Camera Sensor Sizes (Crop Factor, APS-C)

What is a four-thirds sensor?  Is a 35mm sensor measured across the diagonal (like a TV screen) or width etc.?  What is ASPC?

The answers I found are:

    • Sensors are often referred to with a “type” designation using imperial fractions such as 1/1.8″ or 2/3″ which are larger than the actual sensor diameters. The type designation harks back to a set of standard sizes given to TV camera tubes in the 50’s. These sizes were typically 1/2″, 2/3″ etc. The size designation does not define the diagonal of the sensor area but rather the outer diameter of the long glass envelope of the tube.
    • There appears to be no specific mathematical relationship between the diameter of the imaging circle and the sensor size, although it is always roughly two thirds.
      • {The article includes a look-up table for the exact figures}
    • A “35” mm sensor is actually 36 mm on the width;  the height being 24 mm and the diagonal 43 mm.
    • Advanced Photo System type-C (APS-C) is an image sensor format approximately equivalent in size to the Advanced Photo System “classic” size negatives. These negatives were 25.1 × 16.7 mm and had an aspect ratio 3:2.
    • Sensors meeting these approximate dimensions are used in many digital single-lens reflex cameras, in addition to a few large-sensored live-preview digital cameras and a few digital rangefinders.
    • Such sensors exist in many different variants depending on the manufacturer and camera model.  All APS-C variants are considerably smaller than 35 mm standard film which measures 36×24 mm. Sensor sizes range from 20.7×13.8 mm to 28.7×19.1 mm. Each variant results in a slightly different angle of view from lenses at the same focal length and overall a much narrower angle of view compared to 35 mm film.
    • This is why each manufacturer offers a range of lenses designed for its format.
    • Philip includes a diagram comparing sensor sizes.

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