why the camera has to assume 18% grey?

hahastar · Aug 23, 2006

If the built-in light meter can sense the intensity of the light(whatever taht means), then why does it has to assume all the objects it 'sees' are middle grey? Why can't the camera figure out the exposure based on the intensity of the light it received? e.g. if the intensity of the light is very high, then it is white, if the intensity is low, then its black. As long as it clear defines high and low.

Thanks.

darin3200 · Aug 23, 2006

How does the camera know if it's metering off of something black in a bright situation or something white in a dark room?

hahastar · Aug 23, 2006

If it is a dark object in a white background, and we meter the dark object. the camera should see 'dark' as dark. Yes, the white background will be a little bit dark. but at least we got the dark. If the camera see 'dark' as gray, then we will end up have a dull-grey-dark, and a 'dull-grey-white'
Am I thinking this in a wrong way?

usayit · Aug 24, 2006

Everything the camera sees in terms of intensity is relative. Black is black because it reflects little light as compared to the source and the objects around it. White is white because it reflects alot. In a simple reflective meter, the camera will require a base line in order to determine a proper exposure of relative intensities of reflected light. Furthermore, the negative itself has a range from which it can gather "data". Its up to the photographer to determine how to take advantage of that range. In a high contrast situation like a snowy forest, shall we meter to capture the highlights or meter to capture the detail in the shadows.

If you think about it, your eye is the most complicated optical camera in the world. It does all of this much better and gathers much more data than any digital or film camera could ever. It does this automatically without us even a notice.

On the other hand, newer sophisticated cameras today have much more intelligent metering compared to the simple meters of 20 years ago. They take several intensity readings from portions of the viewing field and calculate what it should think is the best exposure based on formulas and matrices. This is much like the behavior you originally posted.

If you are concerned about accurate exposures, try out a hand held meter that can measure ambient (as opposed to reflective) light. If you need to guess something... remember the sunny-16 rule and compensate from there. I shoot a lot with older cameras without a meter... I've either used a relatively new Sekonic flashmate meter or a 20+ year old selenium sekonic meter which miraculously still works accurately. Either way... I have learned a bookful using a handheld and guessing exposures