Easy way to understand middle gray (18%) and over-riding automatic exposure?

Started Nov 14, 2012 | Discussions thread
Barrie Davis
Barrie Davis Forum Pro • Posts: 21,460
Exposure Compensation: Getting started

dennissaid wrote:

Hi guys, I'm doing a short course on the Canon 5d mark 2, and I'm reading up on situations where you override automatic exposures... two examples they used was a SNOWY environment where it is brighter than middle gray and a DARK cat where it's darker than middle gray.

They talked about adjusting exposure manually to compensate for this but it's not clicking for me... can anybody point me in the right direction to learn this??

In auto modes the exposure errors that arise because a metering target is lighter or darker than 18% tone are overcome by using the Exposure Compensation scale...(somethines shortened to EC)

It is important to remember that lighter subjects subjects need MORE exposure than indicated, (positive compensation) and dark subjects need LESS exposure than indicated (negative compensation). Beginners find it very easy to get that the wrong way around.

The principle of Exposure Compensation:-

The idea is that all tonalities of subject (meaning "light" "dark" or anything "in between", or any proportion or "mixture" of tones) should all receive the same shutter speed and aperture... that is...

.....As if they are all being exposed by the same light strength.

This is because it is light strength falling on the subject that is important to correct exposure.. not lightness or darkness of the subject itself.

The amount of Exposure Compensation to use for any one lightness or darkness of mering target has to be learned by experience. However, this is easier than it sounds.

The best way, of course is to carry a grey card around with you while you make comparisons and note down the differences in 1/3 stop increments! A couple of hour's practise should be enough, if you are diligent.

Some are compensations are easy to remember, for instance,

Typical Snow Scene in Sunshine = plus 2+1/3 stops compensation.

You can confirm this snow scene compensation by spot metering a sheet of white paper lying in bright light, shooting that for reference...

....and then shooting a series shots increasing the exposure by 1/3 stop each time until the camera histogram shows the tonal bulge "hard" against the right-hand side, but not bursting through it.

Final point: The histogram remains available for evaluating your exposures for any shot, whether you are trying to ascertain correct pre-compensation (EC) for subjects of light or dark tonal structure... or not!

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