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

[Doug Kerr]

The Sekonic L-398A exposure meter offers two incident light metering configurations. In one, the light collector is a flat disk. In the other the light collector is a generally-hemispherical dome.

In the "disk" configuration, the directivity pattern oi the meter is probably nearly a cosine function,. Thus its meter movement would respond proportionally to the illuminance of the light it observes (with respect to the plane of the collector).

In the "dome" configuration, the directivity pattern oi the meter is probably nearly a cardiod function,. Thus its meter ovement would respond proportionally to what I call the "Norwood quasi-illuminance" of the light it observes with respect to the axis of the dome.

In the manual, the use of the dome configuration is clearly assumed for photographic exposure metering (of the incident light type). The use of the disk configuration is discussed only in connection with using the instrument to determine the illuminance of a light source on a certain plane (as might be done in a survey of workplace lighting).

In fact, though, the use of a cosine directivity (actual illuminace) meter for photographic exposure metering is recognized (for example, in ISO 2720)

The "calibration" of a incident light exposure meter is characterized by the incident light exposure meter calibration factor C, defined in ISO 2720. The manual for the L-398A states that, for the flat configuration, C is 250; for the dome configuration, C is 340.

I note here that the value of C is only pertinent to photographic exposure metering. Even though it is not discussed in the manual, the use of the disk mode for photographic exposure metering must have been contemplated (else there would be no stated C for the disk configuration).

Analysis of the exposure calculator of the L-398A reveals that, for the meter in is dome configuration to exhibit the stated C of 340, the "footcandle" indication of the meter movement (illuminance, although here actually quasi-illuminance) must be essentially correct.

I note that in photographic exposure metering, we normally have no need to pay attention to the "illuminance" indication on the meter movement. Rather we just use that number to transfer the meter movement indication into the photographic exposure calculator.

But if using the meter in the "disk" configuration to measure the illuminance on some surface (not in a phtogephic expsure metering comtext), the we do depend on the illuminance indcation of the meert movement and the expsure calcualtor is not at all in the picture.

But if the above about the meter movement indication in the dome configuraion is true, then in the "disk" confuguration, for the meter to exhbit the stated C of 250, the indication on the meter movment has to be about 1.36 times the actual illuuminance upon the meter.

So this seems to be the situation:

• In the dome configuration, when we only treat the illuminance indication of the meter movement as a way of transferring its indication into the exposure calculator, it neverthless seems that the intent if for that reading of itself to actually indcate the (quasi-) illuminance upon the meter.

• In the disk configuration, where we are actually interested in the illuminance upon the meter,, the indication of the meter movement is approximately 1.36 times the iiluminance upon the meter.

So go figger!

Best regards,

Doug

 

[Doug Kerr]

We note with interest (and perhaps some bafflement) that for the Sekonic L-398A exposure meter, for the incident light mode, the manual states that the value of the calibration constant, C ,with the "dome" collector in place is 430, while with the "disk" collector in place, the value of C is 320.

This calibration constant appears in international standard ISO 2720, which gives the requirements for free-standing photographic exposure meters. No unique value of C is prescribed, nor even suggested. This is said to recognize that, traditionally, manufacturers of exposure meter have used a certain "calibration" they felt would give most of their users a good exposure result most of the time.

So ISO 2720 prescribes, for incident light meters, only that C be within a certain fairly broad range.

ISO 2720 recognized that exposure meters may exhibit either a cosine directivity pattern (typically with a "flat" light collector) or a cardioid directivity pattern (typically with a "dome" light collector). For a reason that I have not yet sniffed out, the range of permitted values of C is different for those two configurations.

We might suspect that the "midpoint" of each of these ranges is somehow the "suggested" value. Of course the midpoint might be either the arithmetic midpoint or the geometric midpoint.

But is we use either, we find that the midpoints of the two ranges have the ratio of almost exactly 1.33 (the value for the cosine directivity being the lesser).

The two values of C for the L-398A are not those midpoint values. But interestingly enough, they have a ratio of almost exactly 1.33 (the value for the disk configuration being the lesser).

It is hard to know what to make of that.

Best regards,

Doug

 

[Doug Kerr]

It is interesting to read what ISO 2720 has to say about the usage of the two directivity patterns for an incident light expsure meter:

******
Possible methods of use are :

1) when a cardioid type receptor is used, to hold the
exposure meter at the object position and direct its axis
towards the camera; a single measurement shall be taken.

2) when a cosine type receptor is used, to take the
Iogarithmic average of two measurements, the first being
taken with the meter axis directed from the object
position towards the amera, and the second with the
meter axis directed from the object position towards the
source of maximum light.

*****
Case 1 is of course "Norwood" expsure metering.

Case 2 is of course one type of "duplex metering". In reality, it is consistent with the type of duplex meering studied by Norwood, if we recognize that the fill light (if any) is at the camera and the key light is "the source of maximum light".

It is not at all clear why the two practices should use differnt values of C (for a given exposure strategy). Of courwe, IS0 2720 doesnlt really directly suggest that they should.

Best regards,

Doug

 

[Ted Cousins]

We note with interest (and perhaps some bafflement) that for the Sekonic L-398A exposure meter, for the incident light mode, the manual states that the value of the calibration constant, C ,with the "dome" collector in place is 430, while with the "disk" collector in place, the value of C is 320.

Bafflement indeed, Doug!

From my copy of the manual:

best,

Ted.

 

[Doug Kerr]

It seems fairly likely that the ranges of permitted values of C in ISO 2720 were derived by consideration of a "survey" of the values of C used by various meter manufacturers.

I suspect that some manufacturers emphasized the "cardiod" (dome) configuration in their meters for actual photographic exposure determination, while others provided only the more traditional "flat" configuration.

It is possible that, for certain common usage cases, considering the use of a single measurement with an incident light exposure meter, that the "cardioid directivity" (dome collector) meter was considered to give a more reliably-appropriate photographic exposure recommendation than the "cosine directivity" (flat collector) meter. (Else why have such an "improvement"?)

If so, then the manufacturers of dome collector meters might well have settled onto a "more aggressive" exposure "strategy" then the manufacturers of flat-collector meters. Such a more aggressive strategy would be implemented by a greater value of C (which would lead to a greater photographic exposure recommendation for a given scene).

We are not speaking of a big difference here. The difference between the midpoints of the ISO 2720 ranges of C for the two directivity configurations amounts to about 0.4 stop.

Just sayin'.

Best regards,

Doug

 

[Doug Kerr]

More careful photogrammetry on the calculator dial of my Sekonic L-398A exposure meter indicates that, if the input to the calculator is in fact the quasi-illuminance experienced by the meter (that is, if the footcandle indication of the meter movement is accurate), the C of the meter would be 312.5.

Given the condition expressed in blue above, this does not in any depend on a certain collector being in place. (Of course different collectors will in general result in different indications on the meter movement for the same illuminance or quasi-illuminance.)

The announced value of C with the "dome" collector in place is 340. That suggests that the meter movement indication with the dome collector in place differs substantially from the actual quasi-illuminance experienced by the meter (about a -9% discrepancy).

Note that this finding does not depend on the state of calibration of my particular meter. It depends solely on the setup of the exposure calculator, which it seems could not change over time.

Best regards,

Doug

 

[Doug Kerr]

Just in case anybody is interested, this is the figure from which I did the photogrammetric analysis of the calculator on my Sekonic L-398A exposure meter:

I set the sensitivity to ISO 100 (it is on the detent point for that value). I set the main dial for exact alignment of, for example 1/4 second and f/4. The vernier acuity of the eye permits this to be done with great precision.

What remains is then to determine the input setting that would lead to that dial setup, which is where the photogrammetry comes in. This was done with CorelDraw.

The yellow circle is a construction artifact I used to estimate the center of the calculator.

We see that the angle between the two successive input values of 10 and 20 footcandles is 11.76°, while the angle from the 10 fc mark to the input pointer is 10.60°. The scale is logarithmic, and a little algebra tells us that the input setting is 18.6 fc.

Using the formula for C from ISO 2720 leads to the conclusion that, if the input setting is indeed the quasi-luminance experienced by the meter, then the value of C for the meter is 312.5

Quod erat demonstrandum.

Best regards,

Doug

 

[Ted Cousins]

Just in case anybody is interested, this is the figure from which I did the photogrammetric analysis of the calculator on my Sekonic L-398A exposure meter:

View attachment 14407

The scale is logarithmic, and a little algebra tells us that the input setting is 18.6 fc.

Nice analysis!

Using the formula for C from ISO 2720 leads to the conclusion that, if the input setting is indeed the quasi-luminance experienced by the meter, then the value of C for the meter is 312.5

And EV=log₂(18.6·10.764·(100/312.5)≈6.00 EV ipso facto

OTOH, if C truly equals 340 then 2⁶/(10.764·(100/340) ≈ an input of 20.22 fc which also points to 6 EV on my L-398A ...

In other words log₂(18.6·10.764·(100/312.5) = log₂(20.22·10.764·(100/340)
i.e. both ≈ 6 EV.

best,

Ted.

 

[Doug Kerr]

Hi,, Ted,

Yes, for each stated value of C, 340 or 250, the exposure calculator will work right if the input setting has a certain relationship to the actual illuminance (or quasi-illuminance) on the meter. In neither case will that input setting be the actual illuminance or quasi-illuminance on the meter.

Best regards,

Doug

 

[Ted Cousins]

Hi,, Ted,

Yes, for each stated value of C, 340 or 250, the exposure calculator will work right if the input setting has a certain relationship to the actual illuminance (or quasi-illuminance) on the meter. In neither case will that input setting be the actual illuminance or quasi-illuminance on the meter.

Joke du jour:

A lower C value tells the meter to recommend more exposure (brighter images), while a higher C value results in less exposure (darker images).

I said "are you sure?"

No, I had that completely backwards—thank you for catching that!

I use AI to examine expensive standards like ISO but remain wary of false answers like today's.

This maybe of interest, some familiar numbers there, like 15.7% reflectance ...

Exposure Equations & Light Meter Calibration

yedlin.net

I see that fancy Sekonic digital meters allow "compensation" - and long ago I did that with my L-398 (not A) by offsetting the ISO. Using the dome, I was consistently getting dark images with my Sigma SD9.

 

[Doug Kerr]

Joke du jour:


I said "are you sure?"

<snip>

"Before we can learn, sometimes we must teach"

Best regards,

Doug

 

[Doug Kerr]

Hi, Ted,

This maybe of interest, some familiar numbers there, like 15.7% reflectance ...

Exposure Equations & Light Meter Calibration

yedlin.net

Interesting indeed. Thanks.

Best regards,

Doug

 

[Doug Kerr]

Hi, Ted,

It appears that the author of that article you cited is working with a Sekonic L-558 "Dualmaster" exposure meter. As he discusses, rather than having interchangeable dome and disk light collectors, as on the meters in the "Norwood Director" family, here the light collector system can be switched from "dome" directivity to "disk" directivity by by retracting the dome ("Lumisphere").

The manual for this model says:

******
When the Lumisphere is extended. (3-D Light Measurement)
This is used to measure people, buildings, and other three dimensional objects.
Measurements are basically made by the method of measuring with the lumisphere aimed in
the camera direction (more precisely, in the direction of the lens axis) at the position of the
subject.

When the Lumisphere is retracted (flat diffuser function)
This is used to measure manuscripts, paintings or other flat copy. It can also be used for measuring
illumination levels (see page 26), or brightness difference (see page 24).
******

This is more of less to be expected.

As the author says, the manual for this meter states that the value of C is 340 in the "dome directvity" configuration and 250 in the "diak directvity" configuration.

The author seems to conclude that the value of C does not really differ between the two configurations. I must admit that I have not in fact read his disucssion of that.

Best regards,

Doug

 

[Doug Kerr]

The refernce to the use of the Sekonic L-558 in its "cosine directvity" configuration (dome retractted) in exposure metering for shots of manuascripts, paitnings, and the like is quite apt. When the subject elements all lie in essentially a plane facing the camera, then the quantiy that should affect the phtographic exspsure choice is the illuminance on the subject. And of course a meter with a cosine directivity should measure that quantiy. The "Norwood" exposure technique (using a cardiod directivity) is not the most appproriate there.

One would think that, for the imprtant case where all the light comes from the directon of the camera that the two metering configurations would recommend the same camera exposure. But for that to be so, the value of C wouod have to be the same for the two configurations.

What remains a mystery is that the Sekonic meters intentonally (suppoedly) have different values of C in their "cadioid" and "cosine" directivity configurations.

So the mystery remains such.

Best regards,

Doug

 

[Doug Kerr]

Why do I feel that. for the same "exposure philosophy", "Norwood" and "illuminance-based" ("traditional") exposure meters should have the same value of the incident light metering calibration constant, C?

I go back to the seminal paper by Donald Norwood in which he reports the results of an elaborate series of subjective tets that built a framework that justified his incident light exposure metering scheme, which he had in fact originally constructed on intuitive grounds.

For each of several human subjects, Norwood started by making a "reference shot" with the both key and fill light sources located at the camera position. The photographic exposure was set to produce what was considered a "good" exposure result, presumably using a "traditional" incident light exposure meter. We do not know what "exposure metering formula" was used for that. but we stipulate to the vague notion that the results were "good".

Then, the key light was moved to successive angles with respect to the camera position, and for each location, shots were taken with several photographic exposures in small increments both greater than and less than the exposure used for the "reference" shot. Viewers were then asked to choose, for each key light position, the shot whose exposure result they adjudged "equivalent" to the exposure result for the "reference" shot for that subject (whatever exactly that would mean).

These choices were "averaged" over the panel of several viewers and the consolidated results tabulated. The results showed that, to get what the viewers adjudged to be the equivalent exposure result to that of the reference shot, the photographic exposure had to be increased approximately linearly with the angle of the key light, reaching 2 times the exposure for the reference shot when the key light was at the 90° position.

From this, Norwood posited that a single measurement, made with a meter whose directivity decreased linearly with angle from its axis, reaching 0.5 at 90°. (The "cardiod" directivity, used in most implementations of this scheme, is an easily-implemented approximation to that.)

But if we go back to the initial situation in which the "reference" shot was thought to give an "ideal"exposure result, the same as a "traditional" incident light exposure meter would have given, this then tells us that the value of C for the "Norwood" meter should be the value of "C" for the traditional meter.

So, if we are to have a meter that can be configured to either the "Norwood" form (with cardioid directivity) or the "traditional" form (with cosine directivity), and assuming that we would like the meter in both configurations to follow the same "exposure philosophy"", the value of C would have to be the same for both configurations.

I think.

Best regards,

Doug

 

[Doug Kerr]

Prior to the work of Donald Norwood, exposure metering in cinematography, for key-fill lighting on a human face, often was done with the "duplex metering" technique. There, two separate measurements were made of the luminous flux density of the light from the key light and the fill light. Those two readings were averaged and the result used as the input to the exposure calculator.

Nowrood developed a scheme in which a single measurement, made with a new kind of meter, wouild directly give an appropriate photographic exposure recommendation.

It is instructive to compare. by numerical simulation, the photographic exposure recommendations we would expect to be given by duplex metering and with a "Norwood" exposure meter in various situations. I have done this via an MS Excel spreadsheet.

For example, for a key-fill ratio of 8:1, this exercise showed that the photographic exposure recommendation via "Norwood" metering very closely agreed with the result via duplex metering (over all angles of the key light location from 0° through 90°) if the values of C for the illuminance-based meter (used in the duplex metering technique) and the Norwood meter were the same.

Best regards,

Doug

 

[Doug Kerr]

My recent work on this matter has suffered from the lack of a credible set of photometric measurements on my Sekonic L-398A exposure meter with both the dome and disk light collectors fitted. The values of particular interest are the actual values of C exhibited by the meter in those two configurations.

I hope to remedy this in the near future. I do not suggest that those results will be highly accurate (I have neither the apparatus nor the patience for that), but they should be sufficiently accurate to illuminate (!) what is generally going on here.

Best regards,

Doug

 

[Ted Cousins]

Apropos of which, on Friday I did measure the two configurations:

I just did the test with my A19 LED desk lamp on-axis 15" from the Lumisphere and from the Lumidisc ... <> the meter readings were the same: 48 fc.

Eagerly, I await your comprehensive and far more definitive results ...

best,

Ted.

 

[Doug Kerr]

Hi, Ted,

You repeated from an earlier post just above:

I just did the test with my A19 LED desk lamp on-axis 15" from the Lumisphere and from the Lumidisc ... <> the meter readings were the same: 48 fc.

Remind me what those meter readings were. I suspect that they are the readings of the L-398A meter movement with the dome and disk collectors fitted.

It is "surprising" that the footcandle readings on the meter movement from the same light source were the same with the dome and disk fitted. (And there is good agreement of both with the illuminometer reading.)

There is some "impurity" to that test setup as the diameter of the light source is not "many times less" than the distance to the meter under test, but I think any discrepancy would be slight.

If in fact for both configurations the meter movement reading is essentially the actual illuminance on the meter, then the C for both configurations is about 314 (based on my L-398A exposure calculator; I would expect yours to be the same).

Thanks.

Best regards,

Doug

 

[Ted Cousins]

I think for now I'll wait for your upcoming test results.

best,

Ted.

 

[Ted Cousins]

I think for now I'll wait for your upcoming test results.

best,

Ted.

Meanwhile, Google AI agrees with my test result for same illuminance head-on:

f you swap between the Lumisphere (dome) and the Lumidisc (flat disc) under a perfectly straight, direct frontal beam of light—and we assume both pieces of plastic allow the exact same amount of light to pass through (equal transmissivity)—the meter reading will actually stay the same, but your final exposure calculation will change. [duh??]

Unlike a digital meter that automatically switches its math when you change modes, the Sekonic L-398A is a purely mechanical galvonometer. The amorphous silicon photocell sits behind the plastic attachment.Because the beam of light is perfectly straight and hits both attachments at a 90-deg angle, the physical shape of the dome vs. the disc doesn't change the footprint of the light.The exact same number of photons hit the sensor, generating the same current.The physical needle will point to the exact same Footcandle value on the dial.

But then it says:

The External Calculator Dial Shifts the Exposure: to actually get your shutter speed and f-stop, you have to manually rotate the large dial ring on the Sekonic L-398A to match the foot candle number the needle is pointing to

makes no sense if the needle readings are the same, eh?

 

[Doug Kerr]

Hi, Ted,

makes no sense if the needle readings are the same, eh?

Indeed.

I think that in some other manufacturers' meters (don't remember just now which) having an exposure calculator much like the one in the Sekonic L-398A, there are two different "arrows" used to set the meter movement indication into the calculator, one to be used with the dome collector in place and one for the disk collector. This is needed, taking into account the probable difference in transmissivity of the two collectors, to actually give the meter the intended value of C for the two configurations.

The 'bot's discussion of the expected behavior of the two collectors is naïve.

Again, why different values of C for the two collector configurations is intended eludes ne.

Best regards,

Doug

 

[Ted Cousins]

Hi, Ted,

Indeed.

I think that in some other manufacturers' meters (don't remember just now which) having an exposure calculator much like the one in the Sekonic L-398A, there are two different "arrows" used to set the meter movement indication into the calculator, one to be used with the dome collector in place and one for the disk collector ...

Interesting and new to me!

... This is needed, taking into account the probable difference in transmissivity of the two collectors, to actually give the meter the intended value of C for the two configurations.

The 'bot's discussion of the expected behavior of the two collectors is naïve.

Agreed ...

Again, why different values of C for the two collector configurations is intended eludes me.

Me too.

best,

Ted.

 

[Doug Kerr]

[Not surprisingly, I seem to have fumbled some of the calculations behind my initial preliminary report. This edited version includes what I hope are the correct values. Sorry for the gaffe.]

This is a preliminary report on the results of photoelectric testing of my Sekonic L-398A exposure meter in the incident light exposure metering mode, with both "dome" and "disk" collectors aboard.

The absolute values of C thus determined are dependent upon measurement of the test illuminance with an illuminometer for which I have no reliable calibration trail. Thus there is the potential of experimental error of an unknown degree in those values.

But the ratio of the two values of C does not depend on the accuracy of the illuminance measurement (so long as it is consistent between the two test cases.).

That having been said, the results are:

• For the dome collector, C=405

• For the disk collector, C=328

The ratio of the two values of C is 1.23.

For reference, I note that the manufacturer states that the values of C for this model are, for the dome collector, 340; for the disk collector. 250, a ratio of 1.36.

Best regards,

Doug

 

[Doug Kerr]

By way of further information, the test illuminance was 1890 lux (per an illuminometer of unknown accuracy).

The indication of the L-398A meter movement with the meter under this illumination were, for the two collectors:

Dome collector: 226.2 fc (2435 lux)

Disk collector: 183.4 fc (1974 lux)

It is not surprising that the meter movement indication is different for the two collector types for a given incident illumination. It would have to be for the meter to exhibit different values of C with the two collector types.

Best regards,

Doug

 

[Ted Cousins]

1) trying to keep up, are your results based on the re-arranged ISO 2720 formula:

C=E·S·t/N² ?

Seemingly not ... for example my 398A meter set to 100 ISO and 320 fc, the calculator dial tells me based on that formula that C ≈ 355, irrespective of the collector type ...

if not, what is the basis?

thx,

Ted.

 

[Doug Kerr]

Hi, Ted,

My apologies for not having earlier explained my methodology. I just ran out of steam!

I speak of the "calculator C" as the value of C the L-398A exposure calculator itself exhibits assuming that the input setting to the calculator (normally taken from the meter movement indication) is in fact the actual illuminance (quasi-illuminance, in the case of the dome collector) on the meter. I reckon that from this formula from ISO 2720 (with added notstion):

where Ec is the input "illuminance" setting to the calculator and t and N are any matching pair of exposure time and aperture (as an f-number) given by the calculator.

In my L-398A, that gives C=314.2 That would be the C of the meter (with a certain collector in place) if in fact the meter movement indicted the actual illuminance on the meter (with that collector in place).

Note that this would not change as the meter light collector might be changed. It is a property of the exposure calculator itself.

To determine the value of C the entire meter exhibits (with a certain collector in place), I measured the illuminance at the place where the meter will be placed with an illuminometer. I then put the meter in place and captured the reading of its meter movement. (I locked the meter pointer with the "hold needle" function and then photographed the meter movement scale for later observaion and photogrammetric analysis.)

Then, conceptually, I set the input of the calculator to the meter movement reading and observed the phtometric exposure recommendation (a matching pair of t and N) from the calculator. I would then reckon C from this formula:

where Et is the test illuminance on the meter.

But in reality I did not do that. I knew the C that the calculator would exhibit based on its input being the actual illuminance (discussed earler). Now knowing the test illuminance and resulting meter movement indication for an actual test, I could take the ratio of the test illuminance to the meter movement indication and multiply that ratio times the known "calculator C" to get the actual C exhibited by the meter as tested. [It took quite a bit of thought to determine "which way up" this calculation needed to be made!]

This avoided the need to physically transfer the meter movement indication to the calculator input (hard to do precisely for some random meter movement indication). Instead, in effect, I made that transfer "on paper" (to a "paper model" of the exposure calculator).

Again it is important to note that the actual values of C I obtained might well be in error due to error in the illuminometer, which I had no way to calibrate. On the other hand, the ratio of the two values of C (with the two different collectors in place), is not affected by error in the illuminometer.

Here are some particulars of the "live" test:

Test illuminance: 1890 lux (per illuminometer)
L-398A (dome collector in place) meter movement indication: 226.3 fc (2436 lx) (meter C=405)
L-398A (disk collector in place) meter movement indication:183.4 fc (2008 lx) (meter C=328)

The ratio of the two meter movement idncations (and thus of the two values of C) is about 1.24.

I think.

Best regards,

Doug

 

[Doug Kerr]

Hi, Ted,

This is how I determined the "calculator C" for my Sekonic L-398A exposure meter.

I set the calculator for ISO 100 (there is a detent on that setting so I presume I had set it "accurately") an set the calculator input until i had exact alignment between (for example) t=1/4 second and n- f/4.

I then photographed the calculator and imported the photo to my technical illustration program, CorelDraw. After drawing some construction circles and lines, and having the program determine certain angles, I concluded the value of the calculator input setting (in footcandles).

Then, from the infamous formula in ISO 2720, I determined that the value of C the calculator exhibits was 314.2 That would be the value of C for the meter with a certain collector in place if the illuminance indicated by the meter movement was the actual illuminance on the meter.

Of course this determination did not in any way depend on some collector being in place, or if there was even a photodetector and meter movement in the picture.

But the value of C the exposure meter would exhibit with a certain collector in place would be that "calculator C" times the ratio of the illuminance indicated by the meter movement to the actual illuminance upon the meter.

Best regards,

Doug

 

[Ted Cousins]

Got it, thanks Doug!

best,

Ted.

 

[Doug Kerr]

Hi, Ted,

I think what you said earlier means that you determined what what I call the "calculator C" for your Sekonic L-398A exposure meter to be 355.

I am not at all perplexed at the difference between that value and the value I determined for my specimen of that meter, 314.

For one thing, interpolating on the logarithmic scales of that calculator is a tricky business, and either of us (or both) may have not done that in the "best" way.

We also cannot rule out the possibility that our two specimens have different calculator "calibrations". Sekonic might well have made an "inline" change in that at some point.

Here is a shot of mine, set for ISO 100 and an arbitrary input that gave a result of exactly 1/4 sec, f/4:

That input setting to give that, seen as "a bit less than 20 fc", was evaluated by photogrammetry as 18.7 fc.

Best regards,

Doug