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5D: In-Camera Meter Calibration?

Amit Chugh

New member
Hi,

I am an amateur and this is my first DSLR. To get better at the exposure, I have been reading Nature Photography Field Guide By John Shaw's. In this book John talks about testing the in-camera meter to make sure that the camera's metering should match the standard f/16 Sunny Rule when using SPOT metering. He then states that once the in-camera meter is calibrated, certain standard rules and conditions can be used to get a better exposure control on the final image.

So, I set my camera to Av, ISO to 100, and f-stop to f/16. Then I tried to meter the roof of an apartment which appears to be middle grey on a bright sunny day, when the roof was front illuminated by the sun. My in-camera meter gave me 1/100s. Which means it agrees with the Sunny 16 rule.

Here are the questions I have with the above knowledge now given about this test:

Q1: It has been stated at DPREVIEW's 5D review that 5D is about 1/3 stop more sensitive i.e. at ISO100, the camera is really running the chip at ISO125. Since the internal ISO is different than what I set on the camera's LCD does this mean that the in-camera metering is wrong because at ISO125 the sunny 16 rule should give me 1/125s instead of 1/100s?

Q2: I also own Sekonic L558 flash meter. When I use this meter to meter the same roof at ISO100, f/16 with 1 degree SPOT reflected light, the meter suggests me 1/125s. If I set the ISO on this meter to ISO125, I got 1/160s reading. I know this meter is calibrated for sure. So, I took one shot of the scene at 1/100s (camera suggested meter value) and the same shot at 1/160s (Flash meter suggested value). Did anyone else did any kind of field test like this? What is your conclusion from this test? I did find that the in-camera metering was better i.e. 1/100s did not need any exposure adjustment in RAW (I shot RAW+Small JPEG and used ACR 3.4).

In short, do you think the in-camera meter in 5D is accurate?

Thanks in advance.
-Amit
 
D

Doug Kerr

Guest
Hi, Amit,

This area is more complicated that one might think.

A short answer is this:

1. The internal meter is generally calibrated consistently with the applicable ISO standrds. That is, the aperture/shutter speed combination it chooses (indicates) is conmsistent with what one would expect for a meter calibrated under ISO standards.

2. In fact, the sensitivity of the sensor, when set to "ISO 100", would probably be rated in a test under the applicable ISO standard at about ISO 125-135

3. The implication of this is that the camera in metered operation will provide an expsure on the sensor about 0.4 stop "hot" compared to what is contemplated by the ISO standards.

4. The ISO standards, taken together, provide an expsure result that, in effect, allows a 1/2 stop headroom to prevent blowing out of the highlights in the case that the scene being metered has a substantially lower average reflectance than is contemplated in the meter calibration.

From here on this is just my conjecture.

5. I suspect that Canon feels, because of the fact that the Evaluative metering system is smarter than the metering system contemplated by the ISO standard (which is just based on the detemination oif average scene luminace), it does a better job of outguessing the highlights. Therefore the 1/2 stop headroom bulit into the ISO standards isn't needed. So evidently they decided to "burn" it.

6. They could have done this by adopting a hotter "calibration" of the exposure metering system than called for by the ISO stadanrd. But if they did, then a photographer who used an external exposure meter (calibrated in accordance with the ISO standard) would get a different exposure recomendation than for the metering system in the camera. This would have caused a lot of consternation.

7. Therfore, apparently, they decided to burn the headroom by underating the ISO sensitivity of the sensor system.

So to answer your question: The expsure meter in the camera is probably "accurate". But that's not the whole story.

If you want your external expsure meter to recommen an expsure that will give a result consistent with what you get with the in-camera metering, you need to set the ISO sensitivity on teh meter to teh face vaolue set on teh camera - oif you set the camera to ISO 100, set teh externqal meter to ISO 100.

If you want some more information on who this works, you might want to look at my tutorial article, "Exposure Meter Calibration", available here:

http://doug.kerr.home.att.net/pumpkin/index.htm#ExpsureMeterCalibration
 
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