In a world of marketing buzzwords like ‘optimized’ and ‘designed for digital,’ what’s really going on behind all the hype?
From DigitalPhotoPro.com
Before plunking down a credit card to purchase a new lens, I think all of us want to know we’re making a good decision. Is this really the best lens for my camera and the type of work I want to do? Or can I use one I already have, even if the lens isn’t specifically designed for digital capture?
Because of the physical variations between film and digital image sensors, there’s a need for specialized optics, especially if you’re using a D-SLR with a sub-full-frame sensor. Cameras in this category include all Nikon D-SLRs except the new D3, the Canon EOS 40D and EOS 30D, the Sigma SD14, the Fujifilm S5 Pro and IS Pro, the Sony Alphas and all Olympus D-SLRs, to name a few.
With full-frame, 24x36mm sensors or larger digital-back chips, the need for specialized optics may not be as pronounced, depending on the lens and the camera, but it’s still there. The fact that we’re seeing an ever-increasing number of lenses being made with digital capture in mind isn’t a superfluous phenomenon.
Steve Heiner, Senior Technical Manager for Nikon, describes the effort this way, “The majority of digital SLR designs in the market today utilize an imaging sensor that’s roughly two-thirds the size of what’s often referred to as a “full-sized” imaging sensor, or 24x36mm—the FX format for the Nikon D3. Because of this, and as an optical-lens manufacturer, Nikon set out to create a line of lenses designed specifically for its DX-format-size imaging sensor. The goal was to achieve a good balance between miniaturization in size and improvement in performance. By limiting use of these DX Nikkors to DX-format digital SLR cameras only, it’s possible to create unprecedented lenses by making the most of their specific characteristics, while delivering a more compact lens with higher performance. This was the starting point for the development of the DX Nikkor series.”
Canon’s Chuck Westfall says it isn’t necessarily a problem to use a lens that was originally designed for a film camera on a digital camera. “Generally speaking, you don’t get any degradation just because you’re using it on a digital camera compared to using it with film,” says Westfall. “It really depends on the lens and the camera.
“That said, all Canon EF lenses announced since 1999 feature modifications in design that improve their performance with digital SLRs without affecting their performance with 35mm SLRs.”
As of 2001, Sigma has upgraded its entire line of lenses, even the full-frame lenses, to be better suited to the digital format. Says Sigma’s Tom Sobey, “The DG type that are for both film and digital, and the digital-only DC lenses for smaller sensors, all have the new DG coatings to improve contrast and minimize issues with ghosting and flare.”
Sobey says many of the wider-angle lenses are also designed to get light to the edges of the sensor to avoid vignetting. He continues, “Because sensors can be more sensitive to light striking pixels at sharp angles, many of Sigma’s newer lenses, especially the wide-angles, have larger rear elements, which allow the light rays to hit the edges of a sensor at a more perpendicular angle so potential issues with chromatic aberrations and light falloff at the edges are minimized.”
While there are exceptions, such as the Hasselblad H3D, Canon EOS-1Ds Mark III and EOS 5D, and Nikon’s new D3, most D-SLRs feature sensors that are smaller than a piece of 35mm film. Imagers typically range from 13mm to 19mm along their shorter side, compared to film’s 24mm. This fact has presented some issues that manufacturers have had to deal with in the design of their lenses. The first, and perhaps the most critical, is the magnification factor and the need for shorter focal lengths (Fig. 1).
Crop Factors & The Need For Shorter Focal Lengths
If you use a 20mm film lens to project an image on a sensor that’s 16x24mm, it will behave like a 30mm. This is because the sensor is 1.5 times smaller than a 35mm piece of film, which is 24x36mm. To maintain a 20mm focal length, you’d need a lens with a focal length of 13mm. D-SLRs with sensors even smaller than 16x24mm will need even shorter focal lengths because the magnification factor is even greater.
Nikon’s Heiner says, “DX Nikkor lenses are designed specifically for digital with this in mind. In creating a lens exclusively for Nikon’s DX-format digital SLR cameras, decreasing the size of a lens to two-thirds of what’s necessary for the FX format, or 24x36mm, would seem simple, but it’s not. There are many obstacles that have to be overcome. Otherwise, it would only make the size of a conventional lens smaller, but not improve aspects of lens performance such as sharpness or vignetting. Many DX Nikkor lenses have focal lengths similar to many non-DX Nikkor lenses, but they’re designed to present a smaller image circle to the DX-format imaging sensor, thus solving the restrictions of more conventional lens designs.”
Since lens focal length is the distance from the optical center to the focal plane, the glass of a simple single-element 12mm lens, for example, would have to be physically inside your camera’s body. This is impossible, obviously, so designers had a challenge on their hands.
The back focus had to be longer than the intended focal length, which required the use of an inverted-telephoto or retro-focus design (Fig. 2 and 4). The optical center on such lenses lies well to the rear of the lens element, and in many cases, completely outside the glass. This allows for very short focal lengths, even though the rear element remains quite a bit further from the image sensor’s surface.
Chromatic Aberration
While the retro-focus design solved one problem, it created another problem typically associated with telephoto lenses—chromatic aberration or “color fringing.” Because lenses disperse the different colors of light, it causes them to focus at different points (Fig. 3). Telephoto lenses and wide-angles with an inverted-telephoto design are especially prone to this phenomenon.
Extra-low-dispersion (ED) and ultra-low-dispersion (UD) glass are the common solutions to this problem. ED and UD lens elements help keep each of the colors focused as close as possible to the same point in the image plane.
Many of Tamron’s Di and Di II lenses have unique aspherical glass elements that substantially improve optical quality while eliminating chromatic aberrations that can degrade image contrast and sharpness. Stacie Errera of Tamron USA says, “These elements consist of LD or AD glass that’s perfectly bonded to an aspherical layer of optical resin. Because of the fragile physical nature of the glass during the polishing and coating process, and the specific temperature required for molding the resin, engineers had to develop a brand-new method of production to make these special glass elements a reality.”
Richard Pelkowski, product manager of D-SLRs for Olympus, points out that dealing with color fringing also is more critical with digital capture than with film because of a sensor’s micro lenses. “Chromatic aberration can be more of a critical issue with digital because you have micro lenses on your sensor that can be prone to producing chromatic aberrations,” says Pelkowski. “So you not only have to get all three colors to line up coming out of your primary optic, but then when it goes through the micro lenses on the image sensor, you’ve got to deal with that as well. So you want to keep problems with chromatic aberration to an absolute minimum with the taking lens, so when the light gets to the micro lenses, it doesn’t get magnified there.”
In the case of Hasselblad, technical specialist Paul Claesson says they deal with chromatic aberrations in a different way. “We, or I should say Zeiss, never had to redesign the lenses to meet the needs for digital capture. We do, however, have our HC line of lenses for the H cameras, which are optimized for the parameters of digital capture, and they’re more than sufficient for film capture as well. Any lens anomalies like color fringing or vignetting at the edges can be handled via software in post-capture.
“Dealing with aberrations this way was a conscious choice by the company,” adds Claesson. “We could have gone one of two different directions. We could have made a lot of these corrections physically inside the lens, which would have increased the size and weight of the lens, and also the cost of the lens to the consumer. Or we could use a good-quality lens and make improvements from a software application. That’s the route that we chose to go.”
Ghosting & Flare
Although chromatic aberration isn’t exclusively a digital issue, ghosting and flare is unique to the format (Figures 5a and 5b). Canon’s Westfall explains, “One of the big differences between a digital SLR and a film camera is the fact that you’re using this image sensor with usually a low-pass filter in front of it. And it’s a shiny surface. So if you’re using a lens that isn’t optimized for an image sensor, there’s a risk—depending on which lens—that there will be some internal reflections that bounce back and forth between the image sensor and the element in question, which could be any element in the lens.
“What we’ve done over the last seven or eight years now is to work on developing coatings and make sure that the shapes of the individual lens elements are such that the flare and ghosting are minimized or eliminated.”
Nikon’s Heiner says, “Well before the new Nikon D3 was announced, our lens designers were busy in advance producing Nikkor lenses with new technologies to optimize their use with both our FX- and DX-format D-SLRs. Some of these include Extra-low Dispersion glass elements for minimal chromatic aberration and aspherical lenses, including large-diameter PGM (Precision Glass Molding) elements to reduce coma and other types of aberration even at the widest aperture. These lenses also benefit from Nikon’s exclusive Nano Crystal Coat—an extra-low refractive index coating that virtually eliminates internal lens element reflections across a wide range of wavelengths and is particularly effective in reducing ghosting and flare.”
In the case of super-telephoto lenses, which have a large protective glass filter in front of the lens elements, both Canon and Nikon use a subtly curved meniscus design that prevents light bouncing back off the imager from returning into the camera as a ghost image. Older designs that used a flat-glass filter were known to suffer from this spectral ailment.
Collimation Of Light Rays
You’ve likely heard different opinions regarding whether light rays need to strike the sensor as straight as possible. Some manufacturers say this isn’t an issue because of a sensor’s micro lenses. Other manufacturers will argue that the collimation of light rays is still an important part of digital lens design, regardless of the sensor’s micro lenses.
 |
| Schneider Apo-Digitar 120mm ƒ/5.6 with a
Rollei #0 shutter release |
Sally Smith Clemens of Olympus, says, “When you think of a piece of celluloid and you think of the silver-halide crystals, they’re like little grains of sugar or sand. They’re dimensional, and they can collect light from stray angles because they’re sensitive all the way around. An electronic imaging sensor has a flat surface, and each of the photo diodes have depth, much like a bunch of shot glasses lined up in a row.
“The light has to actually enter each one of those photo diodes at a perpendicular angle all the way across the sensor, even to the edges. The optical design has to be able to bend the light so it hits the sensor at a more perpendicular angle so the light goes all the way to the base of each diode.”
The fact that most sensors now have micro lenses on each pixel to help direct the light is certainly evidence that collimation of light is a concern for digital capture.
Says Canon’s Westfall, “The bottom line is that with the micro lenses that we have over the individual pixels on the sensors, we’re gathering as much light as we possibly can. So even if light hits the sensor at oblique angles, it’s not going to result in any aberrations like extra chromatics. And it also doesn’t have any strong effect on vignetting.”
Resolution
Although it isn’t likely everyone will ever agree on how straight light rays need to be when they hit an image sensor, there’s consensus on how well light is resolved. Generally, most would agree that it does have to be higher for an image sensor compared to film.
The same could be said of lenses for medium- and large-format cameras that can accommodate an optional digital back or film magazine, or vice versa. Take a Mamiya RZ67 Pro IID, for example. It uses 6x7cm and 6×4.5cm interchangeable film magazines, plus you have the option of using the ZD digital back or others on the market. The ZD back has a CCD that’s 36x48mm, which is 1.25 times smaller than 6×4.5cm (60x45mm). That’s roughly the same as using a 35mm film lens on a digital camera with an APS-H-sized sensor, which is 18.7×28.1mm.
So if you’re shooting with a medium-format camera and switching back and forth between capture formats—film and a digital back—you want lenses that have a big enough image circle to cover 6x7cm or 6x9cm and still have enough resolution for the digital back. The Mamiya Sekor Zoom AF 75-150mm ƒ/4.5 D and Sekor AF 28mm ƒ/4.5 D Aspherical lenses are specifically designed for the demands of both formats.
There are other medium- and large-format lenses that have this capability, but this isn’t true across the board. Paul Cousins, large-format lens specialist of Schneider Optics, says, “If you compare our Digitar lenses to a large-format film lens that projects onto a 4×5-inch piece of film, to get that same image quality on a digital sensor that’s 36x48mm—about one-third the size—the resolution has to be much higher, which is certainly the case with our Digitar lenses. Some film lenses have high enough resolving power for today’s high-megapixel digital backs, but not all of them.
“The problem that the lens designers face is, as the resolving power gets greater and greater, the image circle gets smaller and smaller. So they have to make a compromise somewhere. They have to have a lens that has enough resolution for a digital chip, but also has enough image circle to cover these digital backs. That’s where the balance lies.”
Cousins says different lenses have different-sized image circles, so you always need to verify if a particular lens will match your requirements.
Au Final
At the end of the day—beyond all the technical details of why and how—it’s about quality. What are the best lenses for your camera, either the one you own or the one you rent when you go in the studio or out on location? What will get you the best possible images when you snap the shutter? No amount of editing will ever make up for lack of quality at the moment of capture.
Lenses designed for digital capture have many advantages over analog lenses, to be sure. Not in every case—there are exceptions, as we mentioned. But there are some good reasons why everyone is making them now and why you should consider using them if you haven’t already.
want to do? Or can I use one I already have, even if the lens isn’t specifically designed for digital capture?
Because of the physical variations between film and digital image sensors, there’s a need for specialized optics, especially if you’re using a D-SLR with a sub-full-frame sensor. Cameras in this category include all Nikon D-SLRs except the new D3, the Canon EOS 40D and EOS 30D, the Sigma SD14, the Fujifilm S5 Pro and IS Pro, the Sony Alphas and all Olympus D-SLRs, to name a few.
With full-frame, 24x36mm sensors or larger digital-back chips, the need for specialized optics may not be as pronounced, depending on the lens and the camera, but it’s still there. The fact that we’re seeing an ever-increasing number of lenses being made with digital capture in mind isn’t a superfluous phenomenon.
Steve Heiner, Senior Technical Manager for Nikon, describes the effort this way, “The majority of digital SLR designs in the market today utilize an imaging sensor that’s roughly two-thirds the size of what’s often referred to as a “full-sized” imaging sensor, or 24x36mm—the FX format for the Nikon D3. Because of this, and as an optical-lens manufacturer, Nikon set out to create a line of lenses designed specifically for its DX-format-size imaging sensor. The goal was to achieve a good balance between miniaturization in size and improvement in performance. By limiting use of these DX Nikkors to DX-format digital SLR cameras only, it’s possible to create unprecedented lenses by making the most of their specific characteristics, while delivering a more compact lens with higher performance. This was the starting point for the development of the DX Nikkor series.”
Canon’s Chuck Westfall says it isn’t necessarily a problem to use a lens that was originally designed for a film camera on a digital camera. “Generally speaking, you don’t get any degradation just because you’re using it on a digital camera compared to using it with film,” says Westfall. “It really depends on the lens and the camera.
“That said, all Canon EF lenses announced since 1999 feature modifications in design that improve their performance with digital SLRs without affecting their performance with 35mm SLRs.”
As of 2001, Sigma has upgraded its entire line of lenses, even the full-frame lenses, to be better suited to the digital format. Says Sigma’s Tom Sobey, “The DG type that are for both film and digital, and the digital-only DC lenses for smaller sensors, all have the new DG coatings to improve contrast and minimize issues with ghosting and flare.”
Sobey says many of the wider-angle lenses are also designed to get light to the edges of the sensor to avoid vignetting. He continues, “Because sensors can be more sensitive to light striking pixels at sharp angles, many of Sigma’s newer lenses, especially the wide-angles, have larger rear elements, which allow the light rays to hit the edges of a sensor at a more perpendicular angle so potential issues with chromatic aberrations and light falloff at the edges are minimized.”
While there are exceptions, such as the Hasselblad H3D, Canon EOS-1Ds Mark III and EOS 5D, and Nikon’s new D3, most D-SLRs feature sensors that are smaller than a piece of 35mm film. Imagers typically range from 13mm to 19mm along their shorter side, compared to film’s 24mm. This fact has presented some issues that manufacturers have had to deal with in the design of their lenses. The first, and perhaps the most critical, is the magnification factor and the need for shorter focal lengths (Fig. 1).
Crop Factors & The Need For Shorter Focal Lengths
If you use a 20mm film lens to project an image on a sensor that’s 16x24mm, it will behave like a 30mm. This is because the sensor is 1.5 times smaller than a 35mm piece of film, which is 24x36mm. To maintain a 20mm focal length, you’d need a lens with a focal length of 13mm. D-SLRs with sensors even smaller than 16x24mm will need even shorter focal lengths because the magnification factor is even greater.
Nikon’s Heiner says, “DX Nikkor lenses are designed specifically for digital with this in mind. In creating a lens exclusively for Nikon’s DX-format digital SLR cameras, decreasing the size of a lens to two-thirds of what’s necessary for the FX format, or 24x36mm, would seem simple, but it’s not. There are many obstacles that have to be overcome. Otherwise, it would only make the size of a conventional lens smaller, but not improve aspects of lens performance such as sharpness or vignetting. Many DX Nikkor lenses have focal lengths similar to many non-DX Nikkor lenses, but they’re designed to present a smaller image circle to the DX-format imaging sensor, thus solving the restrictions of more conventional lens designs.”
Since lens focal length is the distance from the optical center to the focal plane, the glass of a simple single-element 12mm lens, for example, would have to be physically inside your camera’s body. This is impossible, obviously, so designers had a challenge on their hands.
The back focus had to be longer than the intended focal length, which required the use of an inverted-telephoto or retro-focus design (Fig. 2 and 4). The optical center on such lenses lies well to the rear of the lens element, and in many cases, completely outside the glass. This allows for very short focal lengths, even though the rear element remains quite a bit further from the image sensor’s surface.
Chromatic Aberration
While the retro-focus design solved one problem, it created another problem typically associated with telephoto lenses—chromatic aberration or “color fringing.” Because lenses disperse the different colors of light, it causes them to focus at different points (Fig. 3). Telephoto lenses and wide-angles with an inverted-telephoto design are especially prone to this phenomenon.
Extra-low-dispersion (ED) and ultra-low-dispersion (UD) glass are the common solutions to this problem. ED and UD lens elements help keep each of the colors focused as close as possible to the same point in the image plane.
Many of Tamron’s Di and Di II lenses have unique aspherical glass elements that substantially improve optical quality while eliminating chromatic aberrations that can degrade image contrast and sharpness. Stacie Errera of Tamron USA says, “These elements consist of LD or AD glass that’s perfectly bonded to an aspherical layer of optical resin. Because of the fragile physical nature of the glass during the polishing and coating process, and the specific temperature required for molding the resin, engineers had to develop a brand-new method of production to make these special glass elements a reality.”
Richard Pelkowski, product manager of D-SLRs for Olympus, points out that dealing with color fringing also is more critical with digital capture than with film because of a sensor’s micro lenses. “Chromatic aberration can be more of a critical issue with digital because you have micro lenses on your sensor that can be prone to producing chromatic aberrations,” says Pelkowski. “So you not only have to get all three colors to line up coming out of your primary optic, but then when it goes through the micro lenses on the image sensor, you’ve got to deal with that as well. So you want to keep problems with chromatic aberration to an absolute minimum with the taking lens, so when the light gets to the micro lenses, it doesn’t get magnified there.”
In the case of Hasselblad, technical specialist Paul Claesson says they deal with chromatic aberrations in a different way. “We, or I should say Zeiss, never had to redesign the lenses to meet the needs for digital capture. We do, however, have our HC line of lenses for the H cameras, which are optimized for the parameters of digital capture, and they’re more than sufficient for film capture as well. Any lens anomalies like color fringing or vignetting at the edges can be handled via software in post-capture.
“Dealing with aberrations this way was a conscious choice by the company,” adds Claesson. “We could have gone one of two different directions. We could have made a lot of these corrections physically inside the lens, which would have increased the size and weight of the lens, and also the cost of the lens to the consumer. Or we could use a good-quality lens and make improvements from a software application. That’s the route that we chose to go.”
Ghosting & Flare
Although chromatic aberration isn’t exclusively a digital issue, ghosting and flare is unique to the format (Figures 5a and 5b). Canon’s Westfall explains, “One of the big differences between a digital SLR and a film camera is the fact that you’re using this image sensor with usually a low-pass filter in front of it. And it’s a shiny surface. So if you’re using a lens that isn’t optimized for an image sensor, there’s a risk—depending on which lens—that there will be some internal reflections that bounce back and forth between the image sensor and the element in question, which could be any element in the lens.
“What we’ve done over the last seven or eight years now is to work on developing coatings and make sure that the shapes of the individual lens elements are such that the flare and ghosting are minimized or eliminated.”
Nikon’s Heiner says, “Well before the new Nikon D3 was announced, our lens designers were busy in advance producing Nikkor lenses with new technologies to optimize their use with both our FX- and DX-format D-SLRs. Some of these include Extra-low Dispersion glass elements for minimal chromatic aberration and aspherical lenses, including large-diameter PGM (Precision Glass Molding) elements to reduce coma and other types of aberration even at the widest aperture. These lenses also benefit from Nikon’s exclusive Nano Crystal Coat—an extra-low refractive index coating that virtually eliminates internal lens element reflections across a wide range of wavelengths and is particularly effective in reducing ghosting and flare.”
In the case of super-telephoto lenses, which have a large protective glass filter in front of the lens elements, both Canon and Nikon use a subtly curved meniscus design that prevents light bouncing back off the imager from returning into the camera as a ghost image. Older designs that used a flat-glass filter were known to suffer from this spectral ailment.
Collimation Of Light Rays
You’ve likely heard different opinions regarding whether light rays need to strike the sensor as straight as possible. Some manufacturers say this isn’t an issue because of a sensor’s micro lenses. Other manufacturers will argue that the collimation of light rays is still an important part of digital lens design, regardless of the sensor’s micro lenses.
|
| Schneider Apo-Digitar 120mm ƒ/5.6 with a Rollei #0 shutter release |
Sally Smith Clemens of Olympus, says, “When you think of a piece of celluloid and you think of the silver-halide crystals, they’re like little grains of sugar or sand. They’re dimensional, and they can collect light from stray angles because they’re sensitive all the way around. An electronic imaging sensor has a flat surface, and each of the photo diodes have depth, much like a bunch of shot glasses lined up in a row.
“The light has to actually enter each one of those photo diodes at a perpendicular angle all the way across the sensor, even to the edges. The optical design has to be able to bend the light so it hits the sensor at a more perpendicular angle so the light goes all the way to the base of each diode.”
The fact that most sensors now have micro lenses on each pixel to help direct the light is certainly evidence that collimation of light is a concern for digital capture.
Says Canon’s Westfall, “The bottom line is that with the micro lenses that we have over the individual pixels on the sensors, we’re gathering as much light as we possibly can. So even if light hits the sensor at oblique angles, it’s not going to result in any aberrations like extra chromatics. And it also doesn’t have any strong effect on vignetting.”
Resolution
Although it isn’t likely everyone will ever agree on how straight light rays need to be when they hit an image sensor, there’s consensus on how well light is resolved. Generally, most would agree that it does have to be higher for an image sensor compared to film.
The same could be said of lenses for medium- and large-format cameras that can accommodate an optional digital back or film magazine, or vice versa. Take a Mamiya RZ67 Pro IID, for example. It uses 6x7cm and 6×4.5cm interchangeable film magazines, plus you have the option of using the ZD digital back or others on the market. The ZD back has a CCD that’s 36x48mm, which is 1.25 times smaller than 6×4.5cm (60x45mm). That’s roughly the same as using a 35mm film lens on a digital camera with an APS-H-sized sensor, which is 18.7×28.1mm.
So if you’re shooting with a medium-format camera and switching back and forth between capture formats—film and a digital back—you want lenses that have a big enough image circle to cover 6x7cm or 6x9cm and still have enough resolution for the digital back. The Mamiya Sekor Zoom AF 75-150mm ƒ/4.5 D and Sekor AF 28mm ƒ/4.5 D Aspherical lenses are specifically designed for the demands of both formats.
There are other medium- and large-format lenses that have this capability, but this isn’t true across the board. Paul Cousins, large-format lens specialist of Schneider Optics, says, “If you compare our Digitar lenses to a large-format film lens that projects onto a 4×5-inch piece of film, to get that same image quality on a digital sensor that’s 36x48mm—about one-third the size—the resolution has to be much higher, which is certainly the case with our Digitar lenses. Some film lenses have high enough resolving power for today’s high-megapixel digital backs, but not all of them.
“The problem that the lens designers face is, as the resolving power gets greater and greater, the image circle gets smaller and smaller. So they have to make a compromise somewhere. They have to have a lens that has enough resolution for a digital chip, but also has enough image circle to cover these digital backs. That’s where the balance lies.”
Cousins says different lenses have different-sized image circles, so you always need to verify if a particular lens will match your requirements.
Au Final
At the end of the day—beyond all the technical details of why and how—it’s about quality. What are the best lenses for your camera, either the one you own or the one you rent when you go in the studio or out on location? What will get you the best possible images when you snap the shutter? No amount of editing will ever make up for lack of quality at the moment of capture.
Lenses designed for digital capture have many advantages over analog lenses, to be sure. Not in every case—there are exceptions, as we mentioned. But there are some good reasons why everyone is making them now and why you should consider using them if you haven’t already.