LX7 - does its IQ compete with e.g. Sony RX100?

Started Apr 6, 2013 | Discussions thread
Cyril Catt
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Re: LX7 - does its IQ compete with e.g. Sony RX100?
In reply to ultimitsu, Apr 8, 2013

ultimitsu wrote:

Cyril Catt wrote: [...] Other things being equal, camera lenses of different focal lengths and different effective apertures form an image of the same brightness on the film or sensor if their relative aperture (or f number) is the same. But an f1.4 lens WILL form a brighter image than an f1.8 lens.

That statement is correct. But within the context of this discussion, "other things" are not equal, namely the sensor size and image circle, thus it is wrong to claim LX7's F1.4 is "faster" than RX100's F1.8

The sensor size and the image circle are of no consequence to the amount of brightness with which the lens illuminates a unit area of the sensor. That brightness is defined by the f-number (a function of both the focal length of the lens AND the relative aperture - which relates to the DIAMETER of the effective aperture). The phrase “Other things being equal” relates to such matters as the general structure of the lens, and the nature of the glasses used.

As the brightness of the illumination on the sensor depends partly on the distance of the diaphragm from the sensor, it is meaningless to consider only the absolute area of the aperture and to ignore the focal length of the lens. An aperture with an area of 10 square centimetres at a distance of 100 metres from the sensor will provide far less illumination to the sensor than a smaller aperture of one square centimetre at a distance of 10 centimetres from the sensor.

As Wikipedia states it: “A lens with a larger maximum aperture (that is, a smaller minimum f-number) is called a "fast lens" because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed.” There are plenty of other similar definitions of the f-number which will confirm that the LX7's lens is definitely "faster" than that of the RX100, despite the fact that the absolute area of its maximum aperture is smaller than that of the RX100.

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Cyril

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