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Surprsingly I could not find information on this. Many lenses have vignetting profiles available. Vignetting changes if you use a cropped sensor, or, in case of zoom lenses, can change with zoom.

Does the T-stop as measured by manufacturers (those that state the it) or others (such as DxOMark, who don't mention vignetting) account for it? Since vignetting is not uniform, this could affect meaningfully using it to adjust exposure.

In the same vein - vignetting changes with aperture, but is it usually proportional? If you observe this vignette chart on this page http://3d-kraft.com/index.php?option=com_content&view=article&id=127 (Ctrl+F = Fall off / Vignetting), different lenses have wildly different rates at which the fall off changes. How does that play into using the T-number?

Finally, if we forego my concerns about the variable effective transmission with aperture and zoom, I had a very amateur question about the T-stop. For the lenses that don't write the T-stop on the aperture ring (i.e. most lenses under $10,000), how do you use it when closing the aperture? Let's say I have a lens that's nominally f/2.8, but the T-number is f/3.1. If I close the aperture to f/4, would the resulting T-number be simply (4+0.3)=4.3, because it's the logarithm, and the EV value is just shifted? Or is it more complex than that?

Many thanks

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    DXO even commingles different zoom settings into their T number rating :(
    – rackandboneman
    Feb 13, 2019 at 8:33
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    @rackandboneman Yep. I assume the lens' transmission itself wouldn't change by a meaningful amount though - if isolated the the vignette. And they do have a separate rating for the vignette in EV. But they don't state outright whether it's accounted for in the T-stop
    – Dwarf Vader
    Feb 13, 2019 at 23:04

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If it is a quality cinema lens with T-rating/stops rather than F-rating/stops it should include vignetting characteristics.

To determine T-rating lens manufacturers use an integrating sphere which is not image/camera based. It measures the total collected light from a known light source after it passes through a lens. T-stop is a relative measure/percentage (as is F-stop really). It has nothing to do with image area, sensor size, etc. That's what makes it a constant of exposure...

If it's DXO's T-rating, then I don't know... it could include vignetting if it is done as an average/matrix metered type exposure. And for normal photography lenses the T-value would vary by lens design/aperture...i.e. it is "more complex than that."

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Even two $10K+ cinema lenses with the same T-stop rating does not guarantee the center of the frame will be the same brightness or the edges of the frame will be the same brightness even when both are used with the same camera - only that the TOTAL amount of light collected by the lens when pointed at a constant light field (a featureless scene with constant brightness from edge to edge) will be the same.

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