Exposure is not how dark or how light an image looks!

It depends on whether you are discussing at the image as a whole, or only parts of the image.

For instance, if you have a backlit model at the beach, the pixels over the model may be exposed exactly a desired, yet the background pixels may be blown out.

One can also look at only the pixels behind green Bayer Pattern pixels. One could even limit the discussion to only pixels on the left or right side of the image.

So yes, depending on the context of the discussion, one might talk about the image as a whole, or separate out the discussion by classes of pixel.

Dividing the pixels by the corresponding Bayer Pattern color does make sense. As these are usually processed with different parameters (for instance the White Balance setting will affect the relative processing of the different color classes of pixels).

Um, Dynamic Range?

Also, knowing that the anti-film bias is alive and well at this place.

Copy/paste my sentence where you claim I left out "per unit area".

For me, there aren't too many variables to set the optimal exposure as defined earlier.

I posted how I and many others set optimal exposure in my "Why are my photos noisy" thread in this forum.

"Optimal exposure", in a sense, may demand controlling light spectrum or using colour filters on the lens. ;)
"Optimal" changes with the circumstances.

If you refer it to me you are mistaken. Not only I shoot and process a lot of MF and LF film myself, I repair and adjust film cameras for my friends. People of my age spent 30+ years shooting film before it became possible to purchase a digital camera.

It depends on the goals of the photographer for the final image.

Mine are pretty simple - get as much light as possible onto the sensor while the shutter is open and within my dof and blur requirements without clipping important highlights.

Try a magenta filter (CC40M) on the lens when shooting at daylight, you will get more red and blue light onto the sensor.

If I need more red and blue light to get the final image I want, I'll keep that in mind.

Here goes optimal exposure ;)

The CC filter trick is a way to get a higher SNR in the final image. I wouldn't recommend it for beginners, though. You may have to make your own color profiles to get the best results. And, of course, the exposures will be longer all else equal.

The definition of optimal exposure will vary greatly among photographers depending on their needs and goals.

There is no one size fits all definition for optimal exposure.

I just simply post what it means for me.

YMMV ;-)

Exposure - amount of light striking the sensor per unit area while the shutter is open.

Optimal exposure - the maximum exposure within my dof and blur requirements without clipping important highlights.

The only point I'm trying to make here is that "optimal" is a peremptory word ;) "Optimized, given the circumstances", if one asks me...

when do you calculate just exposure values only before taking a photograph. or shood i use asa 64 film to shoot a concert tonight ;-)

Sometimes the lighting is unbalanced, often under artificial light. Whenever WB deviates away from around 5000K either the red channel or the blue channel will start to be relatively underexposed. Also, if you are trying to get the background not to blow out in a backlit scenario, then even in broad daylight you could have both the red and blue channels relatively underexposed; conversely, maybe only one or two of the channels is overexposed if you are going for an optimal exposure of the subject in a backlit scene.

I have hardly used my CC40M filter since shooting primarily with a Nikon D300. Nowadays, it's my preferred approach to slowing down my shutter speed by a stop when that's something I want to do, so that filter is still in my landscape kit.

At 5000K with a neutral subject, both the red and blue raw channels will be underexposed wrt to the two green channels, with most Bayer CFA cameras.

Good point. That's why I wrote "relatively;" otherwise I'm "underexposing" practically every time I don't use my CC40M filter, and even more so every time I turn on Auto-ISO.