Lens mount

The purpose of the bayonet mount is to maintain the exact position of the optical elements of the lens relative to the photographic material or photosensitive matrix . At the same time, the mount makes it easy to replace the lens and establish its reliable mechanical and electrical connections with the camera. The main advantage of a bayonet mount compared to a threaded mount is the exact orientation of the lens relative to the camera, mainly relative to its mechanical and electrical connections ^[1] . This is especially important for the mechanical transfer of the value of the installed aperture to the exposure meter , the drive of the jumping diaphragm and the combination of the electrical contacts of modern lenses with microprocessors ^[2] . Electrical contacts are also an integral part of modern mounts. Their size and location are included in the concept of the standard.

The most important is the orientation of anamorphic filming lenses that give different image scales in the horizontal and vertical planes. In addition, the rim of some lenses requires an accurate position for the proper installation of auxiliary equipment: devices for macro photography , follow-up tricks and compendiums . A more technologically advanced and cheaper threaded fastener was replaced by a bayonet mount in the 1950s, as the thread does not provide sufficient mutual orientation accuracy. Another advantage of the bayonet mount is a higher efficiency of replacing lenses and less wear with frequent replacement.

In addition to attaching the lens to the camera body, the bayonet mount can be used to attach light-protective hoods and nozzles to the front of the frame. Some photosystems allow mounting the lens both in the normal position and in the inverted, for macro photography . For this purpose, not only the shank of the lens barrel, but also its front part is equipped with a bayonet ^[3] .

The bayonet is a high-precision mounting unit, consisting of the protrusions of the shank of the lens barrel , which are included in the corresponding grooves of the flange on the camera body ^[4] . The bayonet is locked by turning the lens a small angle (usually from 45 ° to 70 °) to the position when the flat protrusions (“petals”) of the adapter shank are fixed in the flange, going under its corresponding protrusions, most often spring-loaded. In other designs, it is not the lens that rotates, but a special union nut with curly slots covering the petals of the frame. Also, a large multi-thread with a short stroke can be used as a bayonet. Such designs make it possible to establish the connection of the rotary actuators of the jumping diaphragm directly when locking and maintain the orientation of the lens with each replacement. The main characteristics of the bayonet mount - the working length and diameter - determine the available range of focal lengths and lens aperture .

The accuracy of the manufacture of bayonet parts corresponds to the accuracy class of processing the film channel and alignment of the optical path. Particularly high are the requirements for the accuracy of observing the working distance and parallelism of the flange of the focal plane of the camera. The first parameter determines the ability to focus on "infinity", and the second - the sharpness of the image in the field. In addition to high accuracy, the bayonet mount must have a strength sufficient for reliable attachment of heavy lenses, as well as corrosion and wear resistance . Therefore, high-grade stainless steel is often used to manufacture bayonet parts. For light amateur lenses, durable plastics are allowed. In addition to the connecting parts, it is customary to include bayonet drives with a diaphragm drive, a mechanism for transmitting its value to an exposure meter, couplings with an autofocus mechanism located in the camera body, as well as electrical contacts. These contacts are used to transmit parameters to the camera and control the lens, as well as to supply power to the microprocessor. The power supply to the servos of television zoom lenses is supplied, as a rule, by a separate cable, and the bayonet itself does not contain contacts. Modern photographic mounts, specially created for autofocus optics, do not contain mechanical drives, as they are designed for electromechanical aperture and focusing drives integrated into the frame. Some mounts provide for mechanical communication, which in cheap lenses of this standard is replaced by electrical. These include the Nikon F mount , which provides for the drive of a jumping diaphragm and the transfer of its value according to the AI ​​specification. However, the most modern professional lenses with the designation E do not use any of these mechanical connections, and budget optics do not support the AI ​​specification using an electronic interface.

Mounts for cameras

Mounts for cameras are much more diverse than mounts for film and television lenses, developed as industry standards. Some types of mounts can be used in various equipment, both in cameras and in film and digital movie cameras. This is especially true for the types of connections used in systems with close frame sizes.

Movie

Bayonets for film-shooting lenses are developed taking into account the possibility of their use for various cinematic systems based on one media format. Therefore, the specification of such mounts clearly specifies the format of the film, and not the specific frame sizes of the cinema systems. So, most mounts for 35 mm movie cameras can be used for both spherical optics and anamorphic ones. Structurally, filming bayonets are distinguished by a deeper lens fit due to the design features of the mirror shutter . Its rotation in one plane allows you to make the rear segment of the lens is much shorter than that of optics for SLR cameras. The mounts used in television and video cameras are most often indicated by the size of the photosensitive sensors in fractions of an inch , since these formats are derived from vacuum transmission tubes , the image size of which is expressed by their outer diameter. Unlike photographic equipment, television mounts exist as an industry standard, with the exception of a half-inch Sony mount, which differs from the generally accepted analogue. In addition, unlike cameras that support a single type of mount, many types of film and digital cinema equipment are available in several versions with mounts for optics of different standards. Arri PL, Panavision PV and Canon EF mounts are most widely used in modern cinema equipment.

The bayonets of modern filming machines, in addition to fixing the lens and connecting it to the camera, perform the function of shifting the optical axis relative to the center of the film to change formats with an asymmetric frame layout to the Super-35 production format , the frame of which is symmetrical with respect to perforation . For this, the bayonet flange is fixed in the eccentric base, the rotation of which by 180 ° displaces the center of the bayonet ring relative to the frame window by 1.125 mm ^[20] . Such a device is combined with interchangeable frame frames and a clamshell module, making 35-mm movie cameras multi-format ^[21] .

Direct compatibility of various types of bayonet joints is not possible due to the different shapes and diameters of the connecting parts. The installation of lenses with one type of mount onto the camera of another system is possible only through the adapter or after replacing the camera flange. The latter method is applicable to some digital movie cameras that provide such a procedure.

At the same time, full compatibility is not possible in all cases due to the difference in working segments ^[22] . Thus, a lens equipped with a mount with a shorter working distance than the camera mount will not be able to focus on “infinity” during installation, since it is located too far from the focal plane. In some cases, this allows shooting at finite distances, for example, portrait. But in the case of a big difference, such a lens is suitable only for macro photography . Sometimes compatibility is achieved by remaking the frame, but such a procedure is possible only in the factory and is often comparable in cost to the cost of the entire lens. In addition, lenses with a short rear segment cannot be mounted on SLR cameras due to the presence of a movable mirror or mirror shutter. There are adapters with a diffuser lens that compensates for the difference, but this solution is not applicable for short-focus optics and often leads to a significant decrease in resolution due to violation of the correction of lens aberrations . Full compatibility of lenses with a short working length through the adapter is possible only in case of a large difference in connection diameters, when the diameter of the camera mount is much larger than the diameter of the lens shank. This is true mainly for large cinema mounts, on which you can mount photo lenses with a shorter working length.

In cases where the working distance of the lens exceeds the working distance of the camera mount or equal to it, optical compatibility is always possible due to the length of the adapter, but the electrical and mechanical connections in most cases remain inoperative. To obtain partial compatibility of electronic interfaces, special adapters are used, received the jargon name "dandelion".

However, if there is a jumping diaphragm mechanism in the lens, even when using a “dandelion”, its full-fledged operation is impossible due to the extreme complexity of mechanical combination of camera and lens drives of various standards. And in the case of an electromechanical drive of a jumping diaphragm (for example, Canon EF lenses ), working with the adapter is possible, most often, only with the diaphragm fully open. Some adapters for optics with a mechanical aperture drive (for example, Nikon) for convenience of focusing can be equipped with a manual drive that allows you to force open the aperture, which is closed in normal condition to the operating value. All adapters only support shooting in aperture priority modes or manual. Shutter priority and program modes are not supported due to the difficulty of implementing automatic iris operation on a “foreign” camera. The cost of adapters that support aperture automation and its closing is comparable to the price of a budget SLR camera ^[23] . The use of filming lenses for photography is also possible only in aperture priority modes or manual, since film optics are equipped with a diaphragm, which is manually closed.

IMS

IMS ( English Interchangeable Mount System ) - a standard for movie equipment, involving the use of interchangeable flanges with different mounts mounted on the body of single-matrix cameras. The system allows almost unlimited use of lenses of any standards and with any working lengths. Developed by P + S Technik ^[24]

• Lens barrel
• Speed ​​booster

• V. Gutskin. Mounting interchangeable lenses (Russian) // " Soviet Photo ": a magazine. - 1981. - No. 4 . - S. 42 . - ISSN 0371-4284 .

• Salomatin S. A., Artishevskaya, I. B., Grebennikov O. F. 4. Foreign filming equipment // Professional filming equipment / T. G. Filatova. - 1st ed. - L.,: Engineering, 1990. - S. 240—257. - 288 p. - ISBN 5-217-00900-4 .

• From the history of compounds (Russian) . Photographic equipment . Zenit Camera Date of treatment June 11, 2013. Archived June 12, 2013.

• Matthew Smith. Lens Mount Cinematography Mailing List (February 3, 2004). Date of treatment June 16, 2013. Archived June 17, 2013.