How does lens affect metering?

It started off with me trying to check the accuracy of the metering of my old Pentax ME Super, which I thought was suspect. I decided to use my Canon EOS 3 and EOS 400 to compare against the ME Super.

All cameras were set to the same ISO. All were in aperture priority mode and all set to f/8. I had a 50mm lens on the Pentax, EF 50 1.8 on the EOS 400 and a EF 24-105L set to 50mm on the EOS 3. The Pentax and the EOS 400 gave very similar shutter speeds but the EOS 3 was was indicating a slower shutter speed... about a full stop slower.

I then started swapping the two Canon bodies and lenses and it seems that regardless of what body is used, the 24-105L always resulted in a slower shutter speed for the same conditions. Should I be worried? Is there a simple explanation?

The f/ stop of the aperture is just a measurement of it's size relative to the lens focal length, it's not an exact measurement of the lenses light transmittance. For that you need t/ stops which are usually given on cine lenses due to the need for exposures to match between lenses in the same scene. Basically f/4 on one lens might let through a little more light than f/4 on another, the different types and amounts of glass elements will absorb light differently. Also the measurement of the aperture isn't exact, the aperture at f/1.8 in one 50mm might be larger or smaller than f/1.8 in another 50mm, and theses measurements are even more approximate in zoom lenses due to their variability.

what metering mode?

In evaluative, things like vignetting can play a role.

The Ran wrote in post #10879957
The f/ stop of the aperture is just a measurement of it's size relative to the lens focal length, it's not an exact measurement of the lenses light transmittance. For that you need t/ stops which are usually given on cine lenses due to the need for exposures to match between lenses in the same scene. Basically f/4 on one lens might let through a little more light than f/4 on another, the different types and amounts of glass elements will absorb light differently. Also the measurement of the aperture isn't exact, the aperture at f/1.8 in one 50mm might be larger or smaller than f/1.8 in another 50mm, and theses measurements are even more approximate in zoom lenses due to their variability.

Thanks for that, its put my mind at rest somewhat.

Yes, if you read pop photo's lens reviews they always give the actual aperture and focal lengths and they are never exactly whats on the lens.

tkbslc wrote in post #10879966
what metering mode?

In evaluative, things like vignetting can play a role.

I was using centre weighted metering because I figured that was closest to what the ME Super would be using (only fixed metering mode on that).

packpe89 wrote in post #10880056
Yes, if you read pop photo's lens reviews they always give the actual aperture and focal lengths and they are never exactly whats on the lens.

Ok, so I'm assuming that the camera is measuring the amount of light reaching the film/sensor and adjusting the shutter speed accordingly to give the right settings? What happens then when you are using an external light meter and setting the aperture and shutter manually? That won't allow for different lenses transmitting different amounts of light at the same aperture.

Usually the difference won't be great enough that it will matter, it will just be changed in post anyway.