I have not disagreed with you in stating that an incident meter is calibrated to 18% we both say that...
of course they would have to be or incident and reflection meters would give different results.
The practical difference is that an incident meter is not influenced by the tone colour or surface of the subject. any part of the subject that is reflecting at 18% will come out as a mid tone any way. Highlights will be white and deep shadows black. all with out any compensation.
A reflectance meter will establish any part of the scene it is pointed at as an 18% mid tone. if you point it at a highlight or white snow it will come out mid grey and pointed at a dark shadow it will also come out as a mid grey. this is plainly not what is required with out compensation. only if you point it at something that is 18% will all tones come out as you wish.
Just like any tool a light meter needs calibrating The old weston meters did this by providing a series of letters , which indicated the brightness range of negative and transparency film.
This was near enough for most people. However it was always worth finding your own adjustment for setting the film speed. As this would take care of any processing and techniques peculiar to you own equipment and working methods.
The same is true today with digital. Just as people processed films to arrive at different film speeds... So do digital sensors vary. Fortunately they are even more consistent than films ever were. so once you find the limits of your sensor by trial and error or measurement, you can adjust the film speed setting accordingly. this adjustment will remain constant.
Thereafter incident readings need no further adjustment. Though as I said in my previous post it is still necessary to know when it preferable to use duplex readings.
Key tones are an excellent method of using a reflectance meter either through the lens or manual. The method used by many wedding photographers has been to get close to the Bride and groom and take a reading from your own hand. this may not be exactly 18% but it will be a consistent tone every time. and all your shots will have a consistent tonal range. you will have to set the appropriately adjusted iso speed to take advantage of the sensors full capability.
All exposure methods are simply to try to get the available range of light to fit the range of the film or sensor, in a standardised way.
The original Norwood super Director incident meter set the standard in the film industry (I have had one from new) the modern equivalent the Sekonic 308 is still available.
The most accurate light measuring device ever available to Photographers was the SEI photometer spot meter (which has a 1/2 degree spot) I used one at college but never owned one.
Terry, I'm not sure what we're disgreeing about now, except that you said you disagreed with what I said previously
The point I want to make, and I think it's important given that people are still saying that an incident reading is best - usually without any qualification or understanding (neither of which applies to you, I know!).
And that is, an incident meter is calibrated to a basic assumption that 'white' is fixed at a certain level above 18% grey. That level is good for slide film, at about three stops, but white is not a fixed tone. The piece of paper I have in front of me now is white, but so too is the sun. Except that it is about a million times brighter.
Where is white for my digital camera? Let's say for this purpose that it is the point at which it blows to just beyond the dynamic range. That varies with different sensors, and even more according to the post processing (or JPEG presets) that are applied. Equally, in a typical scene you will have white in terms of sunlight shining off cheeks and chins, or white collars and cuffs etc, and I will want to retain just a little detail there right at the extreme edge of the histogram. But the very bright specular highlights that are reflecting off shiny things like chrome, or gloss paintwork or whatever, well I don't care about them - they can blow. And if I was to attempt to retain detail there, I would end up with serious under exposure over the rest of the scene.
So in every situation I am making judgements that the incident meter's calibration knows nothing about. I am trying to squeeze the maximum amount of relevant data on to the sensor, pushing
important highlights right to the edge, and in doing so pulling the shadow end up to a position where a lot more detail will be recorded.
Basically, I am exposing to the right, because with digital I can pull the tones back down again to where they should be in post processing, but I'll have a considerably enhanced image as a result, with much more shadow detail in particular. An incident reading does not allow you to do that, and so therefore your statement that
"Incident meters used correctly set the highlights correctly for any media film or digital" cannot be true.
I'm not talking about half a stop here and there, the difference between the settings suggested by an incident reading with a hand meter, and an optimised exposure setting made after checking the LCD/histogram/blinkies is usually in the order of two or even three stops.
