Originally posted by GUB ....So I was suspecting the meter was essentially seeing a greyscale of the colours....
The meter cannot see anything other than luminance values. Spot metering will be the most accurate/consistent method of reading luminance values. It may or may not be more/less sensitive to certain colours
Quote: Now have a look at the colour wheel once it is greyscaled – the reds and blues have ended up with a tone that is darker than the 18% grey that meters tend to calibrate to.
Meters are not calibrated to 18% they never have been since standards introduced. Average calibration point will be around -3EV from sensel saturation = 12.5%. Try it yourself meter something with texture and from the indicated exposure make several more increasing 1/3 stop each time until you are at least +3.67 EV from meter reading. If meter is 12.5% you will have hit saturation at +3EV, if meter calibration is 18% then you will only have +2.47EV to saturation.
Important to note you will not see accurate clipping response in any (AFAIK) raw editor due to the way images are rendered as covered in a previous post. Adobe add BLE and also add exposure, contrast and black level over their previous standards. Others may differ! You will need ot examine your file with Rawdigger or similar to get a proper indication of clipping
Quote: Could it be that the meter is metering red (and blue) this way and so giving a totally different exposure to them compared to how it would meter white.
The colour sensitivity of the meter may be less than even across all colours but it is the lack of red and blue sensels (1/2 that of green) that really make a difference and is one of the winning points with pixel shift systems.
With respect your conclusions appear to be founded on several misunderstandings:
1. That camera exposure meters calibrated to 18%. Generally this is not the case with 12% being more normal. The difference representing 0.5EV i.e. meter a so called 18% grey card and for a 'normal' subject you will need to increase the indicated exposure by +0.5EV or angle the card so that it is 45 degrees to the light - same effect.
2. 18% grey is actually the middle grey of exposure and should be close to RGB values of 128. There is no colour editing space in common use where 128 represents a mid point. Use L*a*b* to get a measure of mid tone set 50,0,0 and you will find that values change for instance I usually work in Prophoto with raw files and middle grey is about RGB =100, Adobe RGB is RGB=118 and sRGB close enough
3. Not familiar with your raw editor but there is no information about what it adds to raw to first render and image but one thing is certain it does not provide you with a real view of your raw data just its rendering and resulting histogram
4. You cannot rely on the EV controls in a raw converter to mimic what happens with exposure changes in camera. Your camera is an analogue capture device with an A/D converter turning photon count into digital numbers in a linear space. Your raw editor must apply a number of adjustments including, demosaicing, finding white and black points, finding a decent White balance, and importantly apply a gamma curve. When you apply exposure correction in your raw editor you are applying through a TRC (gamma curve) which will roll of highlights and shadows very differently from your original capture.
5. Clipping in JPEG? If you have not clipped in raw then your conversion to JPEG shows clipping then that is an error in your editing. Perhaps I have misunderstood the relevance of mentioning JPEG as the discussion really is raw capture
You may or may not be correct that your cameras meter is a little hot but I would expect that you will get similar if you care to shoot a bright blue subject.
In the other thread I asked if anyone was prepared to actually post a link (dropbox maybe) of red problem for us to look at the actual data - that offer is still open