Incident metering exposure different with flash than ambient

windpig wrote in post #17863563
I'm using a non hot shoe flash. By "exposure compensation" I mean I have to manually adjust exposure to compensate. I'm shooting fully manual.
In other words if I'm my meter says f11 at 1/160, ISO 200 when incident metering flash using my 5DIII and 135L, I have to shoot at F9, 1/160, ISO 200 for proper exposure. When I shoot with the same set up but metering using the over head tungsten light source my incident metering is spot on.

You've lost me now. I thought I understood from your clarification earlier what you were describing, but this new description suggests the exact opposite (i.e it's your flash shots that aren't metering correctly, rather than the ambient shots as you confirmed in post #6).

I don't think anyone's going to be able to explain what you're seeing until we know exactly which light sources are being metered, how the metering is being done (what mode the meter is in) and which sources are contributing to lighting the subject. Your latest comments make it sound as if you're metering only the ambient tungsten light, but taking a flash photo, which would only work if your flash wasn't actually illuminating your subject (at least not significantly relative to ambient), which would be a very unusual situation.

Can you isolate the problem to a flash metering problem or an ambient metering problem? Meter the ambient, shoot with ambient only. Then meter the flash (with overhead lights turned off, ideally) and shoot with only the flash. Which photo is correctly exposed?

CliveyBoy wrote in post #17863693
Ah! It was I who made too many assumptions

No worries

absplastic wrote in post #17863846
Can you isolate the problem to a flash metering problem or an ambient metering problem? Meter the ambient, shoot with ambient only. Then meter the flash (with overhead lights turned off, ideally) and shoot with only the flash. Which photo is correctly exposed?

Sorry that I've not been clear. Above is what I've been doing. Understand that my flash source and ambient source are not the same illumination, I'm adjusting SS and aperture to match what the meter suggests. Ambient is metering 2/3 under.

windpig wrote in post #17863563
Left Handed Brisket may be on to something with the angle of incidence. I think I'll set up the tungsten source lower and see how it meters. 2/3 stop is fairly significant.

glad what i mentioned made more sense than Gabby Johnson's authentic frontier gibberish. :P

I do a lot of copy shot type photos and work hard to keep light even across a 40" x 40" area. The board I use to hang the product is not real shiny, but being white and slightly reflective, it is great at showing hotspots caused by reflections in my light set up. It will do this even if the metering (with hand held meter) is perfect across the space.

Additionally, I have to hang my Color Checker Passport by a string and a clamp to take a test shot. This can put the Color Checker at angles other than parallel to the plane of the board/camera sensor. Despite the Color Checker swatches being flat (as in matte) if i hit an angle that reflects the lighting, the exposure on it will be completely off.

Not sure if the regular Color Checker has the same dimpled black plastic case of the Passport, but the amount and strength of the reflections caused by the plastic are usually a good indicator of a strong reflection.

windpig wrote in post #17864078
Ambient is metering 2/3 under.

OK cool. The next thing we need to know is how you're measuring this 2/3 of a stop. Do you mean that if you push the exposure of the ambient light shot +2/3 stop in post, the 18% grey square's RGB values on the color picker will match those from the flash shot? If so, when you're metering, are you putting your meter's lumisphere directly over that square, pointing towards the light source when metering?

absplastic wrote in post #17864550
OK cool. The next thing we need to know is how you're measuring this 2/3 of a stop. Do you mean that if you push the exposure of the ambient light shot +2/3 stop in post, the 18% grey square's RGB values on the color picker will match those from the flash shot? If so, when you're metering, are you putting your meter's lumisphere directly over that square, pointing towards the light source when metering?

I'm bracketing (shutter speed when testing ambient, aperture when testing flash).
Color checker is clamped to a light stand.
meter is directly over the square.

It looks like it comes down to color temperature.

Here is what Lee Varis says in his book "Skin" regarding compensating for different color temperatures of light when calibrating your camera/lens with your incident meter:
"For my 5DmkII, the compensation for direct sun is +2/3, for open shade it is ) (no compensation), and for tungsten it is +1."

Using my Gossen meter
I set up today using a 36" octa box about 36" from the xrite target.
The modeling light was used for the ambient test.

The flash metered spot on. The modeling light metered 2/3 under.


Just so we all are on the same page. I'm calibrating my light meter to match my camera/lens combinations.
This involves setting an exposure target, taking a light meter reading, then taking bracketed shots. You then import into Lightroom using the preset "zeroed", set WB. Then you hover over the 18% exposure patch and see which image is close to 50%, the white patch should be close to 90% but not over.

If your meter said F8, 1/160 at ISO 100, but the best image was at F10, 1/160 at ISO 100, then you make note of the negative exposure compensation needed. Or, if you've got one of the hoity toity meters, you can program that into it.

Anyway, thanks for the input.