why not f/1.2 by Sony?

Started May 14, 2013 | Discussions thread
tomtom50 Veteran Member • Posts: 3,244
Re: why not f/1.2 by Sony?

ilza wrote:

Have you read F stop blues at Dxomark?


Very worthwhile when considering the usefulness of fast lenses on a digital camera.

It is interesting, but their choice of using T-word where F-word is due is abysmal.

T is transmission, f is a ratio. They are measuring transmission. Since their measurement includes transmission loss in the sensor optics they could have chosen a new term, but between T and f they chose the better.

When T-number is mentioned, one assumes that the light loss is discussed (as opposed to the angle of incoming cone of light hitting a point on a sensor). T-number affects the exposure, F-number affects also the diffraction and the circle of confusion due to misfocus.

The light is being lost, at the sensor rather than the lens. T is traditionally used because it is more accurate for exposure. The Dxomark use carries on that tradition adding a new source of transmission loss that did not exist at the time T was developed.

While what they measure (I assume it is the photon noise) may be, technically speaking, the T-number, the observed difference between the cameras cannot be attributed to difference in light transmission in the lens.  My immediate conjecture is that what they observe is due to vignetting in the microlenses; then this would affect the diffraction circle and the circle of confusion in a similar way to a mechanical limiter of the aperture ring.

Dxomark Uses the sesnsor as the measurement device. They have found some cameras hide transmission loss at wide apertures by boosting ISO surreptitiously. I think they do use noise levels to detecting the ISO spoofing; I don't recall. We can observe the phenomenon by mounting legacy lenses with an adapter. Then the camera does not know the nominal aperture and cannot spoof ISO and exposure can be assessed.

(Well, the difference with mechanical limiter is that the bokeh must be significantly improved with such vignetting… :-))

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