Published transmission tests for original Petzval lenses are uncommon, so a rule-of-thumb estimate is usually the practical approach. For uncoated optics, light loss is often estimated at about 4% per glass-to-air surface, or roughly 96% transmission per interface.

A classic Petzval design has about six glass-to-air surfaces, plus two cemented glass-to-glass interfaces. The cemented interfaces usually contribute little unless the cement has deteriorated. Using 0.96^6 gives about 0.78 total transmission, so around 78% of the light gets through—roughly a half-stop loss.

In practice, that means a vintage uncoated Petzval may have a T-stop about 1/2 stop slower than its f-number suggests. If your camera meters through the lens, this usually doesn’t matter much. With external metering, compensating by about half a stop is a reasonable starting point.

Also, “speed” is not just transmission: the effective speed depends on the actual aperture and lens design. Historic Petzvals could be quite fast for their era, but transmission and true working speed are not identical concepts.