In September 2019, Phase One has released a new camera system, the XT medium format system. It is a mirrorless and therefore smaller camera system than the Phase One XF, and offers so many useful features. A year earlier, the IQ4 series of digital backs has been introduced, including the IQ4 150MP (151 megapixels), the IQ4 150MP Achromatic (151 megapixels), and the IQ4 100MP Trichromatic (101 megapixels). Phase One was so kind and let me test their new XT system with the IQ4 150MP digital back and the XT HR 32mm Rodenstock wide angle lens. The tables below show the basic technical specifications of the Phase One XT camera system, the IQ4 150MP digital back, and the Rodenstock lens family.
After spending an entire week testing the XT camera system and trying it in various different situations, I had so many impressions and experiences while working with the system. The feeling that I remember best is the one I had when first holding the entire system in my hands: When taking the XT system out of the case, my fingers enclosed the grip of a heavy piece of precision engineered metal equipment. For me personally, doing a lot of photography on a tripod, the weight was indeed a positive surprise, because on closer inspection it became immediately clear that this weight is a result from an extremely high build quality. There is pure metal everywhere, most of it on the XT body, but also on the IQ4 digital back, and the Rodenstock lens. The weight of the entire system actually indicates that it uses a greater material thickness than typical cameras, making the Phase One XT an extremely robust camera system. The overall appearance is pretty different from other cameras: The camera body has a large circular guiding rail integrated to allow for a 90-degree rotation between landscape orientation and portrait orientation without removing the camera from the tripod. This adds to the very technical look of the system. The lens also has a rather special shape that is different from most lenses that I have seen before. The XF lenses have a leaf shutter built into a small mechanical case that is integrated into the lens and that protrudes from the lens case. In addition, for a wide-angle lens like the Rodenstock 32mm, the lens barrel is longer than it would be on a single lens reflex camera. The reason for that will be explained later. The IQ4 digital back looks very similar to its predecessor, but a label on the side proudly indicates that it is the 150MP-version that offers an image resolution of 151 megapixels. The gallery below shows some details of the camera system when I first took it out of the box.
The Phase One XT medium format camera system is completely modular from lenses to the digital backs. In the past with analog medium format photography, it was pretty common to have replaceable camera backs where the photographic film was loaded, but for digital camera systems, having a replaceable sensor back is pretty unique. This high degree of modularity makes the entire system a lot more interesting as a long-term investment as it can be upgraded with future generations of digital backs without having to invest in a new camera body. The XT system can also integrate with lenses from other manufacturers, like Schneider Kreuznach Lenses, Mamiya Lenses, Hasselblad V Lenses, or Pentacon Lenses via an adaptor.
It is a mirrorless camera, and therefore smaller than the XF system that I have reviewed in 2016. Admittedly, even with no mirror built into the XT body, the system is still not particularly compact. This is due to the fact that a camera lens cannot sit directly in front of a camera sensor on order to produce an image. There must be a minimum distance (called flange distance) between the rear glass element of a lens, and the sensor. With the mirror removed, there is plenty of space that appars to be useless, but it is required for proper image formation. Now that the XT camera body is totally flat, the lenses themselves have to accomodate that flange distance, explaining their additional size.
There is a quick rotation function integrated into the camera body that rotates the XF system exactly around the optical axis. This really is a great feature for all types of photography where the position of the subject should remain exactly the same in relation to the lens when the image orientation is changed. For instance, if the XF system is used for product photography, and the camera lens is perfectly pointed towards a subject that has been carefully placed in the perfect angle towards the camera lens, rotating the XF system around the optical axis will keep the perspective towards the subject intact. All the tripod mounts that I have used before actually tilt the entire camera around a pivot that is anchored somewhere at the tripod head, rotating the camera but also displacing the optical axis. This seems like a very small effect, but product photographers are going to love this solution.
An even more interesting and useful feature of the XT system is that the body has a shift function integrated both horizontally and vertically. While most other camera systems require specialized tilt-shift lenses to achieve this effect, the Phase One XT system allows shifting of the image circle for any optical system that is attached even if the lens doesn’t have a tilt-shift function itself. Shifting is extremely useful to maintain straight lines for architecture and interior photography but also when stitching two photos for panorama photography. This feature is enabled by moving the back plate where the digital sensor back is attached horizontally and vertically. Therefore, the back plate with the sensor can be moved horizontally and vertically from the optical axis. Since the back can also be shifted horizontally and vertically at the same time, the sensor can also be moved diagonally from the optical axis. To shift the camera sensor unit in either direction there are two shift dials, one on the top of the XT body, and one on the side. The horizontal dial (the one on the top) allows to shift the camera sensor 12mm to the left and 12mm to the right from the optical axis. The vertical dial (the one on the side) allows to shift the camera sensor 12mm up and 12mm down from the optical axis. There are also two mechanical displays indicating the current position of the camera back in relation to the central position.
Shooting with the Phase One XT IQ4 is a special experience! First of all, the camera is surprisingly easy to control. After starting the IQ4 which takes rd. 20 seconds, the touch display on the IQ4 offers an intuitive menu that can either be controlled via the screen or four buttons next to the display. The only electronic button on the XT body is the two-step shutter release that lets the user customize the half-press sensitivity. However, even more useful is the shutter release button on the touchscreen of the IQ4, especially when the camera is in a position where the top of the XT body is hard to reach. I started to use the on-screen shutter release button a lot. For a mirrorless camera system, the screen is also the main viewfinder where the image is checked for composition, exposure and correct focus. One thing that is extremely useful is the visual feedback of the IQ4 screen on the in-focus areas of the image. Therefore, the IQ4 marks areas that are precisely in focus with a customizable color so that the user gets an instant confirmation whether he placed the focus correctly. To check even more on the focus, the IQ4 can zoom into the image (up to 300%) during live view so that focus changes can be tracked directly on the screen, allowing to adjust the focus just down to the pixel level. The live view mode of the IQ4 also allows for an overlay with histograms and clipping warning.
The IQ4 digital back doesn’t come with any automatic or semi-automatic functions like landscape mode, or others. It requires fully manual adjustments of ISO, shutter speed, and aperture. This is certainly due to the fact that the target group of this type of camera system is professionals and not users looking for a snapshot camera. However, due to the live view mode that simulates the exposure of the final image, it is still easy to use for people who aren’t familiar with manual-only cameras. The IQ4 also brings some very nice features: Dual Exposure is a built-in high dynamic range mode composed out of two images that are taken from the same subject and combined to one shot. Another very special feature is Automated Frame Averaging where the user can select a rather fast shutter speed for the exposure brightness, but several minutes for the total time the camera records the scene. This results in a blurry look of any moving subject like a flowing river or a waterfall.
Although the Phase One XT is designed as a field camera and can be used for freehand shooting, I have decided to use a tripod as often as possible. The camera itself mounts to a tripod via an integrated Arca-Swiss-type rail that tightens itself onto the tripod mount by a finger screw. When switching the camera from landscape to portrait orientation, there is a quick release clamp at the lower side of the XT body. Once that is open, the entire camera system including the lens, the body with its handle, and the digital back rotate around the optical axis counterclockwise for up to 90 degree. This is particularly nice when rotating the camera during live view as this shows how precisely the rotation keeps the subject in place.
Using the shift dials for vertical or horizontal shifting of the sensor unit is surprisingly smooth. Both dials feature detents every 5 millimeters that give the user some tactile feedback when operated.
Shifting the camera back down (which shifts the image up) is extremely smooth because gravity helps, while shifting the camera back upwards is a little bit harder but still manageable with one finger. Shifting the camera back either to the left or to the right is equally smooth. When changing the composition during shooting, I found that it was sometimes hard to remember whether I have shifted the back on the previous shots. Therefore, the mechanical shift scales came in very handy, but also the IQ4 indicates how far the camera has been shifted into the X and Y directions.
Changing the lenses is really easy. The XT lenses are hooked in from below, and secured from the top by two locking systems. One of these locking systems consists of just a security snap lock that prevents the lens from falling out. The other locking system consists of two metal levers that apply pressure to the lens so that it sits tightly on the XT body. This not only ensures that the connection is light-proof but also that the distance from the rear glass element of the lens is absolutely precisely in the right position relative to the image sensor. When I worked with the XT system, I found that this secondary locking system was sometimes loosening itself, probably due to vibration. Luckily this didn’t result in the lens falling out, but the resulting images weren’t as sharp as before. Quickly tightening the two levers again by hand fixed that issue.
Regardless of any functions of the XT camera system, one effect has shown relatively soon: Shooting medium format is a pretty different type of photography. After less than 10 photos I realized that the manual-only settings and the large size of the image files (rd. 120 MB per image) slowed down my speed of shooting. Medium format photography is really something different than shooting with a smaller frame DSLR where it is rather normal to shoot 10 to 20 images per minute and to select the best shot afterwards. The Phase One IQ4 requires more thought about the image composition and challenges the user to get the most out of every single shot. It may seem like a very subtle factor, but this became very noticeable to me and the slower process was something I enjoyed very much.
Looking at the final results on the computer screen, the quality of the images the Phase One IQ4 produces was far beond expectation. There is sharpness in every part of the image, even in the corners there is almost no sign of chromatic aberration or decreased clarity. Also, there wasn’t a sign of any noticeable distortion that often occurs on wide angle lenses. It is also worth mentioning that the sensor’s color reproduction is very accurate and natural. What I found particularly impressing was the bit depth that the IQ4 offered. This became clear when I increased the brightness of dark areas in Phase One’s post-production software Capture One. Inceasing the brightness of extremely dark areas was able to bring up so much detail without visibly introducing noise, and made it look like there was no shadow at all. Below are some sample images that were shot with the Phase One XT IQ4 camera system. The images below have not been post-processed (except for resizing them for the website) and show some of the impressive abilities of the IQ4.
To demonstrate the image sharpness, here are two larger illustrations: Demo 1 and Demo 2. Right-click on each demo and select ‘open in new tab’ to open them in full size. Then zoom into the image to view all details.
When handling the Phase One XT camera system, it became clear that the camera is a highly specialized piece of photographic equipment. Almost every part is made from metal, very precisely machined, creating an extremely sturdy product. It is an incredibly powerful camera for certain types of photography including interior, fashion, products, reproduction, architecture, landscape, and others. Although it is designed as a field camera (as opposed to the XF system that was designed as a studio camera), I would recommend to use it with a tripod. If combined with a tripod, the entire system weighs around 4 kilograms. It is clearly not a travel camera, and also no action or sports camera.
Phase One is always the camera of choice when extreme resolutions are required. The IQ4 digital back produces images with 151 megapixels, super smooth tonality, natural color reproduction and very high dynamic range. It offers a very easy to use interface, perfectly reliable touchscreen, live-view with focus indication and histogram overlay, and additional creative features like dual exposure and automated frame averaging.
Unfortunately, what must also be factored into these pro arguments is the little design flaw with the lens mount, as well as the price of rd. USD 60.000 for the Phase One XT with IQ4 150MP digital back and Rodenstock lens.
At the end of my camera demo week, I had to return the XT camera system to Phase One. Thank you very much for allowing me to review the XT IQ4 150MP, an impressive piece of technology.
About the Price
For those who are asking whether a price of roughly 60.000 EUR for a digital camera system can be justified, there is no easy answer. I have collected a number of facts and considerations that try to explain the price from various perspectives:
When analyzing the Phase One XT IQ4 system from an engineering point of view, it becomes clear that Phase One did not just carry one feature like the sensor resolution to the extreme, but they polished and fine-tuned so many elements, and the resulting camera system is a combination of so many high quality factors that are reinforcing each other. The XT IQ4 camera system utilizes every technological way to get as close as possible to the physical limits of image formation. Here is a summary of the physical principles and key reasons that allow Phase One to deliver such a vastly superior image quality:
1] Lens Characteristics: First of all, the quality of the lenses makes a large contribution to the overall imaging performance of the system. The Phase One XT system integrates lenses that are custom-designed by Rodenstock, a lens manufacturer from Bavaria, Germany. Rodenstock has over 140 years of experience with designing all types of optical systems. For the Phase One XT system, Rodenstock created a series of lenses they call XT-HR Digaron, and they combine only the best optical characteristics: They were developed for applications with extremely high resolution sensors, and they provide incredible spatial frequency (a very technical term for sharpness) and contrast throughout all aperture settings, and they are so well adjusted to the Phase One XT IQ4 system that they even compensate for the thickness of the sensor’s protective glass.
Only Prime Lenses
For the Phase One XT system, Rodenstock designed their XT-HR Digaron series as prime lenses only. As opposed to zoom lenses that can change their focal length, prime lenses only have one focal length. This may sound like an inconvenient limitation, but there is a reason behind this concept. If an optical system had just a single lens element to perform the image formation, the resulting image would actually suffer from a number of imperfections called lens aberrations that are caused by the curvature of the lens element. Typical lens aberrations include image distortion (pincushion-type or barrel-type), chromatic aberration (lateral and longitudinal), spherical aberration, coma, astigmatism, vignetting, ghosting, and others. This causes an image to have visible defects like color fringing, or blurring. To reduce the aberrations of an entire camera lens, optical engineers place several lens elements with different physical properties into the optical system so that the lens elements cancel each others’ aberrations out. This is a highly complex design process called lens correction, and it is the reason why some camera lenses include up to 20 individual glass elements. The intensity of aberrations a lens element procudes is also depending on the path that incoming light takes through the lens. This variance makes it almost impossible to entirely correct a zoom lens, because this type of lens is designed to have different paths of light due to the different focal lenghts. For that reason, zoom lenses are some sort of trade-off between flexibility and performance. In contrast, a prime lens has a fixed focal length, and is therefore designed for only one specific path of light. Optical engineers can perfectly concentrate on the one light path that results from the fixed focal lenght, and can therefore optimize a prime lens much more than a zoom lens. This is the reason why prime lenses create sharper, clearer images with higher contrast.
Larger Image Circle
The XT camera system is designed with an integrated shift function that allows the camera back to be shifted horizontally (+12mm, 0, -12mm) or vertically (+12mm, 0, -12mm). To allow this shift function, Rodenstock’s HR Digaron lenses are designed with an image circle that is larger than in typical camera lenses so that the sensor still captures light when shifted away from the central position. It is a typical issue that the image rendering performance of an optical system is not the same across all areas of the image circle. In the center of the image circle, the imaging quality of a lens is typically the best. However, the image rendering performance including sharpness or contrast slowly decreases in the outer areas of the image circle, with areas furthest away from the center suffering the most. Even highly optimized lenses cannot completely remove this effect. However, this means that on the Phase One’s XT camera system, slight lens aberrations only start to appear on the image when the sensor is moved to the outmost shift position where it is closest to the edge of the image circle. In turn, this means if the image sensor is in the central position, it is completely covered by the inner region of the image circle where the lens has its maximum performance. This is actually a really nice side-effect of shift lenses, because they provide for ultra sharp areas even in the corners of an image (where other lenses already show signs of decreased quality). The IQ4’s image sensor measures 53.4 mm x 40 mm. The image circle of the Rodenstock XT-HR Digaron-W 32mm has a diameter of 90 mm. This shows how much space there is available for shifting, and how much edge region is ignored by the image sensor when in its central position.
Larger Lenses are more forgiving
When light travels through an optical system, it is refracted at each lens element until it finally reaches the sensor. In a camera lens that is designed for a smaller photographic system, the lens elements also have a smaller glass surface. Smaller lens surfaces in turn make smaller lenses more vulnerable for production variations or little imperfections. This is primarily due to the fact that small lenses have less surface area to compensate for imperfect areas or defects in the glass. On the other hand, lenses that are designed for large camera systems such as medium format cameras typically have large glass elements with larger lens surfaces. These larger lenses are more forgiving and more tolerant towards small spots with production variations or little defects in the glass, because the relation between a defective area and the entire lens surface is smaller when the lens is bigger. Simply put, in larger lenses small lens defects will not show in the final image, because there is so much more surface area in the lenses to compensate these imperfect spots.
2] Sensor Characteristics: Secondly, the IQ4 digital back with its image sensor and the processing electronics is a unique piece of photographic equipment, and it is another reason for the superior overall imaging quality of Phase One’s camera systems.
The IQ4 150MP digital back uses a backside-illuminated complementary metal oxide (BSI CMOS) sensor that offers an incredibly high resolution of 151 megapixels which captures the extreme sharpness produced by the lens. The active light-receiving area of the sensor consists of 14.204 vertical lines and 10.652 horizontal lines of pixels. This high resolution is stunning: It is more than 10 times the resolution current DSLRs from other manufacturers offer. If a photographer wants to print such an image with a dot density of 300 DPI (dots per inch) and without resizing the original, the print will have a dimension of 120.26 cm x 90.19 cm. However, there is a lot more about the sensor than its resolution.
The size of the IQ4’s image sensor is more than twice the size of those found in high-end DSLRs. Each pixel consists of an optoelectronic receptor, called photodiode or photosite, that captures light and converts it into an electric charge. Once the photo has been shot, this charge is translated into a number representing the brightness. Having a larger image sensor also allows to use larger sized photosites. And this in turn has some substantial advantages over smaller photosites: Small photosites tend to saturate (overfill) faster and when their storage capacity is reached, they simply ignore whether the scene is actually brighter. When shooting a subject with bright areas, small photosites will sooner produce a signal that represents full white (a phenomenon called clipping) even though the real scene wasn’t completely white. Due to their smaller storage capacity, the range between absolute black and full white is rather limited, which results in a lower dynamic range. Larger photosites, on the other hand, have a lot more storage capacity for incoming light, and have therefore a higher dynamic range. This means that they aren’t very vulnerable to clipping, and they can make more accurate representations of scenes with difficult light situations. Another beneficial effect of a larger pixel size is that they reach a better signal-to-noise-ratio which results in the final images having less image noise. Also, larger image sensors offer better low-light characteristics.
The signal that each photosite on the image sensor collects is an electric charge. Each particle of light (photon) that hits a photosite creates an electron in the photodiode’s semiconductor material. If the pixel doesn’t receive any light, the storage remains empty, and no charge is generated. If bright light hits the pixel, it generates countless electrons, and therefore a higher charge. In order to process the image, the charge of each photosite needs to be interpreted, and translated into a number. This is what an analog-to-digital (AD) converter does each time when the camera reads out the sensor and stores the image on the memory card. The bit depth is a number that represents how many different brightness values the AD converter can detect in a single photosite, provided that the photosite has enough dynamic range. Conventional DSLR cameras typically have 12-bit AD converters which can differentiate 2^12 = 4.096 different brightness levels between absolute darkness and full white. They store 12-bit image files where each pixel can have one of 4.096 different brightness levels, at least when the camera stores the RAW files. JPG images are limited to a bit depth of 8 which means that pixels in JPG files can only have one of 256 different brightness levels. While 12-bit is pretty respectable, the IQ4 has a 16-bit AD converter built in, and stores 16-bit images with a 15-bit dynamic range. Again, this applies only when RAW files are selected. This means that each pixel can have one of 2^15 = 32.768 different brightness levels between absolute darkness and full white. This incredibly large bit depth is what produces super-smooth tonal gradations in the final images which are extremely pleasing to the eye, and finer color details.