Of course this all very well however digital sensors do not work in the same way as film as is covered in this link:
http://www.luminous-landscape.com/tutorials/expose-right.shtml
and is updated in this link:
http://www.luminous-landscape.com/tutorials/optimizing_exposure.shtml
and is also explained in this link in this link:
http://www.johnshawphoto.com/ettr-to-the-far-right/
This of course is only relevant if you are shooting in raw and not jpeg and of course it is important to ensure you don't clip your highlights. I really recommend that you read the links that I have posted so that you all get an understanding as to how digital sensors work. For those of you that already understand the concept I apologize and to the OP if I have derailed your thread even more I apologize.

You knew this was going to happen, Rob. If Oskar Barnack were to write an explanation of the workings of the Leica Ia, some know-it-all would tell him he was wrong. It seems to happen everytime someone tries to be helpful.

If anybody is interested in the difference between 18% gray versus 12-13% gray, let them go to:

http://en.wikipedia.org/wiki/Middle_gray

where they will see the different explanations. I'll bet not one person in a hundred could tell the difference the two colors if printed on cards are not shown side-by-side. I'll also bet not one person in a thousand can find the difference on a histogram.

Some people are stupid, some are just rude and some have gigantic egos. Looks like you've run into one or two people who suffer from all three maladies.

Keep up your good work. Bill

NOW BACK TO THE THREAD TOPIC➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜➜

It appears that there is a degree of misunderstanding of what a histogram actually is and exactly what information it displays.

I will reference this to jpeg images for the sake of this thread and for all those ready to correct me on unhelpful technical details please allow me some licence in order to keep this as understandable for as many people as possible.

A histogram is an x,y representation shown as a series of columns representing black on the left brightness with a relative luminance value of 0 and white at the extreme right with its luminance value of 255 while the space between is filled with the remaining 254 columns representing the luminance values of all the shades graduating from black through to white.

The 18% much talked about is “middle grey” which, of course, falls in the centre of the histogram and holds the luminance value of 127.

The important thing to remember here is that any histogram is not an absolute. Any given histogram is tightly related to a particular parametric circumstance. What that means is that a histogram display in a camera displays the range of luminousities within the limits of the dynamic range of that particular camera.

If one camera has a dynamic range of twelve f/stops and another is capable of six and both were used to photograph a tonal sweep, say a surface with an luminance range of fifteen equivalent f/stops in sunlight with exposure set for an 18% grey card then both histograms would look much the same even though camera (a) recorded six stops either side of the middle whilst camera (b) only managed three.

The histogram on both cameras would show that values were recorded in all the columns 0-255 because in these cases the information they display is limited to the actual dynamic range of each sensor.

The 6 f/stop sensor in this case would record as black a luminance value which the 12 f/stop sensor would record as 9% grey.

Likewise the 6 f/stopper would record a shade as being white whereas the 12 would only record the same luminance value as being around 60% grey.

In other words the 12 stop sensor would in one shot record the same range as the 6 stop sensor could if shot as a +3,0,-3 f/stop HDR.

The 12 stopper would have twice as much shadow and highlight detail even though the histograms would probably look much the same.

That’s probably enough for now, I’ll let this sink in and settle for a bit before I continue. Rob.

Pleeeease.... I knew this would happen!!!

I asked that people give some latitude so we don't all get bogged down in the detail. This subject is confusing enough for so many so let's just please stick to the subject CONCEPTUALLY and we can all build on people's knowledge with a minimum of confusion.
The "13%" to which you refer is actually 12% and is the level set as middle grey as the median sensitivity of the sensor to compensate for the non-linear way the sensor reacts to a linear progression in light levels so that relative brightness levels recorded by the sensor more accurately mimic the reaction to light of human sight.
I used 18% not because I do not know what I am talking about but because I was referring to the reflectance characteristics of standard objects which is not the same as parameters selected by designers for sensor calibration.
I hope this clears that up. Regards, Rob.