Not matching light meter values

Bill Burk · Apr 28, 2024

Fatih Ayoglu said:
I meant I’ve bid for a Luma iPhone light meter on eBay,

@Bill Burk what I’m trying to understand is; is my spotmeter un calibrated or is spotmeter reading 18% gray card differently than other meters?

TBH I can just use 1/2 EV slower ISO each time instead of using ND filter. Much easier

The 1/2 stop filter is only for taking spotmeter readings off the 18% gray card.

If you change the ISO. you might forget and leave it on for any other kind of metering purposes where you want to use the actual ISO.

ISO are in third-stops, and the difference from 18% to 12.7% is a half stop. There would be rounding errors.

Even the Sekonic analog L-28c clicks speeds in third stops while its scale is in half stops.

Fatih Ayoglu said:
Yes, so

automatic was 1/12 f/8

Digital Sekonic 1/15 f/8

Analog Sekonic 1/20 f/8

Pentax 1/24 f/8

This was in the room, next to a window looking North, on a cloudy day. ISO was 200

Converting f/stop and shutter speed to the Weston dial…

Automatic 4 fc
Digital Sekonic 5 fc
Analog Sekonic 6.5 fc
Pentax 8 fc

Whichever ones you read the 18% card by reflected light need to be opened up 1/2 stop…

Correcting 18% to 12.7%

Digital Sekonic 3.5 fc (between 1/12 and 1/10 f/8)
Analog Sekonic 4.6 fc (between 1/15 and 1/12 f/8)
Pentax 5.6 fc (between 1/20 and 1/15 f/8)

You say 1/12 f/8 was the “correct” exposure.

The Pentax is reading one half stop higher than correct.

The Sekonics are within a sixth stop of correct.

Bill Burk · Apr 28, 2024

Sputtered black spray paint on a piece of plastic (cardboard box packaging with a window).

Created a bit of gradient with the spray and cut out the corner that changed light meter reading closest to a half-stop.

Hold in front of spotmeter only when reading 18% gray card.

Otherwise you use the spotmeter to evaluate different spots for what they are.

Sirius Glass · Apr 28, 2024

I solved the calibration problems by sending ALL my cameras and meters out to be calibrated at the same lab with standardized sources. Since the I have had no light meter problems.

Bill Burk · Apr 28, 2024

Sirius Glass said:
I solved the calibration problems by sending ALL my cameras and meters out to be calibrated at the same lab with standardized sources. Since the I have had no light meter problems.

But can you get an incident meter and a reflected light meter to recommend the same combination of f/stop and shutter speed for a particular scene?

Until this thread I was happy to have a couple suggested combinations from which to choose because I believed they would both work.

But what if we could have two readings that agree. But if they differ, one of them is wrong.

RalphLambrecht · Apr 28, 2024

Fatih Ayoglu said:
I’ll try as soon as rain stops and we have a clear blue sky. I assume sun should be behind me and fully exposing the light meter some.

Yes, the dome should point directly to the sun.

RalphLambrecht · Apr 28, 2024

Bill Burk said:
But can you get an incident meter and a reflected light meter to recommend the same combination of f/stop and shutter speed for a particular scene?

Until this thread I was happy to have a couple suggested combinations from which to choose because I believed they would both work.

But what if we could have two readings that agree. But if they differ, one of them is wrong.

theoretically, yes by reading withe a reflective meter off a neutral graycard, In practice, the reading changes with the angle of reflection from the graycard quite significantly.

Bill Burk · Apr 29, 2024

RalphLambrecht said:
theoretically, yes by reading with a reflective meter off a neutral graycard, In practice, the reading changes with the angle of reflection from the graycard quite significantly.

Found an old thread from 2016 where we talked about this. One member of the group was insincere, so the discussion degraded.

It ruined my vacation.

These are the photographic equations for reflected light and incident light.

K (reflected light meter) and C (incident light meter) are important.

For the Sekonic L-28c the value of C = 28

This is from Connelly “The incident light method of exposure determination” the paper can be accessed from here:

Index of /benskin/content

Bill Burk · Apr 29, 2024

The math is yet to be produced. But that doesn’t ruin this experiment.

Some meters have a value for K that is OK with 18% gray card. (Minolta Spot Meter K=14).

But the advice printed with the new Kodak gray cards recommend opening up 1/2 stop.

For meters that are calibrated such that opening 1/2 stop from Kodak 18% gray card is recommended, this simple filter could be a good learning tool.

Spatter black spray paint on a scrap of transparent plastic, maybe from a cake box. Try to create a smooth gradient.

Check meter readings through the spray-painted plastic. Find a section where the meter reading drops 1/2 stop. Make a simple frame and label it something like 0.15 ND for 18% to 12.7%

Bill Burk · Apr 29, 2024

@Fatih Ayoglu if you can collect the K values from the spotmeters and the C values for the incident meters.

RalphLambrecht · Apr 29, 2024

RalphLambrecht said:
theoretically, yes by reading withe a reflective meter off a neutral graycard, In practice, the reading changes with the angle of reflection from the graycard quite significantly.

Don't ever let a Photrio thread ruin your vacation; mostly nice people with an opinion.

Sirius Glass · Apr 29, 2024

Bill Burk said:
But can you get an incident meter and a reflected light meter to recommend the same combination of f/stop and shutter speed for a particular scene?

Until this thread I was happy to have a couple suggested combinations from which to choose because I believed they would both work.

But what if we could have two readings that agree. But if they differ, one of them is wrong.

Yes. Either the same or so close there is virtually no difference.

Bill Burk · Apr 29, 2024

Ah but I am starting to realize the root of my half stop discrepancy when calibrating might be that K= 1.0 for Weston meters.

BrianShaw · Apr 29, 2024

Bill Burk said:
Ah but I am starting to realize the root of my half stop discrepancy when calibrating might be that K= 1.0 for Weston meters.

You might have read this already, but re-read of Section 4 might be worthwhile regarding constants and light units.

http://dougkerr.net/Pumpkin/articles/Weston_Master_meters.pdf

Bill Burk · Apr 29, 2024

RalphLambrecht said:
Don't ever let a Photrio thread ruin your vacation; mostly nice people with an opinion.

Thanks! I consider you and Stephen good friends. I was only reminded of my nemesis because I can’t (won’t) read many of the posts in that ten year old thread.

Bill Burk · Apr 29, 2024

BrianShaw said:
You might have read this already, but re-read of Section 4 might be worthwhile regarding constants and light units.

http://dougkerr.net/Pumpkin/articles/Weston_Master_meters.pdf

Thanks,

There’s some good stuff in there. Figure 2 is the 650, the white circle is not a mysterious unidentified incident attachment, it’s just a sticker.

I’ll be sure to remember FC vs CI when interpreting K = 1.0 (somewhere between 10.8 to 10.6 in SI terms).

I consider the Master 6 an important member of that family. It reintroduced the candles per square foot numbering, you can lock the film speed dial, there is a calibration rheostat under the hood. And you can get an Ansel Adams dial for it.

Not matching light meter values

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