The optical format of an image sensor is a traditional way of describing the size of the sensor. It relates to the diagonal measurement of the sensor's active area and is expressed as a fractional or whole number in inches.
Understanding Optical Format
Optical format, often seen specified for camera sensors (like 1/2.3", 1"), provides a quick reference for its approximate physical dimensions. This numbering system originated from vacuum tube camera tubes (like Vidicons) used in early video systems, where the number referred to the outside diameter of the glass envelope of the tube itself. While modern solid-state sensors don't have these tubes, the nomenclature persisted.
Based on the provided reference:
- Definition: The optical format is approximately the diagonal length of the sensor multiplied by 3/2.
- Measurement: The result of this calculation is expressed in inches.
- Rounding: The final value is usually (but not always) rounded to a convenient fraction.
For instance, the reference states: "For instance, a 6.4x4." - This appears to be an incomplete example in the reference text itself (perhaps referring to sensor dimensions in mm), but it highlights the concept of linking sensor dimensions to the resulting optical format number. If we assume "6.4x4" refers to a sensor with dimensions 6.4mm by 4.8mm (a common 4:3 aspect ratio, diagonal ≈ 8mm), calculating the optical format would involve:
- Find the diagonal length of the sensor. For a 6.4mm x 4.8mm sensor, the diagonal
d = sqrt(6.4^2 + 4.8^2) = sqrt(40.96 + 23.04) = sqrt(64) = 8 mm. - Apply the formula: Optical Format ≈ Diagonal Length * 1.5
- Optical Format ≈ 8 mm * 1.5 = 12 mm.
- Convert to inches: 12 mm / 25.4 mm/inch ≈ 0.47 inches.
- Express as a traditional format. 0.47 inches is close to 1/2 inch (0.5 inches). So, a sensor with an 8mm diagonal might be referred to as a 1/2" format sensor.
Why is this format used?
- Legacy: It's a continuation of historical naming conventions.
- Convenience: Provides a rough comparative size without needing exact millimeter dimensions. A 1" sensor is generally larger than a 1/2" sensor, which is larger than a 1/3" sensor, and so on (though the relationship isn't strictly linear or exact due to the rounding).
It's important to note that the optical format number does not represent the actual diagonal size of the sensor itself in inches. A 1" optical format sensor, for example, has a diagonal of approximately 16mm, which is roughly 0.63 inches, not a full inch.
