Revision: 0.4, December 6, 2003 (under construction)

Author: Joseph S. Wisniewski

Home: http://www.swissarmyfork.com/digital_lens_faq.htm

Copyright 2003, all rights reserved

Brief Introduction

1) What defines a "digital lens"?

2) What are the different "digital lenses"?

3) Do we really need digital lenses?

4) How about "Wide Converters"?

5) How well do teleconverters work with DSLRs?

A) Glossary

Acknowledgements

References

Disclaimer

Revision History

By now, the average DSLR owner has heard the phrase "digital lenses" often enough to be familiar with the term, but the explanations, facts, and myths are so conflicting that one really does not know what to think. The common myths and flying factoids include:

• Old film SLR lenses don't have enough resolution for digital
• DLSRs always exaggerate chromatic aberration

Here, then, is my pathetic attempt to shine some light into this area…

(Note: This humble FAQ maintainer is a long time Nikon user, and is much more familiar with Nikon lenses than the competition. I do not wish to slight Canon, Pentax, Olympus, etc. shooters. Please feel free to contribute additional information on non-Nikon digital lenses, or the "digital friendliness" of the existing film lenses. All such information would be gratefully appreciated, and will earn you an immortal place in genuine "electronic print" in the acknowledgments section at the end of the FAQ)

There is no real agreement among lens makers and digital camera users as to what constitutes a "digital lens". A "digital lens" could be described as a lens with one or more of these characteristics: long "throw" (strong retrofocus or telecentric design), reduced coverage circle, reduced chromatic aberration, or increased resolution. It may also have other "digital friendly" features, such as:

• A double lens cap or lens shade that provides coverage for both a full sized film frame and a reduced size digital frame
• A focal length range that provides the DSLR equivalent of a popular 35mm film lens
• The ability to easily use lower cost "common size" filters.
• Baffles inside the lens to reduce flare and ghosting and increase contrast. This is important on telephoto lenses.

Film is sensitive to light that strikes its surface at virtually angle, even angles far from perpendicular. For example, some ultra-wide-angle lenses such as the Voiglander Heliar (for 35mm rangefinder cameras) and the Schneider Super Angulon (for large format cameras) actually reach an angle of 60 degrees from perpendicular in the corners of the image. These lenses are symmetric in construction (which is wonderful for controlling lens aberrations) and their "exit pupil" (the point in space from which light seems to originate) is very near the "optical center" of the lens.

Digital sensors are more selective, being most sensitive to light that arrives perpendicular to the sensor. Most sensors use "microlenses", a tiny lens in front of each pixel of the sensor, to increase their sensitivity. Such sensors normally lose 50% of their sensitivity when the light diverges just 15 degrees from perpendicular horizontally (from the data sheets for the Kodak KAI-11000CM "full frame" and KAF-5101CE "four thirds" sensors). To avoid noticeable vignetting, light should be within 12 degrees of perpendicular, horizontally. Even sensors that do not have microlenses have a noticeable decrease in sensitivity by the time light diverges 20 degrees from perpendicular.

This means that any conventional symmetrical wide-angle lens would not work on a digital camera. Fortunately, for the last 50 years, the popularity of SLR cameras caused the creation of many "retrofocus" lenses, lenses that have "virtual" exit pupils much farther away from the film than you would expect from their focal lengths. This was done to increase "back focus" (the physical distance between the rear lens element and the film or sensor) so that the rear elements of the lens would physically clear the swinging mirror in the SLR cameras. It has the benefit of moving the exit pupil to a point around 50 mm from the film plane. Light from a 50mm exit pupil will strike the film within 19 degrees of perpendicular, a big improvement from 60 degrees for the Heliar. So digital wide-angle photography is possible.

Ordinary retrofocus lenses move the exit pupil just far enough to clear an SLR mirror. A lens can be made more "digital friendly" if the exit pupil is moved farther than this. For example, the Nikon 17-35mm f2.8 lens has an exit pupil that ranges from 98mm (at the wide 17mm setting) to 78mm at the 35mm telephoto setting. The Sigma DG lenses also feature such "extreme" 80mm exit pupils. Camera and lens companies refer to such lenses as "telecentric". This is a slight exaggeration: a true telecentric lens has an exit pupil an infinite (mathematically, anyway) distance from the sensor. This causes the light to arrive perpendicular to the sensor. It is also costly, and creates problems, so 80-90mm "near telecentric" lenses are considered the norm.

This table shows that a lens with an exit pupil 52mm from the film plane (typical of Nikon or Canon wide angles) has the potential for severe vignetting on a 1.0x or 1.3x crop camera, and noticeable vignetting on 1.5x or 1.6x cameras. (1.7x and 2.0x cameras are essentially immune to such vignetting). Increasing the exit pupil to 80mm means that the 1.0x camera may have objectionable vignetting, but 1.3x to 2x cameras will have no vignetting at all.

Lenses designed for 35mm film photography project a circular image at least 43mm in diameter (the diagonal measurement of a 24mm x 36mm film frame). Digital cameras usually have sensors that are smaller than the 35mm film frame, the infamous "crop factor". This is due primarily to the cost of making larger sensors. The nice thing about lens design is that you can take any lens design and simply scale it up or down as much as you want to cover any particular format. So the Tessar (a classic "normal" lens) can be scaled to a 45mm lens that covers a 35mm film frame, or a 420mm lens that covers an 8x10 view camera frame. Weight, of course, is proportional to the "scaling factor" raised to some power between 2 and 3.

The only problem here is that the exit pupil of the lens gets closer to the sensor when you scale a lens design smaller. That is correctable by making a retrofocus lens: altering the rear part of the lens (maybe adding an extra element or two) to make the light appear to come from a point farther away from the film plane. This increases the "back focus", the distance from the lens to the film plane so the lens physically clears the SLR mirror, and moves the exit pupil farther from the sensor, so the lens is more "digital friendly".

But the minor additions necessary to increase the back focus don't add an outrageous amount of size and weight to a lens. There is no reason why a 1.5x smaller "DX" lens can't have a diameter about 66% that of a good 35mm design such as the 17-35mm f2.8, and about 1/2 the weight. Length should still be a bit shorter, assume you can take an 80mm long lens, scale it down 66% to 53mm, then add another 20mm at the back for a couple of achromats to increase back focus, and you're still 10mm shorter than an equivalent 35mm lens.

This "scale and extend" method of increasing the coverage circle should make for a cost-effective lens.

Chromatic aberration is the most annoying flaw in a lens used on a digital camera. It causes different colors to come to focus at different places on the imager. Both Bayer sensor and Foveon sensor cameras perform forms of interpolation (guessing) based on colors. If the lens produces colors that are not properly aligned, this guessing process is disrupted, and the results are annoying. Color fringes are brighter and clearer than they would be on an equivalent film camera. Sometimes they glow like neon or acquire really strange colors. So, it is important to have good chromatic aberration performance, even if this means lens design compromises that increase other (less annoying) aberrations.

Aside from these features, the lens may have other "digital friendly" features. The Sigma 8mm fisheye, 14mm ultra-wide-angle, and 15-30mm zoom have "double lens caps". The "main" lens cap is a "slip on" cap. When removed, it exposes the lenses' built in "petal" lens hood (or the full sweep of the 8mm fisheye). This permits full frame "film" coverage. This cap has a lens cap of it's own, a more conventional 72mm or 82mm "snap in" cap. When the main cap is left on the lens, and just the snap in cap is removed, the lens covers a reduced frame "digital" image. Unfortunately, it's a 1.7x crop image, since these lenses were designed in the late 90's and this feature was added for the Nikon/Kodak, Canon/Kodak, and Nikon/Fuji hybrid DSLRs having crop factors in the 2x range.

DSLR photography demands higher lens resolution than its film counterpart. First, because most DSLRs have crop factors ranging from 1.3x to 2x. To match the resolution of a film camera on the final print, a DSLR needs a lens with 1.3x to 2x the resolution of a film camera lens (depending on crop factor).

Second, there is a very interesting paradox: while digital SLRs in the 6mp range cannot match the resolution of a fine-grained ISO 100 slide or print film, DSLR pictures are frequently enlarged to a greater extent than 35mm film images. This is because the DSLR has increased the popularity of the "digital darkroom", so DSLR users are printing larger prints than were typical in the "film days". A3 and "Super B" 13x19 inch prints are common. And cropping (which requires still greater increases in magnification) is increasingly common. So, combining the magnifications of the crop factor and the larger prints, DSLRs need very high-resolution lenses. Lenses that are "diffraction limited", having resolutions equal to the theoretical maximum for a lens at a particular aperture, are sought after.

Fortunately, many existing prime lenses already have such resolution. These are often "legendary" lenses, such as the Canon 135mm f2.0 L, the Nikon 45mm f2.8 and 85mm f1.4, the Pentax 31mm f1.8 "limited").

One of the difficulties in using regular "film" lenses on a digital camera is that the focal lengths become a bit uncomfortable. For example, every lens manufacturer offers a 24-70mm, 28-80mm, or similar zoom, because it is a very handy focal length range for event coverage, or just general-purpose photography. In fact, this is considered a "standard" zoom lens theses days: the original 50mm "normal" +/- 25mm of zide or telephoto zoom. With crop factors of 1.3x to 2.0x, these lenses becomes more like 45-120mm zooms. The wide end isn't "wide" anymore; it's merely "normal". The Olympus 14-56mm, Nikon 16-55mm, Canon 18-55mm, and Pentax 16-45mm are the first lenses to offer equivalents to the "standard" zoom on a cropped camera.

Additionally, many lenses that have usable focal lengths when cropped are still "built wrong" for those purposes. A 50mm f1.4 (common "film" normal) is a "double Gauss" design. This gives it certain image characteristics that are useful for general photography: high speed, neutral bokeh (neither macro lens harsh nor portrait lens soft), good sharpness, flare, etc. and quite affordable. On a 1.6x crop camera, it acts like an 80mm lens, and would seem like the perfect portrait lens. But the 85mm lenses that film photographer are used to for portraits have very different characteristics. The bokeh is more attractive, the lens is a little sharper and has better contrast.

Even in the "film days", some lenses exhibited an annoying problem, often referred to as a "center spot". This is a large spot, often red or blue, in the center of the image. It is seen whenever the center of the image is dark, and substantial portions of the outer part of the image are light. Portraits where the background is light and the clothing is dark, and concert photography are the most common "center spot" situations.

The center spot is caused by a reflection from the film (or sensor) back into the back of the lens, and then back to the film or sensor, of the aperture of the lens. In fact, the spots are often sharp edged, and clearly show the 7 or 9 sided shape of the aperture.

Digital cameras have been known to exacerbate this effect. In general, digital sensors are less reflective than film (this is why Nikon and Canon cameras do not have conventional "off the film" TTL flashes, but use "pre-flash" systems that measure TTL flash off the shutter curtain, before the shutter opens). The problem is that what little reflection they have is almost entirely "specular reflection", bouncing light directly in the direction it came from, right back into the back of the lens. This is because the CCD (or the IR cut or AA filter in front of it) is a highly polished, optically flat surface. Film is more reflective, but it is a "diffuse reflection", light is scattered all over, instead of "beaming back" into the lens.

Some digital cameras are more susceptible to this effect than others. The Fuji S2 seems to be especially vulnerable (possibly due to having a CCD or filter that is reflective enough to permit true "off the film" TTL). The Kodak 14n is another camera with "spot problems", possibly the most severe in the indistry.

So far, there are several different "digital lenses" either in production, or announced for near future production. Nikon has the "DX" lenses. Olympus has the "four thirds system". Sigma has "DG" and "DC" lenses. Canon has "EF-S" lenses. Tamron has "Di". Schneider offers the "digitar" lenses for digital medium and large format. Each of these offerings approaches digital in a different way.

Some are "ground up" designs, entirely new and optimized for the reduced size digital formats. The Nikon DX lenses and some of the Olympus "four thirds" lenses fall into this category.

Most are "kit lenses", designed to sell at rather low cost with DSLRs. These are often very easy to design. By taking an existing wide-angle zoom lens and simply extending the "range of motion" of the lenses elements that move inside it when it zooms, you can increase the wide-angle zoom capability of the lens, at the expense of the coverage circle. Since the DSLRs do not requite the lens to cover a 35mm "full frame", this means that the extended wide angle range is essentially "free", requiring only mechanical modifications to the lenses. Sigma, Canon, and Pentax have such "kit lenses", and there is some argument that at least some of the Olympus lenses also fall into this category.

Sigma says that the DG lenses are "designed specifically for digital SLR cameras" and that the lenses "feature superior light distribution, so that there is minimal light fall-off or vignetting, even when used at maximum aperture". Aside from this, they are not saying what constitutes "designed specifically for digital". From Sigma’s lens data sheets, and from measurements on several Sigma lenses, we see that the most noticeable design feature is that the DG lenses have large back focus distances. This means the light from the lens is nearly perpendicular to the sensor, even in the far corners of the sensor. This prevents vignetting (dark corners) and reduces chromatic aberrations (color fringes). This is especially important on cameras with large sensors and cameras that use "micro lenses" on their sensor to increase sensitivity.

The Sigma 20, 24, and 28mm DG lenses have exit pupils from in the 80mm range. Equivalent Nikon and Canon lenses have exit pupils in the 50-55mm range. The lenses do not feature reduced coverage circles, or noticeably less chromatic aberration than wide angle lenses from other manufacturers.

It's strangely ironic that Sigma's DG lenses appear to be optimized for performance on any digital camera, except Sigma's own. The original Sigma SD9 camera has the smallest sensor in the industry, and it's one of the few cameras to not use microlenses, so lenses with exit pupils in the 50mm range are not a problem for the SD9. The later SD10 uses microlenses, but they are relatively mild, since the original sensor cells are fairly large. And the sensor size is still small.

This is probably the very first "digital friendly" lens, so it deserves special mention. The Voigtlander Heliar proved that a relatively small, cheap 12mm lens could be built for a rangefinder film camera. In designing the retrofocus 14mm for SLR cameras, Sigma appears to have taken the SRL design one step further. It has two "digital" features, a 65mm exit pupil, where 50 would have done fine (and probably reduced distortions on film a little bit) and the double lens cap. The 14mm f3.5 and f2.8 predate Sigma's "DG" designation. I don't think Sigma would want to try to "re-brand" it a "DG" lens after it was already on dealer's shelves.

Another line of low cost "kit lens" lenses. These have reduced coverage circles. This includes the 18-50mm F3.5-5.6 DC and the 55-200mm F4-5.6 DC.

Nikon has announced that there will be four of these, featuring reduced coverage circles, reduced aberrations, and lens hoods matched to the 1.5x crop of the Nikon D1, D100, and D2 series cameras. They are high quality (and expensive) lenses.

The lenses are part of Nikon's professional line, with multiple aspheric and low dispersion elements to control aberrations, constant apertures, and fairly small zoom ratios. Nikon is expected to announce a line of low cost DX lenses when they launch their "entry level" D70 DSLR in early 2004.

The 12-24mm DX has a reduced coverage circle, since a 12-24mm zoom isn’t really practical for 35mm full frame. It is essentially a conventional 18-35mm lens, scaled down by a factor of 1.5x, with additional enhancements to make it more "digital friendly", and improvements to decrease aberrations.

Another reduced coverage circle lens, Nikon is claiming that this lens will be even sharper and better corrected than their 17-35mm f2.8, a modern legend.

An interesting lens, it delivers a full 180-degree coverage across the diagonal of the image. This is equivalent to the popular 15mm fisheye on full frame film. Nikon is offering an option in their latest software to convert the fisheye image into a "rectilinear" image, corresponding to a true 10mm wide-angle lens on 35mm film.

Nikon ties with Sigma as producing the first "digital friendly" lens that I have direct experience with. The Nikon 17-35mm f2.8 AF-S appears to have been designed to fill the needs of both film and digital photographers. Its aberrations are so well controlled that it defines the state of the art in ultrawide zooms. And its exit pupil location is impressive, ranging from 78mm at the 35mm telephoto setting to 98mm (at the wide 17mm setting). This is long enough to insure virtually undetectable vignetting on a 1.5x crop camera, and excellent performance even on a full frame DSLR.

This may be the most confusing "digital lens" system of them all. Some consider "four thirds" to be an entirely new, digitally optimized system. Others consider it to be a simple repackaging of existing Olympus OM lenses. We’re still sorting it out. Right now, no one that I know of has access to the "open specification" for the 4/3 system (as announced jointly by Kodak and Olympus) so this section will refer specifically to characteristics of the Olympus "E-System" flavor of "4/3". This appears to be a rather advanced digital lens system, having virtually every desirable "designed for digital" characteristic.

Again, keep in mind that these may be Olympus "E-System" specific features, and not parts of the 4/3 system in general.

Olympus refers to the E-System as "telecentric". They define this as restricting the incident angle to within 6 degrees of perpendicular at the corners of the image. While not truly telecentric, this does require the exit pupil to be at least 85 mm from the sensor, over twice what is required for the 21.8mm image circle of the 2x crop "4/3 system".

The lenses also claimed to have very well controlled aberrations and high resolution. The diagrams furnished by Olympus seem to support this, showing complex, modern designs with aspheric and low dispersion elements.

Lens hoods, of course, match the sensor, since there is no film body "counterpart" to the digital "E-system".

Another interesting "digital" feature is that each lens contains a memory chip which carries image correction parameters (barrel and pincushion distortion, vignetting, and, we would assume, chromatic aberration) allowing the camera to correct these distortions digitally, on the fly.

Tamron has two "Digitally Integrated" lenses. According to Tamron:

This is a nice assortment of features. The first two Di lenses are the 28-75mm f2.8 zoom, and a 180mm f3.5 1:1 macro. These lenses are long enough so that reducing the coverage circle would not noticeably improve their size, weight or performance. And f2.8 zooms in that range from 24-28mm up to 70-105mm have exit pupils sufficiently far from the sensor that they do not need additional telecentric redesigns. Nevertheless, these lenses are compact and have good performance, and have gained a strong fan following.

At the moment, there is only one lens in this system, the 18-55mm announced for the Canon 300D (Digital Rebel, KISS) body. It may not be proper to say that this is a "designed for digital" lens, or even a "digital friendly" lens. But EF-S is an interesting technique to increase the wide-angle zoom range of a low cost "kit zoom" lens, and it is only applicable to digital cameras, so we will cover it here. EF-S takes advantage of the fact that a 1.6x crop digital camera can have a much smaller mirror than a 35mm "full frame" film SLR. Full frame cameras have mirrors that are approximately 36mm wide and 33mm deep. This means that no part of the lens can come closer to the sensor than about 35mm, or the moving mirror will strike it. Until now, most DSLRs have used "borrowed" film camera parts, including the mirror. This is because it's more cost effective to use parts that were already tooled and tested for million unit/year film cameras, than to design new components for ten thousand (maybe 100,000) unit/year digital SLRs. The Canon 300D changes that, being the first DSLR actually designed to "go gold" and sell a million units the first year.

A 1.6x crop digital only needs a 22x21 mirror. This allows a lens to approach within 25mm of the sensor. In general, this is not much of an advantage. But for wide-angle zooms, it offers something unique. A wide-angle zoom typically zooms from its longest setting to its widest one by moving an inner group of lenses forward, and the rear group of lenses back towards the sensor. If there's more room to move the back element farther back, the zoom can zoom even wider. This also reduces the coverage circle, but this isn't a problem if you've got a 1.6x crop DLSR. This has allowed Canon to take an existing lens, the 22-55mm f4-5.6 (or at least start with a similar design), and stretch it into the 18-55mm. And this allows them to hit a $99 (US) target for a "kit lens", similar to the 22-55 on film cameras. It is a comfortable lens for fans of 28-80mm lenses on 35mm film cameras, offering the equivalent of 29-88mm coverage.

Aside from this, we would expect a lens hood that is properly matched the 1.6x crop, and not much else from such a lens. Although tests show it has surprisingly good resolution, and early feedback is favorable.

Schneider states that this series of lenses " provides the ultra-high resolution required by today's CCD sensors, and by the next generation as well. This assures the highest possible image quality".

Pentax DA lenses are similar to the Canon EF-S, in that they are oriented around low cost "kit lens". The first is the 16-45 DA.

The answer is a firm, definite "maybe". For reduced size cameras with a crop factor, many "film" lenses are excellent performers. Vignetting isn't much of a problem. It is possible (and really not that complicated) to alter existing sensor designs so that they do not have any vignetting and corner chromatic aberration sensitivities, and will respond just like film.

It may be sufficient to just have a good database of which lenses are the real "winners" on digital, and to have a few "digital" wide angles added to round out the system. The camera manufacturers aren't going to be much help for something like that database, but the online community seems to be heading off in that direction already. Such a listing should describe which lenses have vignetting or CA problems (until altered sensors become the norm) as well as which have sufficiently high resolution to tolerate crop factors and large prints.

Another useful idea is to leave the lenses alone, and tweak the DSLR sensor a bit to make it a bit more versatile in the lenses that it will accept. You need to alter the acceptance angle range without distorting the image, or removing substantial light. The easiest way, for a sensor that already has microlenses in front of the sensor cells, is to alter the microlenses so that the sensor acceptance angle is different. To preserve the light gathering power of the microlenses (and the overall sensitivity of the sensor) the focal length of the microlenses should not be changed. Just their placement should be altered. Near the center of the sensor, their placement should remain the same. Towards the edges of the sensor, they should be moved inward slightly. The change would be gradual, so that there was a smooth variation in the pattern from center to edge.

This would alter the center of their cones of sensitivity (the "chief ray" of their directional pattern) so that they are directed slightly inward at the edges of the sensor, perhaps 6 degrees. This would mean that a sensor that was tolerant of light up to 12 degrees from perpendicular (within the range of the existing Sony, Canon, and Kodak sensors) could now tolerate light up to 18 degrees from perpendicular in the corners. This would be sufficient so that even the largest "cropped" sensors, such as the 1.3x Canon, would work well with existing "film" lenses, like Nikon and Canon primes having exit pupils in the "conventional" 50-55mm range. A lens that already has light nearly perpendicular to the sensor (a "digital ready" telecentric lens, or a long telephoto lens) would still work perfectly well, since the angle alteration would only be 6 degrees, and the sensor is tolerant of 12 degrees.

A second solution, if the sensor manufacture does not wish to alter the microlens pattern, is to use a focusing lens. This needs to be very near the sensor and flat: a Fresnel lens or a diffractive optical plate. Either approach could be etched or pressed into an optical coating on the front surface of the chip, or on the front or rear surface of an additional optical plate, such as the AA filter or IR cut filter. Again, it would be a subtle effect, a 6 degree alteration towards the edges of the frame. A Fresnel lens with a 1.5 micron pitch would be sufficiently longer than a wavelength of light to insure it passed the light instead of diffracting it all over the camera. That pitch would also insure that six Fresnel lines crossed each pixel of a modern 8 micron 6MP APS sized or 12MP full frame sensor, so moiré would not be a problem. It would even protect for sensor pitches as low as 3 or 4 micron, which would be good enough for 24MP APS sized sensors, or 50MP full frame.

Full frame sensors would be amicable to the treatment discussed in section 3.1. It might be preferable to have the altered microlenses (or flat lens) provide 9 degrees of correction in the corners instead of 6. This is still compatible with 12 degree sensors (which seem to be about the norm today) but would provide sufficient correction for "old fashioned" primes with exit pupils just 50mm from the sensor.

The focal reducer (or wide converter) is a device that goes between the lens and the camera to increase the field of view of a lens, just as the common teleconverter reduces the field of view. Such devices are well documented. Astronomers refer to them as "focal reducers". (Then again, astronomers refer to "teleconverters" as "Barlow lenses", so they're a bit of a strange lot. Always scurrying about in the middle of the night…).

Here's a nice "focal reducer" link.

http://astro.martianbachelor.com/CB245/ReducerDesign.html

That's the basic principle. In practice, a single lens isn't going to do well. A pair of achromats seems to be about the minimum that will get the job done (just like teleconverters). More sophisticated optical systems will deliver better results, such as the seven element teleconverter designs popular today.

My own work is split into two designs, one that works well with wide angle lenses (primarily for increased wide angle coverage) and one that works with telephotos (to increase speed, allow shallow depth of field, and restore lenses to more useful "portrait" lengths).

Kodak has a patent on the basic concept of the "wide converter".

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/netahtml/search-adv.htm&r=1&f=G&l=50&d=CR86&S1=4,591,234.UREF.&OS=ref/4,591,234&RS=REF/4,591,234

Claim 3 is very specific:

The key difficulty with this is that prior art (of which there is much) all deals with astronomical "focal reducers", which all reduce back focus. On a telescope, this is not typically a problem, since their optical systems allow a little extra back focus, for placement of accessories. It is a problem on an SLR "wide converter" because back focus is minimal.

You'll often hear people say that extra elements hurt the picture, adding aberrations, or reducing light. This, of course, is absolute nonsense. While extra elements, applied improperly, will cause image degradation, using them right will improve the performance of the optical system. Yes, each element you add to an optical system adds some aberrations, some attenuation, and some reflection (increasing the risk of flare). But each element you add contributes to the system, correcting some of the aberrations introduced by other elements. If the system is properly designed the beneficial correction from each additional element more than offsets its additional attenuation or distortion. This is why you've probably never taken a picture with any lens having less than four elements.

In the case of the wide converter, reducing magnification brings all sorts of benefits. The prototype 0.66x "wide converter", for example, reduces most of the aberrations of the main lens by 33%. This includes flare (per unit area), chromatic aberration, softness (resulting from coma, astigmatism, or spherical aberration), and any aberrations resulting from centration problems (the lenses elements not all being precisely lined up along the common optical axis). In fact, your simple little 4 or 6 element system would have to increase any of these problems 50% in order to overcome the aberration reducing effect of the reduction in magnification.

As far as light loss, even the single coated prototype has a 2% per surface loss, which is 8% for the four surfaces. Since the focal length reduction provides a 112% increase in light density, an 8% loss is negligable. There's still over a full stop of light gain.

Generally, not as well as film. It has to do with "final magnification". Digital photography hits us very hard in this area. First, unless you own an expensive and exotic "full frame" camera like Canon 1Ds or Kodak 14n, you have a crop factor to deal with: 1.3x, 1.5x, 1.6x, 1.7x, or 2.0x. This is exactly the same as if you already had a teleconverter on the camera, enlarging a central portion of the lens's image circle to cover the entire final print. (Although the "crop factor" teleconverter is a more "perfect" teleconverter than any conventional teleconverter optical system you could put between the lens and the camera. Regular teleconverters have to work at a non-optimal position about 50mm away from the final image. Their optical system has to be a compromise, because they can be used with a number of different lenses, each of which has a different rear node and exit pupil location. The crop factor "teleconverter" works directly at the focused part of the image, where all the aberrations are minimized).

Second, you're typically printing larger than you did with film, or looking at it closer (1:1 pixel level on the computer screen).

So, the end result is, if the lens has just enough resolution to look good with a 1.4x or 2x teleconverter, you've already "used up" this reserve of resolution with the crop factor and the larger prints. For many zoom lenses, that's all they've got to give. Many lenses are sharp enough to look fairly good with an additional 1.4x teleconverter, but only a few legendary lenses are sharp enough to get by with a 2x converter, or a stack of teleconverters.

However, the same rule that applies to film use of teleconverters also applies to digital photography: "If the teleconverter means the difference between not having the shot at all, and having a slightly unsharp shot, go for it".

Special thanks to Richard Gosler, for the information on the Schneider "digitar" lenses

KODAK KAF-5101CE Image Sensor Device Performance Specification, July 18, 2002, Revision H.

KODAK KAI-11000M KODAK KAI-11000CM Image Sensor Device Performance Specification, December 9, 2002, Revision 1.

"Frequently Asked Questions...and some helpful answers", Sigma Corporation, http://www.sigmaphoto.com/html/faqs.htm (captured May 1, 2003)

"Tamron USA, Inc - Di Site", http://www.tamron.com/di.htm (captured August 29, 2003)

A lot of trademarks appear in this FAQ. They are all the property of their respective owners. I am not associated in any way with any of these folks. I own no trademarks. I was given the name Joseph Stanley Wisniewski, long ago, by a couple of nice people.

Initial draft

Reformatted document

Expanded several sections

Added sections 2.2.1, 2.2.2, 2.2.3, 2.2.4, 2.3.1, 2.4, 2.5, 2.6, 3, 4, 4.1, 4.2

Added the glossary and acknowledgements

Added info on the Pentax DA and Tamron Di lenses

Added sections 1.5, 3, and 5

Changed major section headings to questions (it is a FAQ, after all)

Added the index, disclaimer, and revision history

Fixed minor section numbering errors

Finished sections 1.4, 1.5, 3, 3.1, 3.2

Added section 1.7, 2.1.2