Why does my camera's meter suggest the same exposure time for lenses with different apertures?

Question

On the same camera body (Canon 1Ds MkII) at the same ISO setting (400) and with no filters attached, two different lenses give the same exposure time (0.4s) at very different apertures. The lenses and the apertures are:

• Canon 17-40mm L @ F/4
• Canon 28mm @ F/2.8

Given the differences between the apertures I would expect the exposure times to be different but they are the same. No filters (ND, polarizing etc) were used. How is this possible?

UPDATE:

More data as per your request:

1. The focal length of the 17-40 was 17 mm, even if through the zoom range the exposure was (almost) the same, including (of course) the value at 28 mm.
2. The scene was entirely the same (a rather dark one, no bright spots) - no light change, no settings change etc. In fact, I saw this sporadically and I tried to reproduce it. So, the camera was steady, shot a frame at 17 mm, change the lens, shot another one at 28 mm with the prime.
3. The camera was set up in average metering.
4. Both lenses were used full open - at F/4 and F/2.8 respectivelly.

TIA for your feedback so far.

Answer

The apertures are different, but at only 1 stop I wouldn't call them very different. :)

Anyways, the only other thing that changes in the equation to make the exposure is the amount of light. If you're metering the whole scene then the zoom lens at the wider angle with the smaller aperture metered a larger area and the resulting average indicated the shutter speed. The 28mm, more open, but with a narrower field of view, metered less information and that resulted in less average light.

If you're center metering then you may have metered on a slightly different spot, a darker one, resulting in no change in shutter speed. In any case, it would strike me that the camera has different meter readings based on the scene it has through the lens.

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