Comparison of Film Camera Metering Systems: External Metering Window vs TTL Mirror Metering vs OTF Real-Time Film Reflex Metering

1. Physical evolution and optical path disassembly of the three major film photometry systems

The location of the photometric element of the film machine directly determines the photometric accuracy and usage taboos:

1. External independent metering window (External Cell / such as Canonet QL17, Leica M6 early stage, etc.):

• The photometric element (selenium photocell or cadmium sulfide CdS) is installed in a small window on the top cover of the fuselage or on the outer edge of the lens filter thread;
• Fatal Achilles' heel: What the component detects is "external ambient light", not the light that actually passes through the aperture and is projected onto the film! If you use a dark filter, use a telephoto hood, or install a macro adapter, the external metering window will not know about it, resulting in serious underexposure of the film!

2. TTL SLR prism internal metering (Through The Lens / such as Nikon FM2, Pentax K1000):

• The photometric element is installed on both sides of the pentaprism eyepiece or on the back of the secondary mirror; light passes through the lens aperture and is directly measured;
• Advantage: Use any filter, macro lens or telephoto lens, and the metering system will automatically complete the light attenuation compensation.

3. OTF real-time film reflection dynamic metering (Off The Film/Olympus OM-2 unique skill):

• During the exposure process, the reflector has bounced up, and traditional TTL is completely blind;
• Olympus installs two silicon photodiodes (SPD) at the bottom of the mirror box, directly aligns them with the computer coating pattern on the first curtain of the shutter curtain and the real photosensitive emulsion surface of the film, and measures the light reflected from the film at the moment of exposure in real time;
• Even if the light is suddenly turned on during a long exposure, the camera can instantly recognize it and close the shutter to cut off the exposure within milliseconds. It is the originator of TTL automatic dimming of flash!

2. Practical photometric blind spots and filter assembly pitfall avoidance rules

1. Filter trap of rangefinder off-axis metering cameras:

• If the metering window is located on the forehead of the camera body (not inside the lens filter ring): When a CPL polarizer or ND filter is installed, the camera metering value will not change at all! You must manually increase the exposure level on the body's ISO dial;

2. Interference of light leakage from SLR TTL eyepiece on photometry:

• When using a tripod for long exposures or taking selfies, keep your eyes away from the viewfinder eyepiece; strong rear backlight will flow along the eyepiece into the light metering element in the pentaprism, causing the light meter to misjudge that the environment is extremely bright, resulting in underexposure of the film! The eyepiece light shield must be closed or covered with black tape;

3. Slight deviation of OTF photometry on reflectivity of different brands of film emulsions:

• The base of traditional black-and-white film is grayer, and the reflectivity of color film emulsion is slightly different. Pay attention to fine-tuning under extremely harsh long exposures.

3. Suggestions on model selection and collection evaluation

• Pursue minimalism and pure mechanics: choose a fully mechanical body without light meter + handheld light meter;
• Pursuing the freedom of filter and macro creation: TTL metering SLR is the first choice;
• Pursuing long exposure of night scenes and macro flash calendering: Olympus OM-2 / OM-4 OTF system is still the supreme palace of film industry technology.

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