Oh yes, focal length can make a big difference in exposure requirements. Good point, Nic.

There will certainly be some slight differences in the nuances of the specific cameras and of course the way you meter will make a difference as well. However, we're talking about f/1.7 to f/2.8. That means that if the metering itself is different then you would see a full 1.5 EV stops difference in the final image! Plus you're saying that the camera with the faster lens is using HIGHER ISO, not just the same... So that means there would have to be over a 1.5 EV difference, which I don't think you would be able to miss. ;) So that part goes back to "yes, you're imaging it", but the focal length difference that Nic pointed out can actually account for a full EV or more, if the difference is as much as a standard-wide to a super-telephoto for instance. We're talking about going into a different class of focal length to see that much difference in metering.

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So here's my quick and dirty experiment. Two cameras side by side. The same type of lens on both, Pentax 50mm F1.4. To make sure the lenses were the same, I checked it again after swapping the lenses. The results were consistent. I set both cameras to ISO 200 and both aperatures to F1.4. Both were set to AWB. I then let each camera meter out the shutter speed. The subject was a white sheet of paper uniformly lit and filling the entire field of view. I took 3 sets of shots. Low, mid and high illumination.

The setup.



GF2, low illumination.



NEX, low illumination.



GF2, mid illumination.



NEX, mid illumination.



GF2, high illumination.



NEX, high illuminaton.



Theoretically, the shutter speed should have been the same for each set. For low illumination that was indeed the case. For the others, they weren't.

In this case, I fixed the variables except for shutter speed. That was allowed to vary and it did. If the shutter speed and aperature were fixed, then the ISO would have varied. As the OP says he has witnessed.

I may have messed up somewhere since this was done quickly in a few minutes. I can't think of anything but if you spot something, please let me know.

I think you should make this comparison in Manual mode by dialing the same ISO, WB, Shutter Speed and Aperture, take picture with each camera and compare not the image shown on camera's screen but in computer.. I think camera with larger sensor can emmit more light (sensor is familiar to solar panel somehow..) so exposure could be different..

You should've taken the pictures, lenshoarder. Your test shows the differences between the metering systems of those 2 cameras, but not the sensors' sensitivity, to see that you'd need to look at the final pictures. The final exposure from GF2 at 1/500 will likely to look different than the NEX picture at 1/250.

Yep that is what I was asking!
And I did search the forum first or at least tried to but I seem never to have luck with the search function!

Shooting wide open ( why else would I have fast lens for low light)
The shorter focal length lens (tamron) on the canon is not stabilized. I will do a side by side today
Aperture priority, manual ISO. I noticed I had to keep bumping up the ISO to get a usable shutter speed. Focal length similar canon may have been a little longer since it is a zoom.
I think I may be expecting too much from the Pany 20 1.7. I really thought I would be able to shoot in low light (inside home with normal lighting) at around ISO 800
Thanks again for all the responses. This group has a wealth of knowledge and it is so nice that folks share that knowledge

ISO has little to do with sensor sensitivity. Every sensor has some base sensitivity to light. This is electrically measurable. Next, in a combination of hardware and software, the signal is amplified to be equivalent to various ISO sensitivities.

The ISO a camera is set to is often inaccurate. I've seen tests at DXO showing as much as a full stop under or over the stated ISO. Most seem to overstate the ISO slightly. The measured ISO on the G3 is slightly under the stated ISO. You can compare this to your Canon here DxOMark - DxOMark by DxO Labs

Fred

I'll also add that the same f-stop on two different lenses won't always give you the same amount of light. There is also a transmission loss of light going through the all the glass. It's usually small enough to be insignificant and not too important if you're metering through the lens. In cinematography lenses are often rated in t-stops which factor in the transmission loss. This allows identical exposure when changing lenses or on multiple cameras.

Different cameras meter differently for a different tone curve target for their jpeg processing. I compared my X100 and E-P2 back to back in a similar fashion, pointing at a dimly lit wall the X100 wants to use 2/3 stop more exposure than the E-P2. IMHO the X100 does tend to overexpose in aperture priority anyway, especially in a low contrast situation.

Also IMVHO with identical exposure values on each camera, chimping the captured image of my bedroom wall looked pretty much the same to me, enough to convince me that the 2/3 of a stop difference that the camera wants to use is just down to its metering and nothing to do with actual amount of exposure.

First, there are different standards for ISO. So ISO may not match.

Second, , vignetting can influence metering.

Third, metering can be different among cameras.

Forth, metering can be different when using an adapted lens.

Fifth, the curve applied to the image can effect exposure.

You are going to have to work harder to come to any firm conclusions. So far you have not identified the variables.