ISO/exposure discrepancy?

I've been noticing that my 1D3 and 5Dc expose differently... and I don't mean the metering. I put it on full manual, and use the exact same settings... and the 1D3 comes out exposed easily one or more stops less than the 5Dc.

I had both of my cameras on f8, 1/200th, ISO 100 with my strobes. The cameras were triggering them independently with transmitters on both. Both were being shot from the same location, I just had one on a tripod and one around my neck for a longer FL.

However, while looking at the playback/histogram, I notice that the 1D3's images are drastically darker. I had to shoot at ISO 200 just to get similar exposure. The 5Dc's were still exposed brighter when I looked at the histogram. I came home and was able to re-create it with the same lens, nearly identical framing, and same ambient lighting.

What gives? I used to think the 1D3 had amazing high ISO performance but maybe it's the case that its light sensitivity is just way off of the standard ASA ratings. I've noticed similar things with the 5D2 underexposing using a proper flash meter reading.

Any other bodies you guys have noticed inconsistent exposure with? Am I crazy? Or is something wrong with one or both of my cameras?

I think to be really sure, you need to make a more rigorous check of the exposure than looking at anything on the playback LCD. For sure, there could be differences, but as much as a full stop seems unlikely.

Place one camera on a tripod and shoot a target of uniform color and tone. Preferably the target should be a middle tone, not white and not black. Think maybe a gray sheet of construction paper. Then change cameras and shoot the same target using the same lens and the same settings exactly.

Load the images to PS or LR and drag the cursor over the target region and read the RGB tones. Then brighten the image from the darker camera until the RGB tones match, and you will have a more accurate measure of the difference between these two bodies.

I used this method once to check various lenses and found about 1/4 stops difference between my brightest and my darkest at the same exposure.

Good plan Jeffrey.

For reference I ended up with f8 ISO 100 1/200th on the 5Dc. For the 1D3 I was shooting at f6.7 ISO 200 1/200 on the 1D3 for the histograms to match with the same framing...

But I'll try to do a more controlled test.

If you have a Sekonic meter like a 358 or 758 you can use the DTS software to accurately measure the ISO speeds of your camera and profile it to the light meter.