Canon FD Lenses - Other Issues Part III


What Those Numbers Mean on a lens

Take a look at virtually any lens designed for 35mm cameras and you'll see several groups of numbers. Once you discover the functions of the various indications you'll find them extremely useful.

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First, let's look at the front of a lens. Typically you will see something on the order of 50mm 1:1.4, 85mm 1:1.8 or other combinations. The first number, for example, 50mm, indicates the focal length which is an indication of the lens' angle of view and image magnification. Anything less than 50mm is a wide-angle, and generally anything more than 50mm is a telephoto lens. As discussed previously, the second number, for example, 1:1.4 or f/1.4, indicates the maximum aperture or widest opening of the lens.

< A newer FDn lens.

The larger the aperture, the greater its speed or light gathering power. A series of f/stops on the aperture ring indicate the diaphragm range (largest to smallest opening). Numbers may typically range from f/1.4 to f/22 or f/2.8 to f/32. There is also an "A" setting after the smallest aperture. This setting is used for automatic exposure operation on certain camera models.

A second series of numbers on the focusing ring indicate distance and range from the minimum focusing distance to infinity. The closest focusing distance varies with the focal length. Shorter focal length lenses - standard and wide-angle - permit shorter camera-to-subject distances than telephoto lenses. The exception to this are the macro lenses which are designed for very close focusing. The focusing distance is indicated in meters and feet.

On the lens barrel adjacent to the distance scale is the depth of field scale. The depth of field scale denotes the zone of acceptably sharp focus in front of and behind the subject. Depth of field is an important factor in controlling the effect of a picture and will be discussed in more detail in the section, "
Depth of Field."

The Function of the Diaphragm

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If you look directly into the lens barrel with the lens stopped down you will see the diaphragm, a set of metal blades for varying the size of the lens opening. More popular known as the "aperture", this opening regulates the amount of light that goes through the lens.

You can adjust the size of the aperture by turning the aperture ring on the outside of the lens barrel. The size of the aperture is indicated by a series of numerical markings called f-numbers or f/stops which are engraved on the aperture ring. The numbers actually refer to the ratio of the diameter of the opening to the focal length of the lens. A small number, let's say 1.4, indicates a large opening. A large number, 16 for instance, indicates a small opening. Thus as the numbers become larger the aperture becomes smaller allowing progressively smaller amounts of light to reach the film. Moving from a given aperture to the next smaller reduces the amount of light by half. In addition to controlling the amount of light, the aperture also controls the depth of field. When your subject is in focus, there is a certain area in front of and behind it which will also be in focus. This area of sharpness is called depth of field and varies depending on the aperture. Depth of field is also affected by lens focal length and shooting distance, and tends to be greater in the background than In the foreground.
* More info on Depth of Field.

Canon's Breech-Lock Mount and What It Does

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Basically, the lens mount fastens the lens to the camera - but there's a lot more to it than that. If you'll look at the rear of a Canon lens mount you will see several levers and pins. These devices serve to transmit and receive information to and from the camera. For example, data to and from the camera's light meter is transmitted through these pins and levers.
In addition, the signal from the shutter release to the automatic diaphragm mechanism is sent through the lens mount/camera coupling system. There are two lens mount types: thread (or screw) and bayonet. Until recently, the thread mount was extremely popular. Not only manufacturers of cameras employing this mount, but independent lens makers also produced thread mount lenses. As cameras became more automatic and sophisticated, however, the thread mount began to be considered obsolete. It was simply impossible to retain the precision needed to assure accurate coupling of camera and lens. Constant lens changing produced wear that caused misalignment. In addition, changing thread mount lenses is a slow process. The bayonet mount is considerably faster, more able to absorb punishment and can retain the required precision.
The full aperture signal pin transmits the maximum aperture of the lens, automatically and without indexing, to the camera body, while the AE switch pin signals whether the lens is set at "A" (for automatic exposure). The aperture signal lever transmits the selected aperture to the camera's metering system. Pressing the shutter release activates the camera's aperture lever which in turn activates the automatic aperture lever of the lens to close the diaphragm to the preset aperture. An additional pin positions the lens correctly when mounted on the camera by mating with a slot on the front of the camera mount. The last pin is in reserve.

The design of the Canon lens signal pin system has made it possible to use the lenses on Canon cameras regardless of which exposure system they employ. They can be used with the
New Canon F-l's full aperture match-needle exposure system, or one of the automatic exposure systems, such as shutter-priority AE with the Canon AE-1, shutter-priority AE and programmed AE with the AE-1 PROGRAM, five mode AE with the Canon A-1, programmed AE with the T-50 or shutter-priority AE and multi-programmed AE with the T-70, used on the autofocus body of T-80 if a non AF lens is used or the eight exposure control modes on the mighty Canon T-90... . One of the important mechanical advantages of the breech lock design is that the levers and pins on the lens make contact with the camera body when the lens and body are first aligned. There is no pin rotation. With other cameras, the lens rotates with pins and levers rubbing against each other.

Old FD lens.jpg
Even if it claims they have no significant compatibility issue between the two but HOW to differentiate between the newer and older version of the FD lenses ? Basically, the differences between the two are the speed of mounting and with the older ones being generally larger and a more robust construction. Both have a locking mechanism 'A' for locking minimum aperture of any FD lens when use with automatic exposure mode in either Program AE or Shutter Priority AE modes.

1) Chrome ring; 2) tiny red mounting index; 3) Orange distance scale, 4) 'A' lock at different ring.

For models that have aperture priority or operating in manual mode, just press the locking button to release the aperture to any aperture setting that you want. Visually, most strikingly is being, the older FD lenses comes with a chrome ring after the aperture ring and the newer FD series have a black ring instead.


1) Black ring; 2) Bold red mounting index knob; 3) Green distance scale, 4) 'A' lock release Button next 'A'. Many offer f22 minimum aperture.
Backview.jpg Backview2.jpg

Sole.jpg sole2.jpg
The New FD lenses are bayonet mounting in mounting or lens removal operation. They're generally lighter in weight and more compact in size as compared (and stronger feel of plastic). The far left is a FD 50mm f1.2 as compared physically with a older FD 50mm f1.4 S.S.C..Most older FD lenses were marked with red S.C or S.S.C. lettering on the filter ring. The inner portion of the new FD mount has also provided with a red lens mounting index guide.

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The Canon FD mount:

1. Perfect fit between lens and camera body.
2. The toughness to withstand constant changing of lenses.
3. Total interchangeablility of lenses with all SLR's in the system.

4. Ease and speed of mounting and dismounting.
5. Complete accuracy of alignment of signal pins, levers and other contacts.
6. Absence of wear to the camera mount and the lens mount - even after long use.

Elimination of Mating Surface Friction

During those days, Canon does has a very valid and convincing reason to sell or promote the FD lenses. Traditionally, the usual approach to lens mounts has been the use of either a bayonet or thread system. Both designs create wear on the camera and lens mounts with eventual loss of seating accuracy. This can result in the deterioration of lens performance. The breech-lock design, which was introduced in 1959 - attrition is virtually eliminated because neither lens mount nor camera mount abrade against each other during the actual mounting process. And with a FD lens mounted on the camera, a locking system holds it in place. However, for some strange reasons, the current autofocus EF mount on the current lenses have 'reverted to standard bayonet design (Which Canon claimed others' lens mount design could caused 'wear and tear' over time to affect accuracy) and dropped the long seemed to be more logical lens mount design than other manufacturers. Anyway, that is not the issue and as along as you are using the FD based system, you will have something to cheer about.

Other than possible issues arose from the possibility of 'wear and tear', one of the other great advantages of the breech-lock system is its inherently greater accuracy of signal pin positioning. At the rear of the FD lenses are five levers and pins that transmit information between camera and lens. This signal system was first introduced in 1971.

Canon could have been anticipated the extremely high level of sophistication of their coming SLR's when it was duly introduced, for example, the system provided for automatic exposure before the development of such fine AE cameras such as Canon's AE-1, AE-1 PROGRAM, A-1, AL-1, T50, T70, T80 and T90 cameras which all have different sophisticated levels in their respective automatic exposure control methods.

The FD Mounts can also be regarded as a bayonet mount but it is still Breech-lock in nature.

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The concept of the non-rotating, mating surfaces of the FD mount is quite unique as none of its competing manufacturers offer similar concept and design. To mount a FD lens lens, you first align the red dot on the lens with the red dot on the body (older FD has a tiny dot at the chrome ring). The positioning pin on the lens nests in the alignment slot on the camera mount. You then rotate the entire lens barrel clockwise until the pin pops out and the lens is locked to the body. The pins and levers on the FD lenses remain stationary, mating with their counterpart when you first align the lens with the camera.

Older FD lenses with chrome ring require to turn the ring to secure them by locking it in place, the newer FD, combining the best of Bayonet and the essence of the breech lock concept, is far more convenient and responsive as compared. It permits single hand operation by just a twist.

2 above illustrates the FD mount system. The rotating portion of the mount is represented by the blue section of the drawing. The yellow section shows the part of the lens mount that does not move when you attach a lens. The various signal transmitting mechanisms are in this yellow area. As we mentioned earlier, mounting the lens automatically positions the various signal pins and levers. As shown in illustration below, a clutch pin allows the barrel on the FD lenses to rotate. The diaphragm, partially closed when the lens is off the camera, opens fully, as indicated at the bottom of the drawing. The actual opening of the blades is accomplished by means of a cam and lever arrangement.

Relative: Compatibility issues with older FL and R series lenses and related notes on operating such lenses with Canon SLR bodies .

mechanisml.jpg Detailed illustration available only in PDF format (468k).

Credit: Courtesy of Canon Marketing Malaysia.
Rotating the lens barrel activates the aperture cam (lower pale blue ring) which pushes on the lever. The lever in turn opens up the diaphragm blades (illustration uses only one blade for sake of clarity). There's an audible click when the lens locks in place. To change lenses, you push a button on the lens mount, releasing the lens, and rotate the lens counterclockwise about 1/5 of a turn.

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Canon FD mount Camera Bodies:
A Series:
AE-1 | AT-1 | A-1 | AV-1 | AE-1 Program | AL-1
T- Series:
T50 | T60 | T70 | T80 | T90
F-1 | New F-1
Canon FL Resources
Pellix | FTQL

Lenses: Canon FL lenses | Canon FD lens resources

Canon EOS SLRs | Canon EF lens Resources


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