Characteristics of a Good Lens Mount
Most industrial cameras, like single-lens reflex and large-format photographic cameras, have interchangeable lenses to allow the user to select a lens that matches the requirements of a particular task. For reliable interchangeability, a lens mount must be designed to assure that any lens with the mount will be located the proper distance from the image plane with its optical axis normal to the image plane. The mount must be able to accommodate large numbers of lens changes without losing positioning accuracy and support lenses of widely varying size and weight. Mounts should also be simple to use, providing some indication when the lens is properly and securely mounted. Most mounts do most of these things but the exceptions are important to understand.
Distances Behind the Lens
Since the basic purpose of a mount is to position the lens properly relative to the image plane, the distance from some reference surface on the back of the lens to the location of the focal plane in air, called the flange focal distance orregister, is the first defining characteristic of a lens mount. In some mounts, this surface is visible and can be easily used to set up a camera. In others, it may be hidden inside a mechanism or recessed and not so easily referenced. For each mount used, locating this surface on both the lens and the camera is important because dirt buildup or wear here can cause lens mis-focusing or misalignment.
Im almost all actual lenses, the reference surface is not the last surface in the back of the lens. Typically, there are other things, either mounting mechanisms or lens elements that protrude behind the reference surface. Some of these may be outside the mount but others fit in the space in front of the image plane. if this space must also accommodate filters or shutters or other optical components, knowing the actual space available is critical. This is especially true when lenses that have significant protrusions are mounted in front of image sensors having protective windows set far in front of the sensor imaging surface. The actual mechanical clearance distance is called the back working distance. This must be clearly separated from the back focal distance, which is the distance from the vertex of the last optical element in the lens to the image plane. In many lenses, metal parts protrude farther than the last optical element so the clearance limit is the back working distance. Of course, if the lens element protrudes farthest, then the two values are the same.
Each lens mount has, in its original specifications, a throat diameter, the size of the smallest opening inside the mount. In a Nikon F mount, for instance, this is supposed to be 45 mm. Compliance with this minimum assures that none of the marginal rays from the lens are cut off by the mount and that any part of the lens that must protrude into the mount will fit. As long as lenses are used only on the mounts for which they were designed, this is not an issue. However, when adapters or extension tubes are used, the minimum diameter may be reduced or the minimum may be positioned in a place where rays are blocked.
For example, most Nikon F-mount lenses when used on a C-mount camera with an adapter will produce uniform images but since the throat diameter for a C-mount is no more than 23 mm, often less, care is required to assure that the corners of the sensor are properly illuminated. If, in addition, extension tubes are required, the Nikon end should be extended, not the C-mount end, to avoid severe vignetting.
Lens Mount Types
Four types of mounts are in common use on cameras - threaded mounts, bayonet mounts, breech mounts and setscrew mounts. Each of these types has its advantages and weaknesses. Of each type, there are examples of better and worse implementations. History has prevented some of the most problematic from disappearing so understanding the particular problems with each mount can minimize the chance that a shortcoming in design will result in damage to the lens or camera. There are at least 100 different mounts that have been in service, many just slightly different from others to prevent cross-utilization by photographers of one brand of lenses on another brand of camera. Many are now only of historical interest. This listing includes only those in common use now.
These are called screw mounts because the mount is threaded and so the lens screws into a hole the camera. Probably the most common screw mount is the C mount. Originally developed for 16mm cine cameras (hence the "C"), this mount has a 1 inch diameter, 32 thread per inch screw. The screw protrudes from the reference surface, typically about 3/16 inch (4 mm) so part of the space between the reference and the image plane is always taken up by the mount itself. Screw mounts have two significant shortcomings - they do not lock in place and they do not orient the lens markings at a predetermined angle relative to the camera. The main advantages are that they have no moving parts and that they are inexpensive. They are also the easiest mounts to make small. This is the only mount type that allows the use of thin shims to make small adjustments in the lens position relative to the image plane. Screw mounts are available in a large variety of sizes as shown in Table 1. In the table, the mount name is given and then the corresponding thread size and flange focal distance (also called the "register) and notes on origin or use.
Table 1 - Screw-type lens mounts
|Designator||Thread||Flange Focal Distance||Notes|
|C||1" x 32 tpi||0.69 in (17.526 mm)||Originally denominated in inches|
|CS||1" x 32 tpi||12.526 mm (0.493 in)||Same as C with a 5 mm shorter FFD|
|Canon screw||M39 x 1 mm||28.80 mm||Close but won't fit Leica screw cameras|
|Leica screw||M39 x 26 tpi||28.80 mm||Don't try to force a Canon screw into this|
|Pentax screw||M42 x 1 mm||46.46 mm||Often mistaken for M42 x 0.75 mm|
|S||M12 x 0.5 mm||1 to12 mm or so||Also called M12 mount - for board cameras|
|T2||M42 x 0.75 mm||55.00 mm||Often mistaken for M42 x 1 mm|
|TELI TFL-II||M48 x 0.75||17.5 mm||Sort of a C-mount for bigger sensors|
|Microscope objective||0.8 in x 36 tpi||several options||Verify that the objective makes an image|
Bayonet mounts are the twist-lock type. The lens is inserted into the camera and twisted 30 degrees or so until it is caught by a rotational latch. The primary advantage of this mount is that it provides a repeatable rotational position for the lens relative to the camera, assuring that the markings are in a useful location and that any electrical contacts required between the camera and lens are correctly aligned. It also provides repeatable axial positioning of the lens, usually through spring tension to assure seating of the flange reference surface. The one issue with bayonet mounts is that they can wear because the surfaces rub against one another every time the lens is inserted or removed. This may be good for the electrical contacts because it keeps them clean but it can cause errors in focus distance if the reference surfaces become damaged. The bayonet end of these lenses should be kept protected from damage and dirt when they are off the camera. Table 2 shows some common bayonet lens types.
Table 2 - Bayonet-type lens mounts
|Designator||Flange Focal Distance||Notes|
|B4||48 mm in air||Used in 3-chip HDTV cameras with prisms|
|Canon EOS||44.00 mm||Has no manual focus ring|
|Diop||1.549 in from boss||For mid-wave infrared lenses|
|Four-Thirds||38.67 mm||Use of mount requires license|
|Micro Four-Thirds||19.25 mm||Use of mount requires license SEE PRODUCTS HERE|
|Hasselblad 500||82.10 mm||For large-format sensors|
|Leica R||47.00 mm||Parts available from Leica|
|Nikon F||46.50 mm||Most common, parts available from Nikon|
Sometimes called Reverse Bayonet mounts, breech mount lenses have a rotating locking ring. This allows the lens to be inserted in its final position on the camera and the be secured with a twist of the locking ring. The major advantage of this is that the flange reference surface never slides against the camera reference surface because the lens itself is never twisted. These mounts are probably the most secure since the locking ring can have a very sturdy latch and can provide stiff springs to hold the lens against the camera and the lens can have a locating pin that fits in a hole in the camera mount to prevent rotation. The breech is somewhat more bulky than the bayonet and costs a little more. Like the bayonet, the breech always positions the lens in the proper rotational orientation with the camera. Breech mounts are now uncommon but are still in use in special situations. Table 3 shows the only current example.
Table 3 - Breech-type lens mount
|Designator||Flange Focal Distance||Notes|
|Canon FD||42.00 mm||You have to make your own parts now|
In setscrew mounts, the lens typically has a groove around a tube at the mounting end. The tube slides into a large nole in the camera front and seats against a reference surface. Then setscrews in the camera are tightened to retain the tube. Most often, the groove is in a "V" shape with the setwcrews aligned to be just above the bottom of the "V" when the tube is properly seated. Thus, when the setscrews are tightened, the lens is drawn into the proper axial position.
One big advantage of this mount is that it allow the lens to be rotated freely into any position the user wants while maintaining proper focus calibration. In fact, some mounts and adapters are made in two pieces with a setecrew section in the middle to allow lenses with other mounts to be freely rotated during system setup.
The setscrew mount is also called a "V" mount amthough some setecrew mounts do not have the "V" groove. Lenses designated as "V" mount must carry additional dimensional information because there is no standard for "V" mounts. The two most important dimensions are the diameter of the tube and the back focal distance of the lens. However, the size of the "V" groove can vary between lens types so the position of the setscrew may also be important. Note that the Hasselblad and Minolta V-mount lenses are bayonet mounts, noty setscrew mounts.
V-mount setscrew lenses are made by several machine vision lens manufacturers, mostly for macro or line scan applications. Due to the lack of standards, evaluate each combination carefully.
if you already have lenses you like, then you will eventually want to use them on a camera with a different mount. For this, adapters are available. Most adapters are useless with modern photographic cameras due to the complex electrical and mechanical coupling needed for motorized focus and iris and sometime image stabilization or to read lens information. Industrial and scientific cameras usually don't have these connections so just a mechanical tube with the right fittings will suffice.
The basic rule for adapters is this - you can mount a lens with a long flange focal distance on a camera with a shorter one but not vice versa. For instance, you can put a Nikon F lens (46.50 mm FFD) on a C-mount camera (17.526 mm FFD), but not the reverse. If you try the reverse, the lens will be too far from the sensor and you will only be able to focus at very short distances, if at all. Adapters have been made with lenses inside to allow shorter FFD lenses on longer FFD cameras, but these are rare.
Be aware of a couple of potential problems with adapters. if your lens has a part that sticks out behind the mounting flange, make sure there is room inside the adapter for it. This is a common problem with C-mount adapters. if they taper too quickly, your lens may not fit. Also, be sure your adapter is sturdy enough to hold the lens. A big telephoto lens is going to need support other than the mount. In many cases, you will mount the lens and let the camera hang on the adapter (but remember to support the camera cables). Never put stress on an adapter, it will make the focus non-uniform. Finally, read the section on throat diameter above. This can be a big problem with some lenses when used on C-mount adapters.
If you need to focus close to the camera, you may be able to use extension tubes to move the lens away from the sensor. Success in this depends on the design of the lens so experimentation is always required. An alternative is to place close-up lenses on the front of the lens. Which works better in any particular situation is a matter for testing. if you need to combine extension tubes with an adapter, it is usually better to use extension tubes on the lens side rather than the camera side to minimize the effects on vignetting. The exceptions may be using S-mount lenses or microscope objectives on C-mount cameras since the C-mount has a larger diameter.
Here are a few places that have much longer lists of mounts, many of which are now obsolete: