Light metering in digital camera

[rahuldg11]

Understanding Camera Metering and Exposure

In this site, i could not understand the "18% luminance". If anybody knows about this and how this is related to over exposure and under exposure, please reply.

Thank you
Rahul

[kencaleno]

It refers to the reflectance brightness % from a, mid-grey area of a scene.

I believe a basic understanding of The “Zone System” will be of benefit here, so I have included my intro to this:

Edward Weston/ Ansel Adams developed the “Zone System”
To control exposure and contrast for black and white film photography
And To pre-visualize finished image after post processing in the darkroom,

There are11 zones black to white-each zone represent 1 EV (exposure value) stop:

0 Total black

I Almost black-no detail

II Dark grey-black -very dark detail in shadows

III Very dark grey-shadow detail-dark animal fur

IV Medium dark grey-dark green foliage, shadow side of Caucasian face
Dark stone-landscape shadow-dark skin-details plainly visible

V Medium grey, (18% reflectance/luminance)-green grass-grey stone-dry tarmac-excellent detail

VI Rich mid-tone grey-Caucasian skin in sunlight-light stone and sand

VII Bright light grey-white with texture-silver hair

VIII Almost white-Delicate texture-no detail-reflected highlights on Caucasian skin

IX Nearly pure white

X Absolute white


Each zone is a tone between black and white

Each zone is either half or twice the exposure of it’s neighbour, depending on whether you are going up or down, For example Zone III is 2 stops darker than zone V, and zone VII is 2 stops lighter than zone V-As you move your exposure one stop (ev) say, from f 8 to f 11,or from 1/60sec to 1/125sec, you will have moved the exposure one zone
Although developed for black and white film, it can still be useful for digital imaging, and once you get used to using it, your resulting images will be superior to anything you have done before..

It can be difficult to understand that when working in color, that black and white and tones of grey can apply. Your camera’s meter sees in black, white and tones of grey in between.

So to help you understand:

Your camera’s built-in meter will average everything it sees as mid-grey (18% or 13%-whichever you believe to be correct) > ZONE V!
The camera’s light meter will measure and average everything it’s pointed at

So it becomes necessary to switch to spot metering-a spot meter will measure only a very small angle (1-2 Degrees)-spot metering will let the camera know the most important part of the scene in front of it.

Ok. Let’s assume you have a Caucasian model against a relatively dark background-So you assess the scene before you, and decide that the model’s face is the most important part of the scene.

Usually Caucasian skin registers at Zone VI or Zone VII, (Even though we are looking at a color scene, the camera only sees light and dark.) When we spot meter for model’s face, this will register on camera as Zone V

We have decided to rate the model’s skin at Zone VII, and this means we must open up 2 EV stops to achieve this:
If we are in manual shooting mode, this will require us to either a) Decrease the shutter speed from, say 1/250th sec to 1/60th sec, or b) 0pen up aperture from, say F8 to F4.
If you are in auto mode/scene exposure mode, set exposure compensation to +2

Always do a final check using your histogram: In some situations, by placing the subject at a certain zone, could result in other areas of the image being under or over exposed.

From this you can see that the camera’s Dynamic range is from zone III to zone, VII with Zone V being middle grey.

Regards, Ken

[inkista]

Here's a simpler way of looking at it.

Your camera's sensor (and your computer monitor) can represent a bunch of values whose lightness "value" varies from black to white.

The much-vaunted 18% reflectance is also known as "middle gray"--it's the center of that black-to-white scale. So, say, if a pixel's "darkness/lightness" value can be set from 0-255, with 0 being black, and 255 being white, then middle gray would be 127.5.

How this all affects metering is that your camera's automatic exposure (AE) system is trying to do something similar to the Zone System for you, only much more simplistically (and stupidly). It does something similar to averaging all the light/dark values over the sensor that it sees, and then it sets the average value to middle gray, and sets the exposure to make sure that's where the average value sits.

This is great if the scene has an even distribution of light to dark values, as most scenes do.

But if, say you're taking a shot of a snow scene, where most of your values are closer to white, an average of something very light/nearly white would be set to middle grey, and everything's shifted towards underexposure. Conversely, if you're shooting a night scene, and your average something very dark/nearly black, and that average is set to middle grey, you get overexposure.

This is why we like having exposure compensation, full Manual mode, and histograms (which I personally find easier to use than the Zone System). The AE system can get it very very wrong.

[rahuldg11]

Thanks for both the replies.I am only a beginner and I was trying to know more about exposure. I learnt a lot from both the replies but I have some questions about what you said.

1. Does the zone system apply for colour photographs also? If so, then how does the sensor average all colours to black and white colours? Specially when I am taking a photo with many colours.

2. Is the colour "middle gray" the factor? Or, is it the 18% reflectance?

3. What do you mean by moving the exposure one stop? Is it increasing or decreasing the f-stop only or the shutter speed only or both?

4. Why should darkness/lightness values be set from 0-255?

5. What does over exposure and under exposure actually mean? I am trying to explain it. Over exposure means that the sensor has been exposed to more light than that is required and under exposure means that sensor has been exposed to less light.
So, if I am shooting a scene with lot of light, like that of a region covered with snow, it should be over exposed, isn't it? And the same thing for a night scene. Is it that while averaging the colours in the scene to middle gray, then the scene become over or under exposed?

I would be grateful if somebody could give me the replies to my questions

Thanking you
Rahul

[inkista]

1. Does the zone system apply for colour photographs also?
No. There are people who will tell you that it does, but really, it doesn't.

2. Is the colour "middle gray" the factor? Or, is it the 18% reflectance?
They're the same thing. I know someone who knows whereof he speaks who insists it's actually 14% reflectance. But that's a whole 'nother can of worms.

3. What do you mean by moving the exposure one stop? Is it increasing or decreasing the f-stop only or the shutter speed only or both?
A stop is a measurement of exposure. It's also called an EV. It's essentially a doubling/halving of the light, irregardless of whether it comes from the iso, aperture, or shutter speed. This is how we swap the values and make tradeoffs between the settings. It's the unit they can all speak.

For iso, doubling the iso (e.g., iso 400 -> iso 800) is increasing by a stop.
For shutter speed, halving the shutter speed (e.g., 1/200s -> 1/100s) is increasing by a stop.
For aperture, going one stop down on the f-number scale (e.g., f/4 -> f/2.8) is increasing by one stop.

Aperture is the most difficult to learn, as the f-number scale is not linear (specifically, it's a square-root of 2 scale), and cameras by default are set to 1/3-stop increments. But the full stop scale runs:

f/1 -> f/1.4 -> f/2 -> f/2.8 -> f/4 -> f/5.6 -> f/8 -> f/11 -> f/16 -> f/22 -> f/32

Essentially what's doubling between the steps is the square of the f-number. This is because the f-number is a ratio of the aperture opening diameter, but the amount of light is proportional to the area of the opening (remember pi-r squared?).

4. Why should darkness/lightness values be set from 0-255?
That was an arbitrary choice on my part. I do the computer geek thing a lot; I always tend to think in eight-bit terms by default. What the values actually are and how the "average" is arrived at is all black-box proprietary stuff.

5. What does over exposure and under exposure actually mean?
Put simply, over exposure is when the image is too light. Underexposure is when the image is too dark. It can be subjective.

So, if I am shooting a scene with lot of light, like that of a region covered with snow, it should be over exposed, isn't it?
No, if you're looking at what the AE system is doing. I was explaining what the AE system was doing. More light is coming in than the sensor thinks is "correct" for the scene (remember, it can't tell you're taking a picture of snow), so it cuts down the light and underexposes. Vice versa for the night scene. Picking middle gray as the 'ideal' exposure level is what's at fault. Sometimes you want to be lighter or darker than that.

Is it that while averaging the colours in the scene to middle gray, then the scene become over or under exposed?
No. It's when the AE system sets the exposure to make sure the average value corresponds to middle gray that the over/under exposure occurs.

[rahuldg11]

Thanks for the answers. They were very informative. I did not know that much about EV and aperture. Now, i have to try to apply the knowledge to my photography.

one more clarification - suppose i take a photograph at night or in very low light. I take it in auto mode. Then wont the photo be dark, without any details visible? why would that happen?
Since there is low light, the AE system should overexpose the photo, so that the average colour is middle gray. am i right?
then what do we call the photograph? it should be called under exposed according to the viewer but if we are talking about the AE system, then we should say that the AE system has overexposed the photo.
Hopefully I have been able to explain what i want to say. please tell me whether i have understood the thing correctly

[Jim Poor]

Check out part of this video for more info on how things like snow effect exposure:

Episode 30 | D-Town TV

[Jim Bryant]

The things you learn here are just amazing.

[fletch]

Originally Posted by rahuldg11

suppose i take a photograph at night or in very low light. I take it in auto mode. Then wont the photo be dark, without any details visible?

No. If you let the camera do everything for you the exposure will average out at middle grey. It may appear very dark in some places if there are other areas that are very bright (such as bright lights). It will always average out at middle grey.

If you take a picture of a black wall the camera will make it grey, if you take a picture of a white wall the camera will make it grey.

Originally Posted by rahuldg11

why would that happen?

It could happen if you reach the limit of the settings your camera will pick in auto mode.

[Jim Bryant]

You don't take the picture at night on auto settings. You set the camera up on a tripod, shutter speed on manuel (maybe starting at 1/4 second), fstop on 5.6, take a picture and then if too dark, adjusts your shutter speed to a lower setting and keep doing that until the exposure is to your liking.