Not all apertures are created equal. I don't pretend to have any technical understanding about the design of optics, but I know from what I've read that a prime lens lets in more light that a zoom when set to the same aperture and focal length.
But I wondered by how much?
I took two pictures. The first was taken with a Sigma 85mm f/1.4 prime, shot at f/8.0. The second picture was taken with a Canon EF 24-105mm IS USM L zoom, shot at my best approxiimation to 85mm and and also at f/8.0. Both lenses had UV filters on them from the same manufacturer. Both exposures were made at 1/50 sec. The camera was a Canon 5D MarkII, on manual setting, and the exposures were made less than 30 seconds apart. The only difference was the lens.
This isn't the most scientific or controlled test, but I figured if I could see differences between the pictures, then I'd be convinced.
Is there a difference? Yes. But I'm not posting the pictures because the differences between them are so slight that I don't think viewing them on the web would be useful. But comparing the histograms definitely shows a small but clear shift to the left (meaning a darker image) with the zoom lens.
If all things were equal, the pictures would be identical. But they're not. The picture shot with the Sigma prime is about 1/4 to 1/3 f stop brighter than the one shot with the Canon zoom.
These differences would probably vary with the specific lenses selected. Quality of glass affects transmission just as the number of elements does. The best zooms on the market in all likelihood have glass that absorbs or scatters less light than the lower cost lenses. And in my experiment, I'm comparing a top of the line Canon L series zoom to a Sigma prime.
Zoom lenses have more glass elements in them than primes. Because of that fact, they transmit less light than primes, which generally have fewer glass elements. More glass = less light transmission. It's that simple. The difference is there, even comparing a Canon L series zoom to a Sigma prime.
For all their usefulness and versatility, zoom lenses represent a trade off. The best zooms can be as sharp as the best primes. But at the same f stop, they don't transmit as much light as prime lenses.
This is something I keep in mind when I plan to shoot outdoors at night, or indoors without a flash.