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We know that the APS-C frame is larger than "4amp 3 inches", and we also know that the size of the APS-C frame image sensor is usually 23.5mm × 15.6mm. The problem is that the diagonal length calculated from the actual size of the APS-C frame is 28.2mm, while 28.2mm is converted to 1.11 inches. What's going on here?

It turns out that what we usually call "1ax 2 inch", "1 inch" and "4 stroke 3 inch" all refer to the "Optical Format" of the image sensor. The optical format is not the true size of the image sensor. It is a "false specification" based on historical heritage, which is about 50 per cent larger than the real size-take "4x3in" as an example, the actual size of the image sensor is 17.3mm × 13.0mm, the diagonal length is about 21.6mm, not 4 / 3 × 25.4 = 33.87mm.

The image above in ▲ uses the early 4ram 3-inch image sensor, namely 18mm × 13.5mm, to calculate the 22.5mm. The principle is the same as the text. The optical size means that the image sensor has the same effective area as the corresponding size of the camera vacuum tube (a bit like the difference between the building area and the used area). Although the camera vacuum tube has long been out of public view, this nominal method is still in use today.

The image above ▲ shows the camera vacuum tube, the size of which is the diagonal length of the black circumference, and the effective part has only the orange-red area in the middle. At present, sensor manufacturers represented by Sony and overseas technology media represented by DPreview have begun to use "Type" instead of "inch" to prevent consumers from directly understanding "optical size" as diagonal length.

▲ Sony products use Type instead of inch in PDF

The text on the ▲ is extracted from https://www.dpreview.com/ articles / 4159871805 / making-sensor-sizes-less-misleading. Why not directly mark the true length of the diagonal? Take the APS-C frame as an example, there are Sony 1.5x and Canon 1.6x two factions, and the size of a specific product is also floating. So it is understandable to replace the specific diagonal length (mm) with relatively short, easy-to-remember "Type" or inches, but there are some deviations for historical reasons. Moreover, seriously, the diagonal length does not fully reflect the size of the sensor. Just like monitors and televisions, the diagonal length is the same and the ratio of length to width is different, so the actual area is also different.

Two incorrect calculations are also attached to the table below. First of all, it is wrong to say that the diagonal size is the half-circumference of the outer circle of the sensor. It can be seen from the column of the table [circumcircle] that this method of calculation is not suitable for smaller size sensors. (44 x 33 is 4:3)

On the other hand, [1 "inch"] leaves behind the basic common sense of "1 inch = 25.4mm". In general, 1 inch less than 2 inches, 1 inch = 18mm, 1 inch and above, 1 inch = 16mm. But this can only be used for rough calculations. In the final analysis, the "optical format" is defined. In order to obtain the accurate value of the diagonal length of the sensor, it can only be measured actually, or calculated by length, width and single pixel size.

Diagonal length ²= (long edge pixel x single pixel size) ²+ (wide edge pixel x single pixel size) ²

This article comes from the official account of Wechat: camera Notes (ID:xjbiji), by Teng Fei et

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