Not matching light meter values

[Fatih Ayoglu]

I still think that is because the dome gets old. You eyes don't see it but it transmit less light. I found that out on my Minolta meters. The 12.5 vs 14 is related only to the reflected meter. Both meter are Sekonic and thus should be calibrated to K12.5 while the Pentax should be calibrated to K14 and they all read the same. For the incident with the sphere the Sekonic calibrated with a C340 while some other brand like Minota is calibrated to C330.

The only issue with that is, the old Sekonic reads more EV than the new Sekonic. There is 30 years difference between both

 

[Alan Edward Klein]

Wouldn't it be best to just calibrate all three to the same known accurate reference source?

 

[BrianShaw]

Wouldn't it be best to just calibrate all three to the same known accurate reference source?

In theory, or in practice?

 

[Alan Edward Klein]

In theory, or in practice?

In practice. Calibrate one professionally and then adjust the other two to it.

 

[bluechromis]

You may get better agreement if you will get a 12.5% gray card and meter off it in reflected light mode, and compare it to the incident light mode.

If you only have an 18% gray, the difference between 18 and 12.5 is expected and means that your meter agrees (if the difference can be accounted for by that much) and you should leave it alone. Treat it as OK if they differ by that much.

Where do you get a 12.5 % grey card?

 

[BrianShaw]

In practice. Calibrate one professionally and then adjust the other two to it.

Who does light meter calibration? How does one “adjust” other than noting the deviation?

In the given situation, wouldn’t it be just as good to take 2 exposures, one per each meter, and see if there is any practical difference?

 

[Chan Tran]

In practice. Calibrate one professionally and then adjust the other two to it.

How do you calibrate the 2 meters the OP has (assuming you have the light standard)

 

[Bill Burk]

Where do you get a 12.5 % grey card?

The right value might be 12.7% but I don't think there is anything commercially available. There's a 12.7% patch on the Sekonic gray card (see post #17). But you are expected to use an 18% gray card in practice.

For calibrating meters you are expected to use a transilluminated panel.


I am having a devil of a time calibrating my Weston Master II meters and my attitude changes daily how I am going to deal with it. Some days I want to print a new scale. Other days I want to use a fly-tying jig to wrap a hundred more winds of wire to the D'Arsonval coil. Other days I want to find the right color temperature for the standard.

I have been able to increase the meter's response to the same brightness by increasing the voltage to a tungsten light source. This both increases the color temperature of the source and generates more infrared. I think the most likely source of the increase is the increased infrared.

For the standard light source, there's a project by Serhiy Rozum where he uses an LED source. He says on his Github site, that the Selenium cell meters read off 1 stop "exactly".... this is from his site:


----
Because of the Color Correction Factor of LED, old light meters like selenium meters, photoresistors or LDR, and some photodiodes read exactly 1 stop lower. More modern light meters, including digital cameras, read LED lights correctly.

Big thanks to Peter Woodford who did numerous tests and helped to identify behaviour of various light meters.

 

[eli griggs]

Has anyone pointed out that some meters have filters installed, pre-manufacture or third party, after-market adjustments or tweaking?

 

[RalphLambrecht]

Hi all,

So I have 3 light meters;

• Sekonic L308X, incident and reflective light meter
• Sekonic L-28C, incident and reflective light meter
• Pentax Digital Spotmeter, reflective light meter

When I measure reflective light, it all shows the same EV value, which is great.

But when I measure incident light while the Lumispheres are on with both Sekonics, L308X measures 0.5 EV less than L-28C. Certainly, I can put the ISO value down on L-28C and be fine with it but wanted to understand why it would create this difference.

Cheers,
Fatih

One who owns a watch knows exactly what time it is; someone with two watches can never be sure.

 

[RalphLambrecht]

Wouldn't it be best to just calibrate all three to the same known accurate reference source?

such as the sun at EV15!

 

[Fatih Ayoglu]

To start with, I have no idea how to calibrate both Sekonics and it might be above my skill set.

I guess the best course of action for the time being would be, shoot the gray card with the measured settings for each lightmeter and compare the result. At the end of the day, this is pretty much what we do with the film as well, shooting through a step wedge and see if our processing is correct.

 

[Chan Tran]

To start with, I have no idea how to calibrate both Sekonics and it might be above my skill set.

I guess the best course of action for the time being would be, shoot the gray card with the measured settings for each lightmeter and compare the result. At the end of the day, this is pretty much what we do with the film as well, shooting through a step wedge and see if our processing is correct.

Yes where are the adjustment for calibration on these meters?

 

[kl122002]

Sometimes the dome got old (yellowed, cloudy, or less transparent) as the aged. It is the problem from the white plastic degeneration.

If the reading is 1/4 stop of less then I won't brother. Otherwise I would look for another dome and try

 

[Fatih Ayoglu]

Sometimes the dome got old (yellowed, cloudy, or less transparent) as the aged. It is the problem from the white plastic degeneration.

If the reading is 1/4 stop of less then I won't brother. Otherwise I would look for another dome and try

As mentioned before, issue is the old Sekonoc shows higher EV than the new one and there is an age difference of 30 years at least between them. The difference is only 0.5EV I’ll shoot a gray card to compare. The most likely explanation I have at the moment is, the old Sekonic has much much larger dome and probably collects more lifht from different angles. We will see, Royal Mail has not delivered that card yet

 

[Stephen Benskin]

Where do you get a 12.5 % grey card?

K isn't about reflectance. It's an exposure constant that factors in various elements that determine how much light strikes the film plane. If the exposure equation only uses f/stop and shutter speed (and film speed), the amount of exposure reaching the film plane or sensor will be lower than the equation predicts. K contains most of the variables as q, which is part of the camera exposure equation, but it includes variables associated with the exposure meter. Through the lens metering measures the actual amount of light entering the camera through the optical system. Hand held exposure meters need to assume average values.

1.16 is in cd/ft2. To convert to cd/m2, 1.16 * 10.76 = 12.48 or 12.5 cd/m2

Camera exposure equation:

OP: Meters need to be calibrated from time to time.

 

[Andrew O'Neill]

To start with, I have no idea how to calibrate both Sekonics and it might be above my skill set.

I guess the best course of action for the time being would be, shoot the gray card with the measured settings for each lightmeter and compare the result. At the end of the day, this is pretty much what we do with the film as well, shooting through a step wedge and see if our processing is correct.

Calibrating the 308
I knew someone who had an L-28C2. There is a dial on the back that you can turn with a coin, to zero out the metre. Does the 28C have something similar?

 

[Fatih Ayoglu]

Calibrating the 308
I knew someone who had an L-28C2. There is a dial on the back that you can turn with a coin, to zero out the metre. Does the 28C have something similar?

Yes it has a screw but that is to make the reading zero, not to calibrate to my understanding. Because certainly it is zero when there is absolutely no light. I have checked few videos, to calibrate L-28c, I need to fully open and expose some circuity to adjust, which I really dont want to do. OTOH calibrating 308X is very easy so I hope it the one which shows the slightly off reading

 

[Andrew O'Neill]

You're supposed to cover your hand over the dome. If the needle does not go to zero, then the dial is turned so that the needle points to zero. I guess that is how that particular metre is calibrated . I would probably just use the 308. My L-588 is super easy to calibrate in the field.

 

[Fatih Ayoglu]

You're supposed to cover your hand over the dome. If the needle does not go to zero, then the dial is turned so that the needle points to zero. I guess that is how that particular metre is calibrated . I would probably just use the 308. My L-588 is super easy to calibrate in the field.

Yes, in fact to make sure there is no light, I made that adjustment under a dark cloth just in case my hand transmit some amount of light.

Where is that gray card now??? C'mon Royal Mail, end this mystery please.

 

[BrianShaw]

You're supposed to cover your hand over the dome. If the needle does not go to zero, then the dial is turned so that the needle points to zero. I guess that is how that particular metre is calibrated . I would probably just use the 308. My L-588 is super easy to calibrate in the field.

That is a zero-adjust procedure, not a calibration per se. Calibration adjusts a meter to a known and accepted standard. See post #13, please.

... and post #35 is worth re-reading, too, as it somewhat validates your conclusion... whether you have a L-588 or a L-558 (or any other collection of light meters).

 

[Alan Edward Klein]

Who does light meter calibration? How does one “adjust” other than noting the deviation?

In the given situation, wouldn’t it be just as good to take 2 exposures, one per each meter, and see if there is any practical difference?

The OP says he's getting a half-stop difference in readings. Assuming one's right, the others off half a stop. Of course, calibration won't help him if he's misusing the meters.

 

[BrianShaw]

The OP says he's getting a half-stop difference in readings. Assuming one's right, the others off half a stop. Of course, calibration won't help him if he's misusing the meters.

No doubt; a very true statement about the importance to correctly use meters, especially when of different designs. But don't assume that only one is off as both could be off by 1/4 stop in the same direction.

Case in point about correct usage: I happily used Weston meters in a vertical position until I compared the readings with a "better and more modern" meter. Then discoverred that the corrrect position fo using a Weson meter is in the horizontal postiion, and a map of the acceptance pattern proved that my "incorrect reading" were because of that. Using that specific Weston meter became much more in agreement with the "modern meter" when the erroneous use was corrected. Truth be told, though... the minor difference in exposure recommendation was rather insignificant to practical image making.

 

[Alan Edward Klein]

How do you calibrate the 2 meters the OP has (assuming you have the light standard)

If there's no way to calibrate, then he'll have to offset the ISO setting by the amount of error in the meter's reading. He should check dark, medium, and light conditions to verify the readings are linear. You could get accurate readings under one lighting condition but not another.

 

[Alan Edward Klein]

No doubt; a very true statement about the improtance to correctly use meters, especially when of different designs. But don't assume that only one is off as both could be off.

Case in point: I happily used Weston meters in a vertical position until I compared the readings with a "better and more modern" meter. Then discoverred that the corrrect position fo using a Weson meter is in the horizontal postiion, and a map of the acceptance pattern proved that my "incorrect reading" were because of that. Using that specific Weston meter became much more in agreement with the "modern meter" when the erroneous use was corrected. Truth be told, though... the minor difference in exposure recommendation was rather insignificant to practical image making.

I said in the earlier post that he should have the first calibrated to an accurate standard. Then calibrate the others to the first.