View single post by jk
 Posted: Sat Mar 27th, 2021 00:57

Joined: Mon Apr 2nd, 2012
Location: Carthew, Cornwall, United Kingdom
Posts: 6601
OK so you want to shoot a white object and a grey object on a black background.

If you use a reflected meter reading e.g. TTL then your camera will see a large black area with two small areas that are lighter.  If you use Nikon matrix metering it does some clever metering so we will assume instead it is a dumb meter.  
So the meter sees the black ground mostly and thinks that correct exposure for this is for a mid-grey (Kodak Grey card or 18% grey card) so it gives say 1/15 at f8 for your ISO200 setting.   However in reality it is a black background which is about 3-4 stops less exposure!  So your exposure should be (3stops less) so 1/125 @f8 with ISO200.

Now swap the black background for a white one.  The meter sees it as a mid-grey again, not white (3-4stops more exposure is required), so gives you an reading of 1/1000 @f8 with ISO200. But the correct reading is 1/125@f8 with ISO200.

Now swap the background for a mid-grey one.  The meter sees a mid-grey again, so gives you a reading of 1/125@f8 with ISO200.

If you had used incident metering then since the light falling on the subject is constant the correct exposure 1/125@f8 with ISO200 is always indicated, (for a mid-grey exposure) is shown whether or not the background is white, black or grey!  

That is the simple story.
Then it is complicated by shiny or matt black, white grey.

Still learning after all these years!