How do we Quantify Exposure?

[Saladsamurai]

First, I am not sure if this should be in the Digital forum or not.

Second, sorry if this question is poorly posed.

I have been doing some reading and also watching some instructive DVDs about my Canon. I am at a point where we are learning about exposure and metering.

They describe the 'Evaluative Metering' function as something to the effect of "using data from many different zones within the frame to calculate what the proper exposure should be."

Now being an engineer, I would like a little more insight into this. What "data" is the camera using for these calculations (light intesity?)? And *what exactly* is it that is being calculated? How do we quantify exposure? That is, when a value is returned to the camera, what does that value represent? And does it have units?

Thanks! And sorry for being so nerdy :blushing:

 

[AlexColeman]

I guess, EV,
Lighting Condition EV100
Daylight
Light sand or snow in full or slightly hazy sunlight (distinct shadows)a 16
Typical scene in full or slightly hazy sunlight (distinct shadows)a, b 15
Typical scene in hazy sunlight (soft shadows) 14
Typical scene, cloudy bright (no shadows) 13
Typical scene, heavy overcast 12
Areas in open shade, clear sunlight 12
Outdoor, Natural light
Rainbows
Clear sky background 15
Cloudy sky background 14
Sunsets and skylines
Just before sunset 12–14
At sunset 12
Just after sunset 9–11
The Moon,c altitude > 40°
Full 15
Gibbous 14
Quarter 13
Crescent 12
Moonlight, Moon altitude > 40°
Full −3 to −2
Gibbous −4
Quarter −6
Aurora borealis and australis
Bright −4 to −3
Medium −6 to −5
Outdoor, Artificial Light
Neon and other bright signs 9–10
Night sports 9
Fires and burning buildings 9
Bright street scenes 8
Night street scenes and window displays 7–8
Night vehicle traffic 5
Fairs and amusement parks 7
Christmas tree lights 4–5
Floodlit buildings, monuments, and fountains 3–5
Distant views of lighted buildings 2

More just a representation of the values needed to achieve medium grey.

 

[Dwig]

...
What "data" is the camera using for these calculations (light intesity?)?

absolute light intensity, in lumens,and color.

And *what exactly* is it that is being calculated?

The pattern of intensity and color is compared to a database and calculations are made to determine the most likely lighting situation and from that the "best" exposure is determined. In some systems, the lens' focal length and focusing distance may also be parameters in the decision making calculations. Exactly what is in the database and exactly how the final determination is made are closely held trade secrets. In broad terms the camera makes its best guess what portion of the image is the subject and what absolute reflective characteristics it has to determine what brightness level to place it in the final image. It recognizes sky and skin tone be color along with who knows how many other aspects of an image.

How do we quantify exposure? That is, when a value is returned to the camera, what does that value represent? And does it have units?

This is a confused set of statements. "...returned to the camera" is meaningless as all this is taking place inside the camera; nothing from the outside is returned to it.

Technically, exposure would be quantified by the number of photos striking the light sensitive material. Since every spot (pixel in digital images) can potentially recieve a different quantity than any other there is not single quantity of exposure that can be specified.

The complex metering decisions balance the various portions of the image and decide on an f/stop and shutter speed that, with the chosen ISO, will result in the best compromise for the whole image. Depending on the metering system and the camera (e.g. modern digital camera) the ISO may be part of the metering system's decision.

To at least partially understand what evaluations must be made, I suggest that you read a good book on understanding exposure and then read "The Negative" and "The Print", both by Ansel Adams. The automated exposure evaluations that modern camera metering systems make are derived from the basic concepts of Adams' Zone System, though without Adams' additional integration of image specific film processing adjustments.

 

[Dwig]

I guess, EV,
Lighting Condition EV100 ...

"EV100" does not mean "EV100" but means "EV if, and only if, you are using ISO 100". The approviate EV value would be different if using any other ISO. A one stop change in ISO requires an exposure change of 1 EV unit. EV0 is defined as f/1.0 at 1 second or any other equivalent f/stop and shutter speed combination (e.g. f/1.4 @ 2 seconds, ...).