On the values of "C" for the Sekonic L-398A exposure meter

[Doug Kerr]

The manual (the one I have) for the Sekonic l-398A exposure meter has a detailed sketch of the meter, part of which is this:

There is no suggestion that this is precisely "to scale".

That having been said, if we analyze the exposure calculator as shown, it suggests that the "calculator C" is 340 (the same as the "announced" value of C with the dome collector in place).

But later in the manual where there are examples pf taking various measurements, there are sketches of (most of) the calculator to this same level of detail. The details there typically seem to imply a "calculator C" of about 315.

Interesting,

Best regards,

Doug

 

[Doug Kerr]

One official Sekonic photo of the "70th anniversary" edition of the L-398A exposure meter shows the calculator in nice detail.

Analyzing the calculator values in that photo suggests that the calculator there has a "calculator C" of 336.

Best regards,

Doug

 

[Ted Cousins]

The manual (the one I have) for the Sekonic l-398A exposure meter has a detailed sketch of the meter, part of which is this:

View attachment 14412
There is no suggestion that this is precisely "to scale".

That having been said, if we analyze the exposure calculator as shown, it suggests that the "calculator C" is 340 (the same as the "announced" value of C with the dome collector in place).

But later in the manual where there are examples pf taking various measurements, there are sketches of (most of) the calculator to this same level of detail. The details there typically seem to imply a "calculator C" of about 315.

Interesting,

Indeed. My 398A dial is quite a lot bigger and more clearly marked than my 398. Also but less relevant to the task in hand, in the 398 manual, there is no mention of a C value of 250 for the Lumidisk and it goes all around the houses on how to use it.

 

[Doug Kerr]

A nice "head-on" photo of the front of a Sekonic L-398A, from the Sekonic site (yes, that model is still offered!), upon photogrammetric analysis suggests that the "calculator C" for the calculator shown is about 412.

Note that this tells us nothing about the value of C for the meter, which as well depends on the ratio between the "observed" (actual) luminance and that indicated by the meter movement.

Best regards,

Doug

 

[Doug Kerr]

Because of the initial topic, this thread has so far focused only on the Sekonic L-398A exposure meter. But is has led us to some conundrums that may go beyond that product and is cousins.

Since I had the manual, I decided to look into the Minolta Auto Meter IV F. This also includes both dome and flat collectors (Minolta calls them "diffusers", which is apt).

As we might expect, the manual,advocates the use of the dome collector for general-purpose expusre metering. As to the flat collector, the introduction to it says:

No surprise there.

An earlier passage reads:

This is consistent with other considerations I have discussed here.

I next note that in the manual I have there is no mention of the value of "C" for this meter. The Google 'bot advises that Minolta has traditionally used a value of C of 320 with a dome collector.

For measurements of actual illuminance, this meter does not give a direct illuminance indication. Rather, the user is instructed to set the ISO value to 100 and, with the flat collector in place, take a reading.

The photographic exposure recommendation as an Ev value is displayed. That is entered into a table that gives the illuminance in lux. Reverse engineering that table in connection with the ISO value to be set suggests that the meter (with the flat collector in place) has a C of 254.

This all seems nearly identical to the announced Sekonic values of C.

The mystery as to substantially different values of C for the use of dome vs. flat collectors remains.

As we often see, the instructions for illuminance measurement say to aim the meter at the light source of interest. That technique measures the luminous flux density of the light beam from that source at that place, which would not necessarily be the illuminance on some target surface (which depends on the orientation of the surface with respect to the direction of arrival of the light.

To measure the illuminance that beam will impart to some surface (real or hypothetical), we must orient the meter so its collector is parallel with the plane of that surface.

(This error results from an all-too-common misunderstanding.)

Best regards,

Doug

 

[Doug Kerr]

For the earlier Minolta exposure meter model, the Auto Meter IIIF, the manual states the value of C with the dome light collector in place as 330, and with the flat collector in place as 250.

As to the measurement of illuminance, the procedure is as I described it above for the Auto Meter IV F. The table for converting the meter Ev reading (with the ISO value set to 100) into illuminance is slightly different than for the Auto Meter IV F. It implies a value of C with the disk collector in place of 250.

******

I note that in the manual for the Auto Meter IIIF the instructions for measuring illuminance with the meter are correct as to the orientation of the meter. (I had commented earlier that in the manual for the Auto Meter IV F the meter orientation for the measurement of illuminance was incorrectly described.)

Best regards,

Doug

 

[Doug Kerr]

The Minolta Flash Meter model series is especially equipped to make various measurements with flash illumination, but it will also make incident light measurements with "continuous" light sources.

Both dome and disk light collectors are provided.

The manual for the Flash Meter VI states the value of C with the dome collector as 330; with the disk collector, 250.

I remain unclear as to the rationale for this difference in the value of C for the two configurations.

Best regards,

Doug

 

[Ted Cousins]

The manual (the one I have) for the Sekonic l-398A exposure meter has a detailed sketch of the meter, part of which is this:

View attachment 14412
There is no suggestion that this is precisely "to scale".

A quick look and it's very similar to my 398A. In particular, f/11 and 1/15 sec are nicely aligned on both but the EV indicator is a bit off on both - showing just over 11 versus an arithmetically calculated 10.8.

Perhaps to do with your comment elsewhere about standard photographic values - e.g. 1/15 sec is not 1 EV different than 1/8 sec even though the radial angles are.

Perhaps there exists a general statement that covers this endless series of not-quite-right instances and which would nicely conclude this ever-growing thread ...

... over ...

 

[Doug Kerr]

Hi, Ted

Perhaps to do with your comment elsewhere about standard photographic values - e.g. 1/15 sec is not 1 EV different than 1/8 sec even though the radial angles are.

Very possibly so. 1/i6 sec and f/11 is nearly Ev 10.91.

Almost certainly the scale position labeled "1/15 s" is actually correct for "1/16 s".

Best regards,

Doug

 

[Doug Kerr]

Also, the aperture scale mark labeled "11" actually represents about f/11.31.

The "actual" values of those two marks comes out to exactly Ev 11.

Best regards,

Doug