A curious form of duplex metering

[Doug Kerr]

"Duplex metering" is a term sometimes applied to an exposure metering technique (principally applied in a key-fill lighting situation)in which an exposure meter (usually with a "flat" collector so it reads true luminance on the plane of the collector) is used to take separate readings while having the meter facing the key light and then while facing the fill light (the other light presumably being off during each measurement, but maybe not).

The two illuminance measurements are then averaged (there being some uncertainty as to whether the arithmetic of geometric average is used) and the resulting illuminance value entered into the exposure meter's calculator to determine the proper photographic exposure (combination of shutter speed and aperture).

******
A curious application (or adaptation) of this concept is taught in the manuals for the Sekonic 398 and 398M incident light exposure meters.

The situation is described thus:

Observatorial Scenes
This refers to the situation where a distant object in the subject
field forms an important part of the pictured scene.
Examples: Observatories, distant views, etc.

I am not sure what the reference to "observatories" means. This is the example shot of this type from the manual:

The introductory text continues:

Distant scenes are subject to haze due to atmospheric effects on
light and can be easily overexposed. For this reason, first use the
Lumisphere and measure by standard method from the camera position.

Then point the Lumisphere [the regular dome collector] toward the sun and measure.
Determine exposure from the mean of the two obtained values.

The explanation that follows makes it certain that it is the geometric mean that is to be taken.

The conclusion is that this mean value is then to be set into the meter calculator (as if it were the "reading" of the meter movement) and the recommended camera exposure (combination of shutter speed and aperture) gotten.

It is not at all clear to me why this is desirable or "how it works".

Best regards,

Doug

 

[Ted Cousins]

I find the Note about figuring the "mean"in the L-398 Observatorial stuff to be thoroughly confusing. There's no such section in the L-398A manual, so maybe Sekonic got some feedback on previous model instructions. I'd post that note, but neither Foxit nor LibreOffice are letting me easily copy the text.

grump ....

 

[Ted Cousins]

<snip>
******
A curious application (or adaptation) of this concept is taught in the manuals for the Sekonic 398 and 398M incident light exposure meters.
<snip>

Then point the Lumisphere [/COLOR][COLOR=rgb(0, 0, 0)][the regular dome collector][/COLOR][COLOR=rgb(41, 105, 176)] toward the sun and measure.

"the regular dome collector" above comes out black against dark gray with my selected Dark Style. I had to select the text to read it.

No biggie, not everyone uses the Dark Style ...

 

[Doug Kerr]

Hi, Ted,

"the regular dome collector" above comes out black against dark gray with my selected Dark Style. I had to select the text to read it.

Ah, I coded that "black" instead of "normal", as I should have (but not easy with this UI). Sorry.

Best regards,

Doug

 

[Ted Cousins]

\

I find the Note about figuring the "mean"in the L-398 Observatorial stuff to be thoroughly confusing. There's no such section in the L-398A manual, so maybe Sekonic got some feedback on previous model instructions. I'd post that note, but neither Foxit nor LibreOffice are letting me easily copy the text.

grump ....

So here is a screen shot instead:

Ignore the numbers in parentheses above - they are for use with the individual aperture slider accessories.

So the only number "transferred to the calculator dial" is normally the foot candle value. On the meter scale, the geometric mean between 80 and 320 is obviously 160 fc, being so marked halfway betwixt the two.

Not so easy for the example values 40 and 80 because there be no handy-dandy intermediate mark. So here one would whip out the pocket calculator that every photography carries and, lo, sqrt(40x80) is about 56.57 ... less easily transferred to the dial for a different scene.

The two bullet points can be ignored IMO. It is not possible to transfer the implied multiple fc values to the calculator dial at the same time, duh.

Like Doug, I don't see the efficacy of pointing the dome at the sun (wherever it happens to be).

 

[Doug Kerr]

Hi, Ted,

\

So here is a screen shot instead:

Thanks

<snip>

Ignore the numbers in parentheses above - they are for use with the individual aperture slider accessories.

Yes. I think the reason they are mentioned here is that these are the second (red) set of markings on the meter movement scale

So the only number "transferred to the calculator dial" is normally the foot candle value. On the meter scale, the geometric mean between 80 and 320 is obviously 160 fc, being so marked halfway betwixt the two.

Not so easy for the example values 40 and 80 because there be no handy-dandy intermediate mark.

But there is in fact a mark on the "illuminance" dial scale halfway between 40 and 80. The fact that this is numerically about 56.6 id not ever needed in the procedure as described.

But there would indeed be a problem if the two meter movement indications were not both on the "handy" marked values (e.g., one was at "40" and the other halfway between 80 and 160.

So here one would whip out the pocket calculator that every photography carries and, lo, sqrt(40x80) is about 56.57 ... less easily transferred to the dial for a different scene.

The two bullet points can be ignored IMO. It is not possible to transfer the implied multiple fc values to the calculator dial at the same time, duh.

The first bullet point is part of the "geometric mean" story. The second is indeed nonsensical.

Best regards,

Doug

 

[Doug Kerr]

Even more mystifying is the passage in the manual for the Sekonic L-398m that in my edition is designated ""4. Illumination adjustment (in my edition, starting at the bottom of page 29).

Here it is, in several screen shots:

******

******

One problem at the opening is why the meter should be set for ISO 100, 24 fps (cine frame speed; the meter considers the exposure time to be had in that situation as roughly 1/50 sec), f/5.6. Presumably these are an example of the applicable parameters, but the text doesn't bother to say that.

It gets worse from there.

Best regards,

Doug

 

[Ted Cousins]

Even more mystifying is the passage in the manual for the Sekonic L-398m that in my edition is designated ""4. Illumination adjustment (in my edition, starting at the bottom of page 29).

Here it is, in several screen shots:

******

View attachment 14559
View attachment 14560
View attachment 14561
******

One problem at the opening is why the meter should be set for ISO 100, 24 fps (cine frame speed; the meter considers the exposure time to be had in that situation as roughly 1/50 sec), f/5.6. Presumably these are an example of the applicable parameters, but the text doesn't bother to say that.

Looks like "determine" means both setting and measuring.

Seems to me that the settings ISO 100, 24 fps: hence f/5.6 is setting the meter sensitivity such that the exposure will be good when the needle is made to point to 500 fc. I don't shoot video, so the foregoing may not be right.

It gets worse from there.

Yep. Considerably so.

best,

Ted.