Polyglot,

If you're only looking at Kodak T-Max film technical data you might get that impression. However if you look at Kodak Tech Pubs F4017 and F-4018 for Tri-X and Plus-X films, you'll see a significant difference between the recommended adjustment in aperture and the recommended adjustment in exposure time. For each of these films you get +2 stops of aperture adjustment at 10 seconds and +3 stops in aperture adjustment at 100 seconds. That would equal 40 seconds of adjusted exposure at 10 seconds measured and 800 seconds adjusted exposure for 100 seconds measured if there were no compounding effect with reciprocity adjustments using time rather than aperture. However, the tech pubs recommend time adjusted exposures of 50 seconds from 10 seconds and 1200 seconds from 100 seconds.

So with Tri-X and Plus-X that's 80% of correct exposure if you apply the aperture correction to time at 10 seconds metered and 66% of correct exposure if you apply the aperture correction to time at 100 seconds metered, the compounding effect of extending by time for reciprocity correction failure.

In the case of the T-Max films, which have less reciprocity failure, Kodak is just rounding down because the reciprocity failure of those films only amounts to about 5% underexposure at the adjusted time for 10 seconds and about 15% underexposure at the adjusted time for 100 seconds. Few people meter that accurately in the first place.

You might want to look at the Schwarzschild formula or Covington's modification of it, or at Gainer's reciprocity formula and variations on that to understand the compounding effect of adjusting for reciprocity with extended times.

Lee