What defines perfect exposure?!?!?

[bigben6]

What I am wondering is more theoretical I guess. When the camera sees a scene it gives a shutter, aperture, and ISO, how does it know those 3 work together. Is there a certain lumens count it needs? color temp? What standard does a camera or a meter use to "know" your exposure is right?

[i speak in math]

Originally Posted by bigben6

What I am wondering is more theoretical I guess. When the camera sees a scene it gives a shutter, aperture, and ISO, how does it know those 3 work together. Is there a certain lumens count it needs? color temp? What standard does a camera or a meter use to "know" your exposure is right?

It averages the scene to 18% grey

[kirbinster]

In less technical terms the perfect exposure is when you capture the full range of light without over-exposing the highlights or clipping the shadows. Unfortunately this is not always possible, and you as a photographer have to make some choices as what is most imporant in the frame.

[bigben6]

Originally Posted by kirbinster

In less technical terms the perfect exposure is when you capture the full range of light without over-exposing the highlights or clipping the shadows. Unfortunately this is not always possible, and you as a photographer have to make some choices as what is most imporant in the frame.

Thats a judgment call, i do get that part! Im looking at what the camera is thinking, the 18% grey is closer to what i want. Is it that simple? It would seem odd because if i take a picture of a blue sky... how does it avg it to grey?!?!

EDIT ok this site seems to explain it well albeit not vary thoroughly:
http://www.cambridgeincolour.com/tut...a-metering.htm

It basically says that

For this reason, in-camera metering is standardized based on the luminance of light which would be reflected from an object appearing as middle gray....
AND

...each camera has a default somewhere in the middle gray tones (~10-18% reflectance)

So if i am understanding, the camera looks at a scene, and then decides IF this was in a grey scale how can i average it to my pre determined standard (~18% grey)

[i speak in math]

Originally Posted by bigben6

Thats judgmental, again im looking at what the camera is thinking, the 18% grey is closer to what i want. Is it that simple? It would seem odd because if i take a picture of a blue sky... how does it avg it to grey?!?!

The sensor doesn't see color, that is interpretted. It looks at the luminance. So primary red, blue and green all appear the same to the sensor.

If you set your camera on a tripod so that all it can see is a white piece of paper filling the viewfinder, the camera will meter so that its 18% grey. Consequently, it will appear underexposed and greyish, not white. If you replace it with a black piece of paper, it will again average it to 18% grey. The image will look nearly identical to the white paper.

If you take a picture of all bright sky with no ground, the camera will likely underexpose it a bit. If you add more and more dark ground to the image, the sky will get brighter and brighter until its blown out -- but the ground will look better and better.

[RLucas]

Originally Posted by bigben6

Thats judgmental, again im looking at what the camera is thinking, the 18% grey is closer to what i want. Is it that simple? It would seem odd because if i take a picture of a blue sky... how does it avg it to grey?!?!

Meter off a blue sky, set the meter in the middle, take the shot and convert it to grayscale then you will see what he is talking about. Do the same with the primaries bright like white and do the same thing. Then try it with darks. It will try to average everything for the midtones.
Kirb is also correct,. Not judgemental at all. It is up to you, the photographer as to what you want to convey. If you want the highlights of an image priority, then expose for it. Same goes for every other value of the scene. If you take a picture of a snow scene, you usually don't want gray looking snow, but if a camera is left to it's own to expose, you would end up with exactly that. It is up to you to "overexpose" to compensate for it.
Hope that makes sense?

[bigben6]

Originally Posted by i speak in math

The sensor doesn't see color, that is interpretted. It looks at the luminance. So primary red, blue and green all appear the same to the sensor.
Did not know that, how does it give me a histogram then? is this a different


If you set your camera on a tripod so that all it can see is a white piece of paper filling the viewfinder, the camera will meter so that its 18% grey. Consequently, it will appear underexposed and greyish, not white. If you replace it with a black piece of paper, it will again average it to 18% grey. The image will look nearly identical to the white paper. I'm gonna try this and hold you too it

If you take a picture of all bright sky with no ground, the camera will likely underexpose it a bit. If you add more and more dark ground to the image, the sky will get brighter and brighter until its blown out -- but the ground will look better and better.

Good points, thanks, im gonna go experiment!

All i meant was that what she is talking about is a judgment call for each photographer just like you said, no knocking her at all! She is right you have ot take creative rights into your own hands, i agree in full! I just meant that its not happening in the camera.

I think i have a good grasp on it now, though i wanna try the white/black peice of paper thing!

So new question then, rather than adjust my white balance for shooting in bright snow, couldent i just say.... over expose to get the same effect?!?!?!

[i speak in math]

Originally Posted by bigben6

Good points, thanks, im gonna go experiment!

Remember that histogram you are seeing on camera is for the interpretted jpeg, not the raw data. The camera has to figure out the colors, develop the image using the selected jpeg settings and then break that down into a histogram

[RLucas]

Originally Posted by bigben6

Good points, thanks, im gonna go experiment!

The histogram is also based off the luminance for each channel. R G B. Not hue.

[ravncat]

Originally Posted by RLucas

The histogram is also based off the luminance for each channel. R G B. Not hue.

Though many cameras only base the histogram from just the G channel, others are weighted with the R and B channels - it's better to use RGB histograms if you can.