Metering Question

If I set up one strobe at a 45 degree angle to a Colorchecker Passport, point the flash meter (Sekonic L-358 ) at the strobe, strobe fires and I'm metering 1/3 stop overexposed.

Same strobe. Same position. Same metering technique. However I'm now just using the modeling lamp and changing the L-358 settings for an ambient measurement. Now there is no overexposure -- actually, perfect exposure.

In both cases, I imported the files to Capture One to evaluate exposure.

Does this make any sense? Can a flash meter be more/less accurate with flash vs. ambient readings?

No one?

Perhaps you need to restate your question because as I read it, it doesnt make sense to me.

Metering with an incident dome you have to aim the dome at the camera in the subject position to get the camera reading. If you aim the dome at the light you are reading the light that is hitting the subject from the light source.

If you are using the Passport, another way to check your exposure and get maximum dynamic range in a raw image is to open the aperture until the white square is blinking in the info histogram. The stop down until you are 1/3 stop below the aperture with the blinkies.

Since the white square is intended to be 238 you will now have a raw file exposed to the right. with about a stop and a half headroom in the highlights. And virtually no noise in the shadows, even at high ISO setting.

As for your question about reading ambient, it probably has to do with the fact that there was enough "other" light along with the modeling light to provide a proper exposure.

Metering for the strobe output is independent from metering by the constant output of the modelling lamp. They are not inherently linked at all.

I've tried to reword my post to clarify my question (conundrum).

Alex_c70 wrote in post #9026839
I've tried to reword my post to clarify my question (conundrum).

Same answer as post #5. It sounds like situation 2 is ambient only (modelling light), whereas situation 1 is flash + ambient. Light is like water...situation 2 is pouring water in from one source, situation 1 is pouring water in from two sources. Situation 1 'overflows' while situation 2 is 'just enough'. Nothing puzzling about that, unless I am still not understanding your puzzlement.

What is your indication that the flash exposure is overexposed? And I take it that your shutter speed and aperture are adjusted when you take the shot without the flash.. Still not clear on what you mean.

How did you evaluate exposure in C1?
Presuming you have metered both accurately and adjusted the camera settings (which adds a variable)
Then are you reading the gray scale squares along the bottom of the Passport?
If so, are you confirming the RGB readings based on the color space you are using in C1?
Remember, Strobe the aperture is the only camera variable. With Ambient shutter is an added variable.
So, shutter may be 1/3 stop fast or slow, or aperture may be 1/3 off in one direction and shutter 1/3 off in the other direction. Result is "good" ambient exposure and "hot" strobe exposure caused by camera not mis-read meter.

And then there is the probability that the meter is not spot on either. So now we have three veriables.

And since the modeling light is 2700K and the strobe is ~5600K did you do a white balance adjustment before or after doing the exposure check? That can impact apparent exposure in C1 and other raw processing software.

And finally, a 1/3 variation could be you standing is such a manner to have impacted the reading based on what you are wearing. Or, just slight difference in the angle of the meter dome relative to the light source.

Now we're up to about 5 or 6 variables.

I would consider 1/3 variation in two meter reading to be within the normal margin of error. That's why I tend to take three readings in succession and average them.

Then use the histogram to check for placement of the white square on the passport for final confirmation that I am not going to blow highlights while keeping maximum SNR headroom in the shadows.

Are you trying to say that you make an exposure by setting your camera based on the incident flash meter reading and the image you capture is overexposed by 1/3 stop, but when you make an incident light reading with just the ambient light and set the camera accordingly, you get an image with the proper exposure? The expression "metering 1/3 stop overexposed" is what is throwing me.

My interpretation of the question is;
First exposure reading was in flash mode. Got a reading set the camera and made an exposure. Result was 1/3 stop over exposed based on C1 evaluation.
Second exposure reading was in ambient mode. Got a reading, set the camera and made an exposure. Result was properly exposed based on C1 evaluation.
Same meter position relative to light and subject (Passport) for both readings.

My thoughts about what the potential causes for the variance are above.

Wilt wrote in post #9026915
Same answer as post #5. It sounds like situation 2 is ambient only (modelling light), whereas situation 1 is flash + ambient. Light is like water...situation 2 is pouring water in from one source, situation 1 is pouring water in from two sources. Situation 1 'overflows' while situation 2 is 'just enough'. Nothing puzzling about that, unless I am still not understanding your puzzlement.

Correct, situation 2 is ambient only (f 8.0, 1/2 second, ISO 100).

Situation 1, though, is just flash (f 8.0, 1/125 second, ISO 100). Modeling lamp is on, but doesn't register at 1/125.

So my meter seems to read ambient accurately, but flash as being 1/3 stop over exposed.

gonzogolf wrote in post #9026918
What is your indication that the flash exposure is overexposed? And I take it that your shutter speed and aperture are adjusted when you take the shot without the flash.. Still not clear on what you mean.

Using the middle gray patch, I get RGB values of around 140. When I bring the exposure down 1/3 stop, RGB values are almost exactly 128.

The shutter speed and aperture settings were:

Ambient: f 8.0, 1/2 second.
Flash: f 8.0, 1/125 second.

Jim M wrote in post #9027916
Are you trying to say that you make an exposure by setting your camera based on the incident flash meter reading and the image you capture is overexposed by 1/3 stop, but when you make an incident light reading with just the ambient light and set the camera accordingly, you get an image with the proper exposure? The expression "metering 1/3 stop overexposed" is what is throwing me.

Exactly. You expressed it better than I did.

Using the middle gray patch, and exposing the image so the RGB values are as close to 128 as possible

Yep.

I'm using sRGB.

I think this may be it! I wasn't considering the shutter as an additional variable.

I was thinking that if the meter's not spot on, that the degree of error would be consistent...but the shutter!

I did a white balance adjustment, in both cases, before exposure check.

I

This sounds like a very good idea!

As does this. Thank you.