liquid lens and driver IC), I think it might be some time until they come out for real cameras.
The advantage of this technology are:
non-movement glasses for zoom or AF
less glasses inside the lens
faster electrical response
non-motor on the glass
silence AF mode
EDIT: As to the organic sensor I only know that Fuji will make it, has the name suggests it uses organic components and Fuji claims that this sensor can surpass the quality of any other FF sensor, more information here.
Finally the global shutter, quoting:
a Global Shutter captures all of the information from every photosite on a chip all at once
this means that it can remove the typical effects from current sensors using the rolling shutter like exhibit smearing, skew, wobble, and partial exposure. More info here.
There are certainly still improvements being made to lenses. The quality control of the glass being produced has improved over time, and also there have been a lot of improvements made to coatings to reduce reflections/glare.
Besides the actual lens elements, there are also a lot of other components of a lens that are still being developed, on great example is IS/VR systems, which have become far more intelligent in recent years.
For a good overview of some of the high-end technology that goes into lenses check out the Canon L Series page. (Yes is is more or less just a big ad for canon, but the page does contain a lot of interesting information.)
Nikon has a Optics Technology section that has a lot of good info as well, but not as much history (and not quite as flashy).
