New High-Resolution Sensors and Compatible Lenses
Published on : Monday 06-09-2021
TFL-Mount lenses are a better choice for APS-C sensors as they are smaller, lighter, less expensive, say Nicholas Sischka and Adarsha Sarpangala.
In recent years, the machine vision industry witnessed many advancements in imaging sensor technology, mainly driven by the ever-increasing demands for higher resolution and performance to support the requirements of newer applications. It is essential for optics manufacturers to also increase their offerings by pushing the limits of the optical and mechanical designs to keep up with this sensor evolution.
From a fundamental perspective, optics technology has not changed since lenses were first designed and manufactured. Essentially, the ability to design and manufacture more precise optical components and systems has improved greatly, but lens design fundamentals are still the same. It is now commonplace to see sensors in the industrial marketplace with over 20 megapixel (MP) resolution at a reasonable price and with good performance, all packed into a C-Mount housing, representing a 2000X increase from the first CCD sensor invented in the 1970s. This magnitude of technological advancement over such a short duration of time has not been experienced in the field of optics since then.
There are two ways to increase sensor resolution: decrease pixel size while keeping sensor size constant, or increase sensor size while keeping pixel size constant. There are trade-offs with each of these methods; small pixels tend to produce low signal to noise ratio (SNR) when compared to larger pixels, but larger sensors tend to be more expensive. However, a current general trend is increasing sensor sizes even as pixels continue to decrease in size. The machine vision industry is currently at an interesting time, with sensor sizes maximising the capabilities of C-Mounts. A C-Mount is a camera mount standard that is defined as a 25.4mm (1”) threaded mount with a back flange distance (often referred to as flange focal distance or simply flange distance) of 17.526mm.
The maximum diameter of an optic in a C-Mount housing is around 17mm, despite the opening being 25.4mm. This is due to the fact that the lens is held in place by retainers within the main lens and focusing inner barrels. Each of these steps reduces the size of the available clear aperture of the optical elements in the lens assembly. As the size of the optics is decreased, the angle at which the light must leave the exit pupil of the lens is increased. This is not an issue when sensors are smaller than the size of the optics, but as sensor size increases, it becomes more difficult for the optics to pair well with the sensor inside of the C-Mount. As this angle is increased, the corners of the image become much darker due to cos4(θ) roll-off. These compounded effects limit the size of a sensor in a C-Mount camera to a 1.1” format (17.6mm diagonal).
The third-generation release of Sony’s Pregius sensors came with the IMX342, which is a 32MP resolution in an APS-C format (28mm diagonal). While this sensor is far too large for a C-Mount it is also too small for an F-Mount, which is the next size up and has several optical problems associated with it. This leaves an interesting product gap in the industrial market. An M42 mount is a potentially logical option, and cameras already exist with M42 lens mounts, though there exists no commonly accepted standard to which the camera industry adheres that makes this option viable (with varying flanges and thread pitches). However, the TFL-Mount is the perfect mount for a sensor of the APS-C size. A TFL-Mount is M35x0.75mm, with a 17.526mm flange distance; this is the same flange distance as the C-Mount. For these reasons, the TFL-Mount can be thought of as a larger diameter C-Mount.
The TFL-Mount has several advantages over the F-Mount for an APS-C sensor size. These advantages are overall size, flange distance, and the way that the lens is secured into the camera lens mount. F-Mount lenses tend to be large when compared to C-Mount lenses. This is mostly because they cover larger sensors. As the size of lenses increases, they become more expensive as well. As a general rule of thumb, the cost for a single lens element increases directly with the square of the radius. Extrapolated over several elements, it is easy to see why larger lenses are more expensive.
Another major advantage of the TFL-Mount solution over an F-Mount solution is the flange distance. As mentioned above, the TFL-Mount can be thought of as a larger diameter C-Mount because they share the same flange distance of 17.526mm. The F-Mount has a flange distance of 46.5mm which limits the type of optical design form that can be used. This is especially true for shorter focal length lenses, which tend to have shorter back focal lengths (the distance from the last optical element to the image plane) as well.
Making a short lens with a long back focal length (BFL) forces the lens to be designed as a reverse telephoto lens, which is a lens with a focal length that is shorter than the overall length of the lens. Forcing a lens into this design paradigm inherently causes trade-offs with the design in terms of resolution. In certain circumstances, this can be remedied by using lenses with larger rear protrusion, meaning that the lens protrudes into the camera housing. However, lenses with large amounts of rear protrusion usually require a substantial reduction to their diameters to properly fit inside the camera body, leading to the same cos4(θ) issues listed above.
The shorter flange distance of a TFL-Mount not only contributes to an overall shorter system, but allows the optical designer much more freedom when designing to maximize the lens resolution. This smaller flange distance, coupled with the fact that TFL-Mount lenses will be designed for smaller sensors and be less impacted by field-dependent aberrations, means that lenses designed for TFL-Mount cameras will be smaller yet higher performing, and will be more cost-effective.
Lastly, an F-Mount is not a threaded mount; rather, it is a bayonet mount. Bayonet mounts are great for commercial photography, as they allow the user of the camera to quickly and efficiently swap lenses for different scenarios and allow for the easy integration of electronic features (i.e., iris/focus control) since they are a clocked mechanism. However, for the majority of applications in machine vision, these advantages are not really beneficial. Lenses are rarely (if ever) replaced, and if they are, replacement is not time sensitive. Iris control is useful in corner case applications, but placement of the iris is usually fixed and relying on micro or servomotors with moving parts to focus a lens in a factory environment will likely lead to worn out parts.
Moreover, the major disadvantage of the bayonet mount is simply the nature of the mount itself. As sensors are made larger, the amount of allowable tilt with respect to the optical axis decreases. Keeping the amount of tilt small is imperative to ensuring high optical performance, especially with low f/# systems, which are required for many high-resolution applications. The bayonet mount allows for more tilt in the imaging system, and does not pair the lens and camera together in an optimal way.
TFL-Mount lenses are certainly the better choice for APS-C sensors as they are smaller, lighter, less expensive, and higher resolution than F-Mount lenses. While TFL-Mounts are gaining popularity in the machine vision marketplace, it will be some time until they are as universal as the F-Mount. However, TFL-Mounts are a promising new take on the optics and cameras that are used every day. Sensors will continue to evolve, and camera lenses will continue evolving right along with them. Sensors that are too large even for a TFL-mount (and more suitable to a TFL-II) are already on the market (such as the Canon 120 MP sensor), and lenses will need to be designed that can handle these ever-increasing resolution requirements.
Nicholas Sischka is Manager of Sales Operations of Imaging Business Unit at Edmund Optics' Barrington, NJ, USA office. Specialising in optics for vision systems, Nicholas supports imaging and machine vision customers with application knowledge and design assistance for modification or customisation requirements. Additionally, he has successfully trained EO’s global vision partners on the specifics of how optics interface with cameras and lighting. An accomplished author, Nicholas has written multiple articles on such diverse topics as using optical filters to enhance image contrast and liquid lenses for machine vision. He holds a B.S. in Optical Sciences and Engineering from the University of Arizona, College of Optical Sciences.
Adarsha Sarpangala is Imaging Business Key Account Manager in Edmund Optics India Pvt Ltd, Bengaluru office. He manages the machine vision business of Edmund Optics for the Indian market. Adarsha has around 10 years of experience in the machine vision industry including selection of cameras, lens, lighting and machine vision solutions. For any queries on the articles or machine vision applications he can be contacted on firstname.lastname@example.org