# Can help me understand metering features?



## eninja (May 26, 2015)

1dx - 100,000 pixel RGB metering sensor
my 6d - iFCL metering and 63 zone dual layer metering sensor
my 70d - i can't find writen on feature list

All I know, it can be said that the output of metering is the histogram.
Just a measure of brightness of scene. I don't understand why you need an advance metering system,
why it is emphasize as a good feature. I mean its just averages the maximum brightness, 100,000 pixel RGB wont be much any difference than 63 zone, is it? I mean I can still nail metering with my 6D with a good error margin. So what's with the 100,000 RGB? Does it really play a big factor on AF process? How does it contribute to AF algorithm.

I hope my questions are right.

In my thougths, 18megapixel sensor area is covered with another 100k bigger pixel sensor.
I imagine a 100k flat LCD, on it, displays a pixelated moving image (which is the scene).
and this image is "intelligently" process to detect motion, face and etc.
Is it similar to like this? or only my illusion. 

Thanks.


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## meywd (May 26, 2015)

no expert on the subject, but a better metering sensor/system benefit is in situations where you don't want to go full manual, like when you are shooting BIF, if the bird is white the exposure need to be less than if the bird is black, if bird is in the sun should be different than the bird in shade, ofc it will be different even with a T3i but is it accurate, or as accurate as the 1DX.


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## neuroanatomist (May 26, 2015)

I would suggest that parts of your thinking are not correct. The output of metering is not the histogram, it is the exposure; the histogram is a measure of scene luminance at that exposure. It's also not correct to say that metering simply averages the brightness across the scene in all cases. That's true in average metering mode, for center-weighted average it's sort of true, and for partial and spot its averaging across a smaller area. But for evaluative metering (the default mode), the metering system compares the scene against a 'library' of scenes in an attempt to decide what you're shooting, biases that based on the selected AF point(s), and bases the exposure on that information. In that mode, it seems reasonable that more data input would drive a better output decision by the metering system.

Another point is that the RGB metering sensor is detecting the full spectrum, whereas the iFCL system detects mainly green, with some blue sensitivity that helps it compensate for fluorescent illumination. When people have two-color vision, we call them colorblind. That's a misnomer, but nevertheless a metering system that sees the full spectrum will be more accurate across a diverse array of lighting conditions.

As you indicate, the other benefit of the high density RGB sensor is to drive focus tracking based on color and shape, which the 1D X and 7DII can do, but the 5DIII cannot.


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## Sporgon (May 26, 2015)

neuroanatomist said:


> Another point is that the RGB metering sensor is detecting the full spectrum, whereas the iFCL system detects mainly green, with some blue sensitivity that helps it compensate for fluorescent illumination.



Ha ! I didn't know that. Maybe that explains why the 6D (for me at any rate) seems to underexposed some scenes with blue - blue flowers for instance. Personally I prefer the way the 5DII handles it, probably because I've got used to the way the meter handles things. 

I seem to remember that when the 6D was introduced there were complains of metering 'problems'.


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## neuroanatomist (May 26, 2015)

Sporgon said:


> neuroanatomist said:
> 
> 
> > Another point is that the RGB metering sensor is detecting the full spectrum, whereas the iFCL system detects mainly green, with some blue sensitivity that helps it compensate for fluorescent illumination.
> ...



Yep - the 5DII has a 35-zone one-color metering sensor, the 7D introduced the 63-zone iFCL metering sensor which is 'dual layer' (one for red/green = mostly green, one for blue/green).


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## eninja (May 26, 2015)

It took me time to digest info.
You guys are saying, these stuff - RGB, iFCL, dual layer sensor, - is primarily use on auto mode, to set the "exposure" according to the user desired exposure (exposure compensation)??

The more accurate the metering feature, the more accurate the "exposure" that colors won't be clip (missed to process), for standard exposure (exposure compesation = 0).

You can also say that, if my metering sensor is not to advance, then I can just lower EC? to prevent clipping, thats the thing it was made to do in the first place, to have right exposure, prevent clipping.

So for two different metering sensor, on auto mode, both may show the same exposure level on the camera.
but when photo finally taken, we may see that the camera with less advance metering sensor,
some of the colors where clip (base from rgb histogram), but with more advance metering no clipping as desired.


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## Marsu42 (May 26, 2015)

eninja said:


> Just a measure of brightness of scene. I don't understand why you need an advance metering system,



You don't on mirrorless or with mirror up - for example Magic Lantern has an "auto ettr" feature that gives you 100% spot-on correct metering in live view.

With the mirror down, traditional dslrs have to duplicate the metering though as the sensor is blind. It's not only the hardware, the software has to match it and get a lot of testing. It has to decide (without color data unless you have rgb metering) what to clip if the scene's dynamic range exceeds the sensor. And phase metering is always done wide open, so vignetting has to be calculated if the actual shot is taken stopped down. 

My understanding from reading reviews is that the different dslr lines get different optimization, for example "amateur" cameras rather play it safe while "pro" cameras put more burden on the operator - but I could be wrong. 



eninja said:


> I mean I can still nail metering with my 6D with a good error margin.



I find the 6d eval metering very dodgy (in comparison to my 60d), I imagine Canon didn't put a lot of work into optimizing it for the various lenses and standard scene types. With my 17-40L this gets really hilarious, it has an error margin of +-1ev and usually needs +1 to +1.5 ec ... until it doesn't and you overexpose severely on occasion. Or it could be just that with an uwa, you always shoot a lot of different lighting and the camera has a tougher job to eval it.

The other problem with the 6d is that eval metering has a very strong tie to the active af spot, which usually is the center as the outer points are so mediocre. So make sure to always use expo lock, or try center-weighted metering if eval is still too unreliable for you.


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## Jack Douglas (May 27, 2015)

Maybe a little off this topic but with my newly acquired 1D4 I was getting some weird exposures for BIF, like one correct followed by one very over exposed, within say 1/10 of a second. Is this likely tied to the timing of when the exposure is set or some other option?

Also, I'd appreciate more insightful comments about the differences in cameras and how the whole process is accomplished, along the lines of the original question. Another example confusing me - what does the 1D4 do with exposure when you have it tied to the spot focus point and you have the option of surrounding focus points being activated?

Jack


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## CanadianInvestor (May 27, 2015)

neuroanatomist said:


> I would suggest that parts of your thinking are not correct.



Man, just reading neuro's lucid explanation set things right for me. Alles klar.


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