(This post is from an email conversation with two friends. I thought the content might be useful for others.)

Q:  [Friend 1] and I have talked in the past about brighter lenses. And at the time, he and I both had a theory that a brighter lens took a better (brighter) picture even at the same settings as a not-as-bright lens. So as an example, a 50mm prime @ f5.6 took a better picture than an 18-200 set at 50mm (or is that 75mm?) @ f5.6. What are your thoughts? I haven’t spent any time testing this theory in a methodical way, but I still have a gut feeling that it’s true.

A:  Lenses have two different metrics for defining how much light they are letting through. The one we’re familiar with is the f-stop. It is simply the focal length of the lens divided by the effective diameter of the lens. That’s why it’s written f/2.0 for instance. The lens’ aperture is f (the focal length) divided by two. What that means is that any simple lens whose ratio of the focal length to the lens stop is the same as another’s, regardless of absolute focal length or lens diameter, will let through the same exact amount of light.

Then there’s the T-stop. T stands for transmission. The above paragraph was about theoretical light transmission. The T-stop is the real transmission of a lens. The T-stop deviates from the f-stop mainly because of the number of elements within a lens. Whenever light travels through a surface of a lens, a small amount of the light is reflected away. With normal uncoated glass surfaces and refractive indices, that amount can be up to 5-10% of the light per surface. That causes two different issues. One is that your image is measurably dimmer than it should be, especially when you have a number of elements. Second is that reflected light has to go somewhere, and where it goes is both random (reduced contrast) and focused (flare) depending on the other elements and internal blackening of the lens.

The fix for that is multicoating on the lens surfaces (which can reduce the reflectance to less than 1% per surface) and minimizing the number of elements.

Your theory is correct that some lenses are brighter than others at the same f-stop. What causes it, though, is not the different speed of the lenses, but the number of elements in them, type and number of layers of coating, and lens design.

Thus, if you compare two hypothetical lenses, a 50mm f/1.4 and a 50mm f/2.8 (focal length doesn’t actually matter here, but if you were comparing pictures of the same scene side-by-side, it would be best to use identical focal lengths to make the comparison easier), that both have six multi-coated elements, pictures taken with both of them at f/5.6 should be identically exposed.

But if you took your 50/1.4 and your 18-200 @ 50mm and took an identically exposed photograph at f/5.6, it is very likely that the one with the zoom would be noticeably darker, possibly by half a stop or more. That is because the 50mm prime has seven multicoated elements and the 18-200 has 16 multicoated elements.

I haven’t read a lens review in a magazine in a long time, but I remember that they used to list the transmission of the lens so you could see the actual light loss.

Note that all of this stuff is automatically compensated for whenever you use your TTL (through the lens) meter. Although the exposure may be different between the two lenses at the same f-stop due to this T-stop reality, your TTL meter is seeing the scene through the two different lenses, so it will expose either of them correctly.

Lenses that are designed for use with studio lighting, and / or hand-held meters (like movie camera lenses) are even marked in T-stops instead of f-stops so that the meter reading from a hand-held meter will result in a correct and consistent exposure across different lenses.