Valori t-stop

“ It doesn't open anything because camera lenses aren't measured or marketed with T/stops. Unless you are shooting video for movies or TV where you need every clip to be identical in exposure, then you don't need to worry about t/stops. „

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It matters. How much light a lens let's through is an important consideration in photography if you're in any sort of low light situation.

For example, the canon 24-105 f/4 L has a tstop of 5.1. The canon 24-70 f/4 L has a tstop of 4 (both values per dxomark). If you only looked at fstop, you'd think these two lenses performed equally well in low light, but you'd be very wrong. The 24-70 f/4 lets in about 1.7x more light than the 24-105 f/4.

I'm sure many people have cross shopped those two lenses. I'm also sure some of those that picked the 24-105 would not have done so if they knew how much more light the 24-70 let's in. „