By definition, adding a converter lens does not affect exposure value (i.e., EV) because the added converter lens changes the focal length and magnifies the entrance pupil by the same ratio. However, glass elements can absorb and reflect some incoming light. High transmitting ratio glass elements and better multi-coating technology can reduce absorption and reflection, respectively. However, this light loss is inevitable.
The following setup was used to measure light loss of the eight wide angle converter lenses.
Here are the measured results. Note that, as mentioned above, all numbers are measured differences which are averaged and rounded/truncated to one digit after the decimal point. Note also that only light loss at the center is measured for each lens.
|Nikon WC-E80 0.8X||0.1 EV|
|Nikon WM-E80 0.8X||0.1 EV|
|Olympus WCON-07 0.7X||0.2 EV|
|Olympus WCON-08B 0.8X||0.1 EV|
|Panasonic LW55 0.7X||0.1 EV|
|Raynox HD6600pro-55 0.66X||0.1 EV|
|Sony VCL DH0758 0.7X||0.1 EV|
Except for the Olympus WCON-07 that has a light loss of about 0.2 EV, all others have a light loss of around 0.1 EV. Since camera meters have 1/3 EV step, they perhaps could not recognize this 0.1 EV or 0.2 EV difference unless the original measure is at the border of 1/3 EV. The difference between center and corner can vary from less than 1 EV to 2 EV depending on the lens. This can be visualized by shooting a uniform blue sky as shown on my FZ-30 user guide pages.