Evolution of Lenses

by Michael J. Hussmann
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Статьи > Техника/Технологии
Дата публикации: 2012-02-03
Страниц: 7
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10 2/2008

Technology Lenses The Evolution of Lenses by Michael J. Hussmann C-type and HC lenses are separated by a turn of the millennium, changes electronically. HC lenses support Photos: Jürgen automatic as well as manual focus: Holzenleuchter in the way lenses are designed, and shifting requirements to comply with rather than transmitting the torque modern camera designs. Still, a great 20th century lens design makes for a from a focusing motor in the body through to the lens using a clutch; fine lens even today, so what are the differences between C-type and HC the focusing motor is integrated into lenses, and how do comparable lenses fare head-to-head? the lens itself. The interface between the focusing motor in the lens and the auto-focus system in the body is Over the decades, Hasselblad has lenses is equal, if not superior to that tured to Hasselblad’s specifications, electrical, but a complex system of worked with a number of highly of the C-type lenses; seeing is believ- but their engineers played no part in gears allows for a manual override skilled partners for manufacturing ing – with an H System body, a pair the actual lens development. While – the photographer can intervene at lenses: Fujinon, Kodak, Rodenstock of old and new lenses, and the CF previously Carl Zeiss engineered any time without having to switch and Schneider. But if any lens manu- adapter, anyone can copy the test. everything including the shutter, from auto-focus to manual focus facturer is specifically associated with lens design today is a collaborative mode first. Hasselblad, it is certainly Carl Zeiss. effort between Hasselblad and Fuji- Auto-focus isn’t just a matter The division The V System’s Zeiss lenses contrib- non, with Hasselblad gradually of convenience or focusing speed; of labor uted a great deal toward establishing assuming a larger role. For example, with HC lenses it is a prerequisite Hasselblad’s fame and reputation. With the current arrangement work on the latest addition to the HC for leveraging Hasselblad’s intimate Indeed, they continue to be popular between Hasselblad and Fujinon, lens portfolio, the HCD 4/28, started knowledge about the performance with many photographers today. In Hasselblad is much more in charge of out in Gothenburg, Sweden. Using of their lenses to achieve higher 2002, when Hasselblad launched the lens design and manufacturing than powerful software for optical design, focusing accuracy than possible with H System, it came with a new and they have ever been. The V System the characteristics of the new lens either manual focus or auto-focus growing set of lenses manufactured lenses were designed and manufac- could be explored in great detail well alone. The methods for improving by Fujinon, a subsidiary of Fujifilm before any glass was ground. Fujinon auto-focus accuracy are subsumed in Japan. While Fujinon is a respected in Saitama City, Japan, then took up under the label Ultra-Focus. lens manufacturer meeting the high- Synopsis the task of refining the lens design est standards, the brand name didn’t and building a prototype series that • The CF lenses made by Carl Focusing have quite the reputation as Carl was put through its paces at Fujinon’s Zeiss did contribute a great deal accuracy revisited Zeiss. Photographers who bought and Hasselblad’s lab. to the Hasselblad V System’s into the modern H System retained This division of labor also extends Photographers might wonder why fame, and they can still be used a preference for the tried and proven to the manufacturing stage. The cen- Hasselblad is investing resources into with today’s H System cameras. Zeiss lenses they had been using, and tral shutter and the iris diaphragm achieving minute improvements in • With the HC lenses jointly de- even some newcomers to Hasselblad are built and assembled in Gothen- focusing accuracy. After all, there veloped with Fujinon, Hasselblad cameras felt they had missed out by burg, then sent to Fujinon where wasn’t anything wrong with focusing is more in control of lens design joining too late in the game. the lenses are ground and the final accuracy in the past – or was there? and manufacturing than ever. Before we delve into the various assembly of the optical, mechanical, Auto-focus systems based on phase issues around designing, manufactur- • Digital photography requires and electronic parts takes place. detection, the technology used in all ing and evaluating lenses; it should higher focusing accuracy than current DSLRs, are working as well as be stressed that as far as Hasselblad analog photography. Auto-focus ever; however, the change from sil- systems need to be augmented What’s in a lens is concerned, Zeiss lenses aren’t just ver halide film to electronic sensors a legacy of the past, but continue to by measures ensuring optimum The C-type lenses are purely opto- also changed the rules of the game. be an asset today. Not only is the V sharpness under any conditions. mechanical devices made up of 1 When an image is opened in an System (for which Zeiss lenses were • Whereas CF lenses were lenses, tubes, helical mounts, and a image editing application such as designed) still a viable camera system optimized for infinity, HC lenses mechanical shutter. A modern HC Photoshop, the first preview is of the in its own right; these C-type lenses are optimized for more typical lens is a more complex design: the image displayed at 100 percent; each can also be used on the latest H Sys- subject distances, and yield a mechanical shutter is replaced by image pixel is mapped to exactly one tem cameras, thanks to the optional superior result overall. an electronically controlled shutter display pixel, whatever the number The CF 3.5/100 by Carl Zeiss, used for the image shown on the left, CF lens adapter. With the adapter fit- • A truly fine lens needs to for added precision and reproduc- of pixels. The greater the resolution, had once set a standard for len- ting between the lens and the body, deliver crisp and sharp detail tibility (see page 32). In a Zeiss lens, measured in megapixels, the greater ses to be measured against. The metering is done at full aperture and and at the same time, a good focusing is manual and it only con- the magnification at 100 percent image on the right was taken with manual focusing is assisted by the bokeh; a pleasing rendition of cerns the lens itself – there is no – on a typical 96 ppi display, an the H-system’s HC 2.2/100 which camera’s AF system providing focus out-of-focus detail. communication between body and image taken with an H3DII-39 would not just meets and partly exceeds confirmation. Cocking the shutter lens regarding the focus distance. measure about 1.9 x 1.4 meters if those standards, but offers all the takes a flick of the wrist, tossing a lever The H System components, on the one could see it in its entirety. The amenities one has come to expect on the adapter. While we are going to other hand, are tightly integrated, high magnification factors so read- from a modern autofocus lens show how the quality of the new HC with lens and body exchanging data ily available in digital image editing ONLINE 3/2009 11

Technology Lenses CF100 f/8 1:10 HC100 f/8 1:10 CF100 f/3.5 1:10 HC100 f/3.5 1:10 CF 3.5/100 f8 1.2 m HC 2.2/100 f8 1.2 m CF 3.5/100 f3.5 1.2 m HC 2.2/100 f3.5 1.2 m 100 100 100 100 80 80 80 80 60 60 60 60 40 40 40 40 20 20 20 20 0 0 0 0 0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30 Tangential contrast drops fast; The HC lens shows good con- At 1.2 m from the subject and fully Even at f3.5, the sagittal con- only sagittal contrast profits from trast over most of the image circle, open, the CF lens only shows good trast of the HC lens shows next to stopping down slightly improved by DAC contrast near the center no drop off towards the edges Diverging values for sagittal and The sagittal and tangential con- The curves for sagittal and tan- At the corners, tangential con- tangential contrast can create dis- trast curves stay close together; gential contrast begin to diverge trast drops, but rises again thanks tortions in the corners only at 40 lp/mm do they diverge about 5 mm from the center to the digital correction At the very center of the image, At the center, contrast would drop Within a small central area the In the center part of the image, the CF lens still performs great due to focus shift, but Ultra-Focus CF lens shows excellent contrast the HC lens performs almost as – even at short distances compensates for this effect even at 1.2 m from the subject well as its CF counterpart At distances typical for studio software have created a distorted is slightly off, this previously went The five aperture blades of the The octogonal blur disks of the work, the HC 2.2/100 comes into perception of image quality. Previ- unnoticed, but it won’t escape the CF 3.5/100 turn out-of-focus spe- HC 2.2/100 have a faint lining indi- its own; here, the CF 3.5/100 ously, photographers had to deal attention of a Photoshop user today. cular highlights into pentagons cative of a slight over-correction suffers from an optimization with constant magnification factors And there’s another reason why strategy focused on performance defined by the desired print size or focusing with digital cameras needs at infinity. That the HC lens is the loupe magnification when view- to meet tighter constraints: sensors more than one f-stop faster can ing slides or negatives. When a film are much less forgiving of small come in handy, too vendor introduced a new product focusing errors than film, especially with finer grain, the improvement in color film. The emulsion layers of the layers will show a sharp image. pixel or the next, and this depth image quality was immediately vis- color film can reach a thickness of 20 A superimposition of a sharp image defines the plane where the lens ible. A higher resolution sensor, on microns, implying that there can’t in one layer and two slightly less has to produce a sharp image. With the other hand, will produce images be optimum sharpness within all the sharp images in the other layers, does regard to focusing, a sensor is like a that will inevitably be viewed at 100 layers. SLR auto-focus systems strive reduce the contrast of fine detail, but film with an extremely thin emul- percent again, and thus at a higher to optimize sharpness in the middle, the general appearance of sharpness sion. There are several small effects magnification than a lower resolu- green-sensitive layer, so sharpness in doesn’t suffer. that once went unnoticed, as the tion image. It’s like getting a more the blue- and red-sensitive layers on With CCD or CMOS sensors, focusing errors they caused were well powerful loupe along with the fine top and below is minimally compro- sharpness degrades much less grace- within the margin allowed for by the grain film, so the apparent grain size mised. Any small focusing errors will fully, even when they too, exhibit a emulsion layers. Digital photography stays the same. And the increased only shift the position of the sharpest layered structure. At a certain depth raises the bar for focusing accuracy, magnification magnifies any imper- image towards the red- or blue-sensi- into the sensor assembly, it is decided and meeting this challenge requires a fections in the image – if the focus tive layers, so that even then one of whether a ray of light will reach this detailed model of the lens so that its 12 ONLINE 3/2009

A good bokeh, i.e. the pleas- ing rendition of out-of-focus detail can be just as important as sharpness and contrast. The CF 3.5/100 (left) and the HC 2.2/100 (right), both at f4, fare quite well in this respect ONLINE 3/2009 13

Technology LEnses HC100 f/3.5 infinity CF100 f/8 infinity HC100 f/8 infinity CF100 f/3.5 infinity CF 3.5/100 f8 infinity HC 2.2/100 f8 infinity CF 3.5/100 f3.5 infinity HC 2.2/100 f3.5 infinity 100 100 100 100 80 80 80 80 60 60 60 60 40 40 40 40 20 20 20 20 0 0 0 0 0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30 Being optimized for infinity, the The HC lens shows good results; Even fully open, the CF lens Up to 20 lp/mm, the HC lens per- stopped down CF lens shows no tangential contrast drops a little produces good contrast; excellent forms nearly as well; at 40 lp/mm, weaknesses whatsoever from 10 mm onwards from the center to 20 mm contrast drops a little faster Near infinity, the CF 3.5/100 has a The CF 3.5/100 makes a good land- Tangential resolution is improved Again the optimization for infinity Despite its different optimization slight edge, performance-wise, but scape lens with excellent by the digital correction of results in outstanding perfor- goals, the performance is still the HC 2.2/100 can hold its own resolution across the whole image chromatic aberration (DAC) mance at long distance settings quite good even near infinity A view of Gothenburg’s harbor, issues to be resolved by Fujinon, or focus shift, focusing would be slightly not far from the Hasselblad to a substandard choice of optimiza- off at larger or smaller apertures. building, provided the near tion compromises that needs to be With small apertures, the increase infinity detail for a comparison adjusted. After the simulation model in depth of field is sufficient to mask between the CF 3.5/100 (left) and has fulfilled its role in improving the this effect, but with a fully open the HC 2.2/100 (right) design and the lens goes into pro- lens, a high-resolution CCD might duction, the model assumes a new reveal a discernible lack of sharp- role informing the camera electron- ness. Since the offset between the ics and the raw conversion software optimum focus position for a given about the peculiarities inherent in aperture and the position found by that particular lens. the AF sensor with the lens fully 2 For fine detail of a given orienta- open is known, an H System camera tion, spatial frequency, and distance; can easily compensate for focus shift, the software can predict the contrast ensuring maximally sharp images at levels produced by the lens at any any aperture setting. focus position and aperture setting. Ultra-Focus, of which the com- From these data points, tables of cor- pensation for focus shift is a part, rection coefficients are calculated shows how knowing your cam- that allow the auto-focus system to era system inside out pays off with behavior can be accurately predicted source will propagate through the compensate for aperture-dependent improvements in image quality. under all possible circumstances. lens elements and the diaphragm, focus shift. An ideal lens would con- This principle extends to the post- taking all the effects of refraction and centrate parallel rays of light to an exposure part of the workflow where diffraction into account. The result incredibly small focus point, but in Hasselblad’s Digital Auto Correction Real and of a simulation can be presented in reality, spherical aberration turns (DAC) takes care of fine honing the virtual lenses a multitude of ways; from a repre- this point into a fuzzy blob. Light image quality in FlexColor or Pho- Well before Fujinon begins grinding sentation of the image formed in rays entering a spherical lens far from cus. DAC relies on the lens model for glass into the desired lens shapes, the image plane, to various diagrams the optical axis are refracted more predicting residual imperfections, the lens exists as a simulation model showing MTF curves, or mapping strongly than those entering near namely chromatic aberration (level on a computer. This virtual lens contrast changes with the focus set- the axis, so there is no common focal I), distortion (level II), and vignetting depicts all the characteristics of the ting. point. Even when spherical aberra- (level III). Thus informed about what real thing, not just the desired ones. Apart from taking these results as tion is corrected for, this correction differentiates the image as it is from Performance parameters have been tips for further improvements in lens will never be perfect; actual lenses the image as it should have been, the specified as goals for optimization design, the simulation models retain will be either under- or over-corrected raw converter can apply the appro- and any residual aberrations, any their usefulness long after the devel- and modern lens designs will tend to priate corrections, relocating pixels compromises necessary for achiev- opment is complete. When the lens over-correct. When stopping down, to their rightful place, either for all ing the design objectives are just as prototype arrives from Japan, it is rays far from the optical axis are three color channels individually accurately modeled. The lens design subject to Hasselblad lab tests on an excluded and the point of optimum (DAC level I) or collectively (level II), software can, for example, probe the optical bench; comparing its actual contrast will shift, resulting in back- and adjusting each pixel’s brightness virtual lens with an equally virtual performance to the performance focus with under-corrected lenses (level III). pin-hole light source, set at a certain predicted by the simulation soft- and front-focus with over-corrected DAC level I, the correction of distance and angle to calculate how ware. Discrepancies detected at this ones. The TTL auto-focus sensor uses chromatic aberration, also removes a the light rays emanating from this stage can either point to production f6.7, so without compensating for major cause for the loss of sharpness 14 ONLINE 3/2009

and resolution towards the edges point, all the rays hitting the pixel approximating the Japanese ぼけ disks with sharply defined bright lower, but still quite high contrast of the image. After the correction, originate from the same point in the meaning, among other things, edges, even to the point of causing at the edge. Huge drops in contrast the resolution of the digital image scene; if the image is out of focus, the “blur”. What constitutes “good” double images, and thus an unpleas- pointing to an uneven distribution of can be even better than that of the rays originate from different sources. or “bad” bokeh – the terms “pleas- ant and distracting kind of bokeh. sharpness in the image area are to be optical image in the sensor plane. The mixture of light from different ant” or “unpleasant” might be more Modern lens designs typically favor avoided. Interpreting an MTF chart With silver-halide photography, the sources in the scene introduces blur. appropriate – is open to debate, as are a slight over-correction of spheri- may seem like a daunting task at first, performance of the lens defined an Even for the in-focus case, the perfect the main factors influencing bokeh. cal aberration to improve sharpness as a multitude of curves are crammed upper limit of image quality one double cone of light – all the light When specular highlights in the for subjects in the plane of focus, so into a single chart, but it takes more could attain; with digital photogra- emanating from a point that reaches out-of-focus background appear as the lens designer has to be careful in than a single curve to paint a com- phy, it is just the starting point. lens converges in the same point in blur disks, these disks are images of keeping a pleasant bokeh while at plete picture. For one thing, MTF the image plane – is an idealization. the aperture and their shape cor- the same time optimizing contrast. charts differentiate between the With an uncorrected lens, aberra- responds to that of the diaphragm. contrast of patterns in different ori- What makes tions such as astigmatism, spherical From polygon shaped rather than entations – sagittal patterns of lines a fine lens Appraising and aberration, or chromatic aberration round blur disks one can easily deter- running from the center towards comparing lenses Unless a camera is used for repro- would contribute to turn the ideal mine the number of aperture blades. the edges, and tangential patterns of duction, photographic subjects will point into a fuzzy blob spreading These shapes are rarely distracting, 4 The prime gauge for appraising lines running perpendicular to the generally be three-dimensional. The over several pixels, reducing the though, and increasing the number the quality of lenses are the so-called former: the MTF curve for sagittal plane of focus cuts out a slice of the contrast at the edges between areas of blades doesn’t improve bokeh as MTF (short for modulation transfer patterns is generally represented by scene that will be rendered sharp, so of different brightness or color. The much to be worth bothering. Still, in function) charts. Short of painstak- a solid line, the curve for tangential there is a contrast between sharpness effect of aberrations increases with extreme cases the aperture can be a ingly comparing a variety of images patterns by a dashed line. and unsharpness at different depths the distance from the image center, factor: catadioptric (mirror) lenses taken with different aperture set- Corresponding curves for tangen- into the scene that can be used to resulting in a gradual loss of contrast, that enjoyed a short-lived popular- tings, an MTF chart is the best way tial and sagittal patterns should be as good effect. The requirements for a sharpness, and resolution towards ity during the 1980s had an annular to get an impression of the strengths close as possible or at least run in par- good lens are thus twofold: within the corners. It is the task of the lens opening due to their secondary mir- and weaknesses of a lens, though allel. MTF curves are also measured the depth of field, subjects should be designer to guarantee a high degree ror, rendering unsharp highlights only with regard to its sharpness for different spatial frequencies: pat- rendered sharp, i.e. with a maximum of correction for all kinds of aber- as bright rings and creating double – there is no way to predict bokeh terns of fine lines have a high spatial of contrast even in the finest detail. rations to maintain good contrast images of out-of-focus lines. Both from an MTF chart. For realistic lens frequency, whereas the spatial fre- Subjects out of focus should appear across all of the image area, and for kinds of optical artifacts were highly design tasks, it is actually the only quency of patterns of thick lines is unsharp, but in an aesthetically all or at least most relevant focusing distracting. way, due to the impossibility of con- low. The contrast of high frequency pleasing way, a quality that is much distances. Quite often, lens designs The biggest factor influencing trolling every factor to the necessary patterns corresponds to the ability of harder to pin down exactly. Still, you have been optimized for infinity, bokeh is the amount of spherical precision required for making image resolving fine detail, the contrast of recognize it when you see it, and it whereas in a typical studio setting, aberration. A perfectly corrected lens comparisons a viable alternative. low frequency patterns to the over- can make a real difference between subjects are just a few meters away. would have blur disks with uniform The modulation transfer function all contrast in the image. Generally, lenses of comparable sharpness. Another aspect to keep in mind is brightness and sharp edges. When indicates how the contrast (and thus high contrast across all spatial fre- For each CCD pixel, there is a cone the repercussions lens corrections some of the spherical aberration stays the sharpness) of alternating black quencies is desirable, but some loss of light converging on it, and from have for the imaging of out-of-focus uncorrected, the edge of the disk gets and white lines vary from the center in high frequency contrast towards the pixel’s point of view, the differ- detail. fuzzy, but the bokeh would still be of the image to its edges. Ideally, the the edges is acceptable as long as low ence between sharpness and blur 3 The characteristic way in which a pleasant, maybe even more so than contrast should start out as high as frequency contrast stays high. Soft- hinges on the origin of those light lens renders out-of-focus subjects is with a perfectly corrected lens. An possible at the center, sloping gen- focus lenses with their deliberately rays. If the image is in focus at this called “bokeh”, an English spelling over-corrected lens would create blur tly towards an inevitably somewhat under-corrected spherical aberrations HC210 f/5.6 1:10 HC210 f/8 1:10 CF250SA f/8 1:10 CF250SA f/5.6 1:10 CF 5.6/250 SA f8 near HC 2.2/100 f8 near CF 5.6/250 SA f5.6 near HC 4/210 f5.6 near 100 100 100 100 80 80 80 80 60 60 60 60 40 40 40 40 20 20 20 20 0 0 0 0 0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30 Stopping down improves sagit- Sagittal contrast is excellent at The curves for the resolution Contrast remains high for all tal contrast, but this doesn’t do as 10 to 40 lp/mm; tangential contrast of sagittal and tangential line spatial frequencies; only tangen- much for tangential contrast is improved by DAC level I patterns quickly diverge tial contrast for 40 lp/mm suffers HC210 f/8 infinity CF250SA f/5.6 infinity HC210 f/5.6 infinity CF250SA f/8 infinity CF 5.6/250 SA f8 infinity HC 4/210 f8 infinity CF 5.6/250 SA f5.6 infinity HC 4/210 f5.6 infinity 100 100 100 100 80 80 80 80 60 60 60 60 40 40 40 40 20 20 20 20 0 0 0 0 0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30 Pitching the HC 4/210 against the At infinity, this fine lens doesn’t At f8, the HC lens performs Even fully open, the CF lens Sagittal contrast is comparable; CF 5.6/250 SA yields surprising need help from stopping down, as almost as well as the CF lens that shows exceptionally high contrast tangential contrast drops slightly results, given the price differential there is little to improve is three times more expensive across the whole image beyond 20 mm ONLINE 3/2009 15

Technology LEnses Lenses – At a glance exhibit reduced contrast for low spa- 2.2/100 and the HC 4/210, with their these two lenses illustrates how shift- tial frequencies while retaining some CF counterparts. These are the CF ing optimization goals can make a 1 The 100-percent syndrome contrast with high frequency detail. 3.5/100 and the CF 5.6/250 Super big difference. The CF 3.5/100 holds Stopping down reduces some Achromat, two of the finest lenses the crown at infinity, but contrast On a computer, the aberrations and improves contrast made by Carl Zeiss and thus provid- at shorter focus settings is heavily image opens at 100 percent, with each by cutting out peripheral rays, so an ing an adequate frame of reference. compromised. Designing the HC image pixel mapped analysis of lens performance needs The MTF curves used for illustration 2.2/100 for optimal performance to a display pixel. to take the aperture into account. were derived mathematically from at shorter distances corresponding The higher the pixel Typically, MTF measurements will simulation models of these lenses. more closely with how the lens will count, the higher the be made with the aperture fully actually be used, did result in a better magnification and open (or, if two lenses are to be com- performance overall. the smaller the part CF 3.5/100 pared, the largest aperture common of the image visible at And HC 2.2/100 to both) and after stopping down to any one time CF 5.6/250 Super Ach- the optimum aperture before diffrac- 5 With the focus set to infinity, the romat and HC 4/210 tion kicks in. Many lenses perform distance for which the CF 3.5/100 reasonably when stopped down, but was optimized, the Zeiss lens proves The general philosophy behind HC 2 Focus shift the difference between a good and a its excellence even fully open. Up to lens design has been to forego excep- mediocre lens often lies in the con- 20 mm from the image center, there tional peak performance at a certain trast delivered when fully open. is only a negligible drop in contrast setting if it comes at the expense of MTF charts were originally across the whole range of spatial fre- severe compromises at other set- invented for 35 mm cameras, so quencies, and still good contrast at tings. Rather, the aim was a more contrast is measured from the center 30 mm. At the same aperture (f3.5), even high performance level giv- of the image to about 21.6 mm, the the HC 2.2/100 performs just as good ing consistently good results at any radius of the image circle. Medium- for 10 and 20 lp/mm; only at 40 lp/ focus setting. When comparing the format cameras have larger image mm and outside the central part of CF 5.6/250 Super Achromat to the circles and their radius ranges from the image does the CF lens have a HC 4/210, another factor comes into With over- or under-corrected spherical aberration (the latter is illustrated 27.6 mm for the H3DII-31 to 30.5 small edge. The HC 2.2/100, on the play, namely price: in its day, the CF here), rays entering the lens at different distances from the center don’t con- verge at the same point. Optimum focus shifts as the lens is stopped down mm for the H3DII-39 and -50. For other hand, is about one f-stop faster 5.6/250 SA did cost three times the purposes of comparison, it is not and gives good results even at f2.2. price the HC 4/210 does today; even the absolute value of the distance When the lenses are stopped compared to the ordinary CF 5.6/250 3 Good bokeh, bad bokeh from the image center that is rel- down to f8, sagittal contrast evens it was twice as expensive. How does evant, but its relative value as a out, with both lenses showing excel- the relatively affordable HC lens fare percentage of the radius of the lent results across the whole image against the high-priced CF lens rep- image circle. Spatial frequencies are circle. Tangential resolution of the resenting the best of Carl Zeiss’ lens generally specified in line pairs per HC 2.2/100 drops a little from 10 mm design, especially since the former is millimeter, referring the resolution onwards towards the edges, but DAC one f-stop faster? of lines in the focal plane. level I, i.e. the digital correction of At f5.6 and focused at infinity, the But the image in the focal plane, chromatical aberration, partly com- Super Achromat maintains excep- well hidden within the camera, pensates for this; as indicated in the tionally high contrast across the isn’t what is relevant here; it’s the MTF diagrams by the red curves. whole image. But the HC 4/210 isn’t Unsharp specular highlights should get rendered as disks; under-corrected resolution in the eventual print that Focused at 1.2 m, the HC 2.2/100, far behind; actually sagittal resolu- spherical aberrations create still quite appealing fuzzy blobs, while over-correction yields bright rings that might give rise to double contours counts. For the sensor sizes of H or V a typical portrait lens really comes tion is quite comparable with only System cameras, 10, 20, and 40 line into its own. Even at f3.5, its sagittal tangential contrast dropping slightly pairs per millimeter (lp/mm) would contrast shows next to no drop off beyond 20 mm. Stopped down to f8 4 Reading MTF charts be appropriate – the measurements towards the edges, and just a slight the performance levels become even at 10 lp/mm are indicative of overall decrease in tangental contrast that, closer. At a shorter range, the image 100 contrast while 40 lp/mm measure- again, gets improved by DAC. The CF changes dramatically. The HC 4/210 80 ments reveal the resolving power 3.5/100 can still excel at the image maintains an even level of sharp- 60 for fine detail. For lenses from 35 center, but sagittal and tangential ness from the image center right to 40 mm systems, the spatial frequencies contrast begins to drop off quickly, the edges, still improved by DAC, 20 need to be higher, due to the higher so from 10 mm onwards the HC lens whereas the Super Achromat per- 0 magnification requirements: start- is clearly superior. Stopping down to forms worse practically everywhere 0 10 20 30 ing from a 36 x 24 mm in the focal f8 doesn’t do much to change this. outside the very center. The contrast MTF curves for sagittal patterns are represented by a solid line, the plane, the image needs to be magni- The CF 3.5/100 shows good of lines in the sagittal and tangential curves for tangential patterns by a fied by another 50 percent to reach contrast with sagittal lines, but for orientation diverges, introducing dashed line. For preventing distor- the same final print size, compared tangential lines, the results show lit- distortions near the edges. Stopping tions, both lines should be close or to a medium-format camera, so the tle improvement, if at all. Moreover, down to f8 improves sagittal contrast at least run in parallel spatial frequencies used for compari- the increasing difference in sagittal with the Zeiss lens, but doesn’t do as son have to be raised by 50 percent and tangential contrast introduces much for tangential contrast. The 5 Optimization goals as well, i.e. to 15, 30, and 60 lp/mm. distortions near the edges – point- HC 4/210, while already superior on Furthermore, different sensor sizes like detail is spread out into short its own, performs even better if DAC CF lenses (green) are optimized for also translate to different depths of radial lines. At the image center, the is applied, improving the resolution infinity, HC lenses (blue) for more field; at the same aperture setting, CF lens still bests its HC counterpart, of fine detail considerably. Lens performance typical distances, resulting in a the depth of field of a medium-for- but this is due to a slight focus shift Again, the HC lens shows a better better correction overall and still mat lens is more shallow than that of that is resolved by the auto-focus; performance overall, with the added good results at infinity a 35 mm lens. The medium-format thanks to Ultra-Focus, it doesn’t benefits of a faster lens that is also lens should be stopped down by one show up in practice. At 10 mm the more reasonably priced. f-stop to account for this difference. HC lens breaks even and takes the 2 meters Distance Infinity In the following, we are going lead at greater distances from the For further information please visit to compare two HC lenses, the HC center. The comparison between www.hasselblad.com 16 ONLINE 3/2009

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