Why do Canon sensors tend to underexpose?

While using my grey card & metering in AV mode, the histogram on my two 7D's showed as being underexposed by 2/3rds of a stop. Does Canon deliberately calibrate their meters to behave like this, which seems strange as digital sensors tend to perform better when overexposing? I know that FEC can be used to compensate this, but why doesn't Canon get it to meter correctly while assembling their cameras?

Miki G wrote in post #15559155
While using my grey card & metering in AV mode, the histogram on my two 7D's showed as being underexposed by 2/3rds of a stop. Does Canon deliberately calibrate their meters to behave like this, which seems strange as digital sensors tend to perform better when overexposing? I know that FEC can be used to compensate this, but why doesn't Canon get it to meter correctly while assembling their cameras?

"metering in AV mode"

the "metering' is evaluative, spot, partial, center, etc.

When you say "FEC" are you referring to "Flash Exposure Compensation", as in you are shooting with a flash?

tonylong wrote in post #15559287
When you say "FEC" are you referring to "Flash Exposure Compensation", as in you are shooting with a flash?

I should have typed "Exposure compensation" instead of FEC, sorry. I'm shooting a grey card with natural light.

Regardless of which metering mode is being used, when I meter from a grey card, it is always showing on the histogram as being 2/3rds of a stop underexposed. I have also found this to be the case when editing shots previously taken. The histogram in the software also shows that the images are slightly underexposed.

Its true that Canon do meter to a slightly different "mid grey", but I'm surprised its as high as 2/3rds stop different.

How are you determining that "showing on the histogram as being 2/3rds of a stop underexposed"?

Here is a shot of a PhotoVision target per the camera meter...note the positions of the three spikes on the histogram for black with detail, midtone, and white with detail. My Canon 40D meter matches exactly what the Minolta incident hand held meter indicates, and it also happens to match exactly what my Minolta Spotmeter F reads when pointed to the same midtone grey.

The central spike is always 2/3rds of a stop to the left for some reason. I have to add 2/3rds when shooting to bring it to the central position so that my whites become white (without blowing the highlights).

Miki G wrote in post #15559619
The central spike is always 2/3rds of a stop to the left for some reason. I have to add 2/3rds when shooting to bring it to the central position so that my whites become white (without blowing the highlights).

Many discussions have been held in the past over the years about the fact that the 18% gray card target results in not a true 'midtone' density, but that the 12% gray card target does.

Nevertheless, the ISO standard definition is what all meters are calibrated to. Yet, the ISO standard for both reflected meters and for incident meters have equations which incorporate a manufacturer-chosen Constant -- wouldn't that really be a 'variable', if the value is not fixed?! -- which causes different brands of meters to not match each other, and the variation between the meters can be about 1/3 EV!

That brings us back to a truism often cited, that meters offer a SUGGESTION. and not an absolutely right measurement, given the fact that shutter speeds vary, aperture sizes vary, even the light transmission of different lenses will not all be the same, so that our cameras cannot necessarily be absolutely accurate in reproducing the settings shown via the dials.

Good post Wilt. You can have several bodies (same camera model) and many times the exposure readings won't be the same. Just learn the quirks your camera might have and adjust.

Do this test. Take a picture of a grey card in spot or partial metering mode. Take note of the settings. Put the camera in manual and expose for three stops higher EV. Look at the histogram. Is the spike at the right? Then its correct. The meter goes for 12.5%, exactly three stops below the sensor's maximum, perfectly capture a scene with a 6 stop range. Is the scene greater than 6? Add negative EC. Is it less? Add positive EC.

Miki G wrote in post #15559155
While using my grey card & metering in AV mode, the histogram on my two 7D's showed as being underexposed by 2/3rds of a stop. Does Canon deliberately calibrate their meters to behave like this, which seems strange as digital sensors tend to perform better when overexposing? I know that FEC can be used to compensate this, but why doesn't Canon get it to meter correctly while assembling their cameras?

The metering mode you're in may make a difference.
Evaluative means the camera is assuming it is looking at a scene and it's algorithms have made a calculation.
I would expect spot, partial and Centre weighted should all read the same from a uniformly lit card(Edge vignetting might affect Centre weighted).
Just a thought

My technique of shooting in manual mode based on ETTR while viewing the histogram in LV mode, after neutralizing all of the "shooting style" settings, does wonders for rendering all these types of questions totally moot.

Thanks to everyone for their suggestions and information. Very much appreciated.

Wilt hit the nail on the head.

kfreels wrote in post #15562373
Wilt hit the nail on the head.

Yeah, as much as I've heard people say that their camera "underexposes" or "overexposes" I haven't seen the "proof". What Wilt's post shows is what I've found by testing -- if you meter and expose something to be "centered"/medium, then the result will be slightly to the left on the histogram.

To me, the best way of testing this out is to fill your frame with a solidly-colored wall. Adjust your settings so that the meter "needle" is centered (in Manual, Av, Tv or P) then take the shot and check the histogram. Like I said the "spike" should be just slightly to the left (this is standard). If it's not, well, that means there is a problem and it could be fixed by Canon. When doing this filling the frame with a solid tone/color, the metering "mode" shouldn't matter, and in fact you could set the camera to Auto/Green Box and get the same results, assuming you don't have an in-camera setting that will "mess with" your shots.

When you are shooting a "scene", of course there will be a variety of tones, so that's where you learn to work with your metering and exposure methods.