• Members 1466 posts
    May 16, 2026, 7:05 p.m.

    By which I mean that the recommended exposure is low compared to that derived from a commercial meter used for determining building lighting requirements.

    For example, my ebay BTMETER BT-5000A reads about 430 lux under my desk lamp, compared to my recently-acquired Sekonic non-selenium L-398A's 860 lux at the same distance.

    The reason is that the Sekonic constant C (340) is not a calibration constant per se. It is a value deduced from a large sample of test shots for Sekonic models. For a Sekonic, the exposure value recommendation is EV=log2(E.S/C) ... where E = illuminance in lux, S = ISO. So, the Sekonic recommended 8 EV Exposure Value.

    But the BTMETER read 430 lx, i.e. 40 fc - for the which needle position, the Sekonic would have recommended 7 EV, double the exposure.