The new Phase One 72mm Mk II lens is a completely new and very innovative optical design. Other than the focal length and compatibility with both iXH and iXG, this new lens shares nothing in common with its predecessor. It’s groundbreaking performance is owed to its novel nature, specialization to CH imaging, and it’s first-of-a-kind asymmetrically controlled floating element. This article will dive deep into the origin story and technical details, but let’s start with the end of the story first: this lens makes FADGI 4-star imaging faster and easier to achieve across a wider range of materials, with less work on the part of the user. It is, simply put, the best lens ever developed for Cultural Heritage imaging.
Before we get to that story, let’s get pedantic: in the strictest scientific sense, absolutely no lens is 100% as sharp at the edge as it is in the center, and absolutely no lens has a 100% flat focus field. That is, all lenses drop off in quality at the edge of the frame, and all lenses have focus planes that bend slightly toward or away from the camera. But this is like saying that all drinking water has a non-zero amount of arsenic in it and bananas are radioactive. In practice, the question is how much the quality falls off (50%? 1%? 0.1%?) and how close to flat the field of focus is (10 cm high at the edge? 0.1mm at the edge?). If the answers to these questions are extremely positive for a given lens, then in shorthand (especially in marketing contexts), we say that the lens is “equally sharp throughout the frame” and is “flat field” in design.
The Schneider 72mm lens (aka 72mm Mk I) was and is a very good
lens; in fact, it was easily the best lens of its category. It was designed by Schneider Kreuznach for use with technical cameras in applications such as art reproduction, architecture, and landscape photography. Because of the emphasis on these areas of work, and owing to its symmetrical and elegant optical design, it performed very well in cultural heritage imaging applications. Used in the DT RCam and Phase One iXG at resolutions up to 100mp, this lens holds detail to the edge of the frame in a way that surpasses FADGI 4-star quality requirements. However, for larger subject matter (e.g. 30x40″) the focus field is only just-barely flat enough to produce excellent quality in the corner of the frame. This places special onus on the user to make sure the camera is especially planar (not tilted out of being parallel with the subject) and at the right aperture, and the focus was set perfectly. Again, this lens is second only to the new 72mm in quality, and can be properly lauded as excellent, but with 150MP+ sensors on the horizon we need a new lens would be needed to maintain our strict quality standards.
Brand new optical design, by Phase One
The new Phase One 72mm Mk II lens was designed by Phase One’s industrial team and manufactured out of Phase One Japan’s state-of-the-art lens fabrication center. Developing a new lens is one of the lengthiest types of R+D projects an imaging company can undertake, so it’s no surprise that initial work on this lens began shortly after DT and Phase One formed our alliance in 2013. At that point, it was clear that as sensors got to 150 megapixels of resolution and beyond, the Cultural Heritage market would need a lens designed exclusively for CH needs. What makes CH needs unique is the physical size and flatness of the subject matter, and the need for detail to be sharp throughout the entire captured frame. For example, compared to landscape photography, CH institutions very rarely need to capture subject matter larger than a few meters in size (and when they do, they often capture it in smaller tiled sections that are then stitched together). Compared to a portrait photographer where the subject (the face) is often in the inner half of the frame, the subject often extends all the way to (just shy of) the corner of the frame. But there are also needs that are less stringent in CH contexts. For example, portrait and street photographers demand lenses with very large maximum apertures that allow shallow DOF and blurry backgrounds, while CH institutions almost always use their lenses stopped down (e.g. to f/8 or f/11). Likewise, most kinds of photography call for lenses that can be focused very quickly to allow shooting in dynamic scenes while CH institutions are fine with focusing taking several seconds as long as it is incredibly accurate. By focusing the design of the Phase One 72mm Mk II lens exclusively on the needs of CH institutions the design team was able to engineer a lens that excelled at those needs (but would be a pretty crummy lens for a street photographer, fashion photographer, sports photographer, etc., etc.).
One design choice that must be called out is the novel use of an asymmetrically controlled floating lens element. Inside of most lenses is a family of glass elements that move together as one unit; when you focus the lens the entire family moves at once. It’s rare, but sometimes a lens will incorporate what is known as a floating element, a glass element that can move independently of the rest of its family. For example, the excellent Rodenstock 105HR Macro is labeled with different magnifications on a ring that you had to turn, separately from the focus ring, to optimize performance for different sized subject matter. By changing this intra-lens glass element spacing the user is effectively modifying the lens design to account for a new subject size; it’s a powerful tool that allows a lens to perform optimally over a much wider range of material. In a few cases, manufacturers have even designed autofocus lenses that used a floating element. For example, the Schneider 120mm LS BR lens uses a floating element that allows it to perform well all the way from infinity (very large objects) to 1:1 magnification (macro for small objects). But such lenses only have one autofocus motor, so the movement of the floating element has to be proportional and symmetrical to the movement of the overall lens, which is a huge constraint.
Razor sharp from edge to edge
The Phase One 72mm Mk II lens features a unique asymmetrically controlled floating lens element. As the user changes PPI (e.g. from 300ppi for a 46″ maps to 2000 ppi for a glass plate negative) the floating element of the lens is moved independently of the lens focus and camera position. This sounds complicated, and it is… for the engineers. But all three movements (the camera on the stand, the lens on the camera, and the floating element inside the lens) are automatically orchestrated by Capture One CH. The user just asks for a PPI, and the software does the rest. The end result is a lens that exceeds FADGI 4 in quality, everywhere in the frame, without muss or fuss, for subject matters from 6″ to 60″ and at up to 2000ppi. Best yet, no extension tubes are needed, so the entire range can be handled without touching the camera, switching lenses, or adding/removing extension tubes. For very small items (e.g. most film or small natural history specimens) the 120mm ASPH lens is still preferred, but everything else can be done, at the highest possible level of quality, with a single lens.
This is the lens you’ve been waiting for.
Note: the Phase One 72mm Mk II lens is compatible with both Phase One iXG and Phase One iXH platforms.
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