Clarification on Spot Meter

[Das Burschlein]

Can anyone clarify a few things about the spot meter for me?  What percentages correspond to the Canon meter readings?  How many percentage points represent 1 stop?  The way I figure it, if the scale from 1% to 100% represents black to white, I would think that a gray card would read 50% because that would be in the middle of the scale.  Highlighted Caucasian skin would be about +2 on Canon's scale (two stops more exposure than a gray card). Now, I've read in other threads that highlighted Caucasian skin would be 70% on the spot meter.  So if 70% is two stops more than 50%, one stop must equal 10 percentage points.  And if I were to apply the Zone System, zone 5 would be 50%, zone 6 would be 60%, zone 7 would be 70%, etc.  Am I correct in thinking so?

Thanks!

[Audionut]

Middle grey is 18%, but your light meter may meter differently.
A good article regarding the zone system.

I'm not sure what scale percentage matches exposure (other then 18% = middle grey), and I prefer to use raw based exposure feedback anyway.

[garry23]

For what its worth, like Audionut, I only use the Ev feedback. But, then again, I don't do videography, 'only' photography.

I've looked at the code, and in fact started to change it to give a 1-deg spot capability, ie the metered area would change according to FL. The current ML spot is, if I remember correctly, +/- 10 'pixcels' (I assume from the LV resolution ?).

Thus the standard ML spot gives you very tight metering.

My main use of the ML RAW Ev spotmeter is when I bracket. I use the ML spotmeter on the darkest area where I wish to see 'detail' and place this in the Ev 'zone' of my choice, based on 0Ev is 100% 'saturated'.

I would recommend using a grey card and 'calibrating' your ML spotmeter, ie knowing your spot is -x EV down from saturated allows you to 'place' the metered area in a zone of your choice.

[Das Burschlein]

Thanks for your responses!
Ok,  yesterday I did a quick test.  Using Canon's meter I exposed a gray card for normal exposure, so Zone V (not shooting RAW). I switched to ML spot meter and the reading was 49.  I did the same comparison up and down the scale and got the following:
1. (-3) Zone II = 11%
2. (-2) Zone III = 20%
3. (-1) Zone IV = 34%
4. (0) Zone V = 49%
5. (+1) Zone VI = 69%
6.(+2) Zone VII = 83%
7. (+3) Zone VIII = 96%
Then I re-read the article by Gisle Hannemyr (ftp://http://dpanswers.com/content/tech_zonesystem.php) and noticed that he has the percentages that correspond to the various Zones and my numbers come very close to his.  I think this shows that the proper reading for a gray card (18% reflectance), in order to place it on Zone V, should be 49/50%, the middle of the scale, and not 18%. I think the reason for this is that a camera meter is measuring tonalities, not actual reflectance.  When I think about it, if a meter is going to measure the percentage of reflectance an object has, it would have to know the strength of the light source.  So I think that the scale 0% to 100% simply represents the tonal range from black to white (much like 0-255), not the actual reflectance of an object, and we expose any given object in a way to place it where we want it on that scale.

[garry23]

You may wish to rerun your tests in RAW and with the ML Spot Ev meter.

Ev readings allow you to better link to camera settings, ie stops of change.

[Das Burschlein]

I'll have to give that a try.  However, how would I interpret the results to correlate them with JPEG?  I shoot primarily video and am not ready to get into RAW until it is more stable.  Besides, I'll have to invest in a larger and faster card before I do.

[Audionut]

Ok,  yesterday I did a quick test.

Your results look sound. 

6D meters 1 stop to low (0EV meters to zone IV), and be mindful that picture style settings affect the displayed results.