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When comparing the speed of two lenses, you almost exclusively hear about the maximum aperture. This of course makes sense, since at a certain lighting condition, a larger aperture in general allows a faster shutter speed.

But is this really the whole story? I guess the relevant property to compare is the amount of light actually hitting the sensor when the lens is wide open. Therefore, the transmission of the lenses should be important to include in the comparison.

  • How much does the transmission in today's lenses affect the overall exposure?
  • Does the transmission differ much between primes, which generally have simpler structure, and zoom lenses, which are more complicated.
  • How much does the massive coating of modern lenses affect the transmission measured in Ev?
  • Is the transmission of lenses revealed by the manufacturers?

For example, a 35/2 prime is only a third f-stop slower than a 18-35/1.8 zoom. Can the difference in transmission make the prime "effectively faster" although it has a smaller aperture? (Of course it can theoretically, I mean practically with modern lenses.)

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You can look at DxOMark's Lens Ratings, and particularly the Optical Metric Scores, which include a T stop measurement. I don't put too much stock in DxOMark's overall numbers (which don't have much practical impact for real use), but if you're interested in this particular thing, here's a way to tell.

Manufacturers do not typically give this number, so the measurements DxOMark gives are probably your best bet. Fortunately, they have measured a lot of lenses.

As you can see from the numbers, it's pretty typical for the light loss to be about a third of the stop from the nominal f number, although sometimes it's almost a full stop, and occasionally it is almost nothing — and you are right that in the latter case, it's usually prime lens designs. But not always; for example, the Sigma 18-35mm F1.8 DC HSM is measured as having a T stop of 1.8.

I think coatings can make a difference here, but that's clearly not the only factor, or the main one.

Another big factor is certainly going to be rounding, not just for precision but because a lot of camera and lens numbers are actually given as "close-enough" standardized values — see How to test actual focal length? for example. This is for a number of reasons, starting with variance in focal length with distance, overall irrelevance of being exact in real use, and almost certainly also because when more impressive numbers can be written on the box without really lying, of course they will be.

As for overall impact: sure, it can make a difference, and sometimes every bit counts, but overall, if the difference is less than a stop, it's not worth sweating over.

And of course, depth of field isn't affected.

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  • \$\begingroup\$ beaten by 22 seconds... \$\endgroup\$
    – Matt Grum
    Mar 14, 2014 at 13:05
  • \$\begingroup\$ Thanks Matt! I googled t stop and found this tutorial video on the subject, which was really helpful to me. I thought I'd share it for future users. \$\endgroup\$
    – Daniel R
    Mar 14, 2014 at 13:16
  • \$\begingroup\$ Sometimes, the difference between f and t stop can be very significant, e. g. look at the SAL-135F28. Due to it's optical construction, the depth of field is that of an f/2.8 lens, but it's as bright as an f/4.5 or better t/4.5 one. \$\endgroup\$
    – Vertigo
    Mar 14, 2014 at 18:07
  • \$\begingroup\$ @Vertigo Interesting example, although important to note that this is a specialty lens with intentionally reduced transmission, not a normal case. \$\endgroup\$
    – mattdm
    Mar 15, 2014 at 10:31
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You're right that transmission contributes to the light gathering ability of a lens.

To compare like for like you need to look at the T-stop, which is a measured value of total light transmission (on a log scale similar to the f-stop).

Manufactures rarely specify the T-stop of photographic lenses (as opposed to cine lenses) but you can find values on some review sites, such as DXO labs.

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