Is a Sigma T2.0 equal to F1.8?

[Tom S]

Is the Sigma 18-35 t2.0 PL mount the same as the Sigma 18-35 f1.8 EF mount?

 

[Dan Kanes]

Optically , yes.

F stop is a theoretical number based on ratio of pupil size to focal length.

T stop is a practical number based on transmission of light through the ratio of a pupil size to focal length.

So, there is always some light loss because of various factors and a T number takes those into account.

 

[Tom S]

Yes, I know the difference between t and f stops. I want to know if this Sigma 18-35 is the exact same glass in PL and EF mounts, or is the EF a 1/3 stop faster than the PL mount?

 

[Phil Holland]

Yes, I know the difference between t and f stops. I want to know if this Sigma 18-35 is the exact same glass in PL and EF mounts, or is the EF a 1/3 stop faster than the PL mount?

Same exact glass. Wildly different bodies, mechanics, and mounts.

 

[Dan Kanes]

Long story short, to hammer it down again:

Same glass, same transmission.

Mainly that’s for everyone else reading this who may not Yet know what a T stop is vs. an F stop, it’s something I hear asked about several times a week. I find it best to share not only the facts but why/how things work as it helps build a better foundational understanding for the community at large.