Is the Panasonic 25mm a real f/1.4 lens?

Started Feb 17, 2014 | Discussions thread
toporossa Regular Member • Posts: 127
ND filters...

DXO defines t stop well in layman's terms :

Thus, T-stop takes into account every reflection or absorption due to the lens. It can be considered as an effective (versus theoretical) way of measuring the aperture.

Since T-stops are measuring the effective light gathering ability of a lens they are more relevant for exposure times/shutter speeds, choice of ISO and so on. But for depth of field, the f-stop is relevant, while the T-stop is completely irrelevant. You can proof that quite simply by putting a ND filter on your lens. Light gathering will be reduced (greatly, depending on the filter) but DOF will stay the same - that's the main reason why these filters are used instead of reducing the aperture.

(Theoretically sharpness could be another reason, but usually lenses are sharpest in their middle apertures)

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