Light Loss at Center

Light Loss at Center

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.

  1. Create a large white window on a good flat LCD monitor.
  2. Point a 1-degree spot meter at the center of this large white area, take five direct measures, and compute the average.
  3. Mount a lens in front of the spot meter, take five measures again, and compute the average.
  4. The difference between the direct measure and the measure with a lens is the light loss in EV. Since the spot meter being used is only accurate up to 0.1 EV, the resolution of this difference is 0.1 EV.

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.

Lens Light Loss
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.