14mm fisheye on 1D Mark II

Apertures are derived by dividing the exit pupil into the focal length of the lens (50mm lens: 25mm exit pupil. 50/25 =2, ie f2). Then you use the diaphragm to "half" that aperture. It's not a measure of transmission but a mathmatical formula. Movie lenses ARE marked in T-Stops (transmission) and are specifically tested to make sure that T11 is really T11 before they blow a few thousand dollars worth of film.

But any digital capture will handle 1/3 stop of overexposure without blowing out the highlights unless there is overexposing to begin with. What does your histogram look like on an average scene with the fisheye and say a moderate wide angle? If the peak of the histogram is already on the right hand side then you are cheating towards overexposure already.

There will be fall off from a fisheye or super wide angle but normally not enough to notice. If your exposure is right down the middle for both the fisheye and the wide angle then I suspect that this more of post production situation where you are reading the highlights as being blown out when they are not.

If you still get overexposure on the fisheye at f8 and OK results from the wide angle at f8, then f8 on your fisheye isn't f8. I have a 90 mm f8 lens that use on my 4x5, which is the eqivilent of a 19mm lens on 35mm. When I look though the ground glass it look MUCH darker than my 300mm f9. Yet I know that when I select 22 on either lens it will give me the same exposure. F8 is f8 is f8, unless it's marked f8 and is really f5.6 (point, something).

all i can say is it's much more pronounced at f2.8 than it is at f8. at 2.8 it could be as much as a full stop or more, especially in bright, non-cloudy/diffused conditions. at f8 it's not nearly as noticable. the histogram might display a similar curve on both the fisheye and say a wide angle - a little towards the right but no where near clipping.

anyway, they are great lenses and i'm getting much more usage out of them than i thought i was going to.