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

Started Apr 6, 2013 | Discussions thread
ultimitsu Veteran Member • Posts: 6,650
Re: LX7 - does its IQ compete with e.g. Sony RX100?

Cyril Catt wrote:

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.

But we are not comparing a fixed unit area, are we? we are comparing the entire image.

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.

That is correct. because the FOV is vastly different, meaningful comparison can only be done where FOV is the same or at the least similar.

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.”

That is only true where FOV and sensor size are the same or similar.

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