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Battery-less Sekonic hand-held light-meter versus commercial (non-photographic) ...

T

Ted Cousins

Member
Most battery-less photographic light-meters loose sensitivity over time as the sensor ages, especially Weston 'Master' Models up to model V.

A couple weeks ago, I found a Sekonic Model L-398A cheap on ebay and bought it because it still current and has an amorphous silicon sensor - much less susceptible aging than Selenium.

Pardon the pic quality:

l-398A- 100 ISO.jpg


I already owned a 4% accurate commercial light-meter ...
BTMETER.jpg


It has a sunken semi-dome filter ... I'm guessing for cosine response ...

About 14" from a cool white office desk lamp, the Sekonic 398A tells me 80 foot-candles (fc), 8Ev. The BTMETER tells me 39 fc.

Assuming that the l-398A hasn't been dropped, is the reading reasonable, I wonder?

Others:
My good-ish Japanese-made Weston Master6 tells me 26 fc. My original Sekonic 398 at 100 ASA with the dome on tells me about 65 fc, might be75 fc with the flat disk, couldn't be bothered to mount it.
 
Doug Kerr

Doug Kerr

Well-known member
Ted,

[The L-398A is my favorite photographic exposure meter.]

When you did the comparison using the L-398A, did you have the flat receptor on? This is the one prescribed for making incident illuminance measurements (not in the photographic context).

I note that we might expect the flat receptor to give nearly a cosine response, which is what is appropriate for illuminance measurement. The dome receptor used for photographic exposure metering purposes is often erroneously thought to give a cosine response, but in fact it gives roughly a "Norwood" response. The story behind that is fairly complicated, but I discuss it at length in an article on The Pumpkin. Here is a link to its listing:

The Pumpkin

Index page of Doug Kerr's The Pumpkin
dougkerr.net dougkerr.net

However, there are often considerable discrepancies between the readings these meters give in the "illuminance" mode and what wholly non-phtographic meters (as typically used in illumination engineering and such) will show.

I never found our the reason for that.

The problem is discussed in Append1x B of my article, "The 'Norwood Director' family of photographic exposure meters". Here is a link to its listing on the index page of The Pumpkin:

The Pumpkin

Index page of Doug Kerr's The Pumpkin
dougkerr.net dougkerr.net

Best regards,

Doug
 
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