This may be what has happened.
Its odd to me but theres only a 2.33 stop difference between spot metering the white poster board and taking an incident reading.
Metering in full midday sun, without clouds, orienting meter and white poster board the same way, meter set to ISO 64:
- Spot on white board 1/60th at f45
- Incident with dome facing sunlight 1/320th at f8 (1/10th at f45)
Why would the pure white poster board be only 2.33 stops from an incident reading, instead of 5?
Incident:
Spot:
Hi Balsao,
Just to give you a quick point.
Interpreting step wedges for exposure is as simple as counting steps between what you got versus what you want.
Your exposure is simply off by that many steps.
You can still interpret the remainder of the scale for contrast.
If you have no measuring device, you can print and count how many steps you can see on the print.
This may be what has happened.
Its odd to me but theres only a 2.33 stop difference between spot metering the white poster board and taking an incident reading.
This may be what has happened.
Its odd to me but theres only a 2.33 stop difference between spot metering the white poster board and taking an incident reading.
Metering in full midday sun, without clouds, orienting meter and white poster board the same way, meter set to ISO 64:
- Spot on white board 1/60th at f45
- Incident with dome facing sunlight 1/320th at f8 (1/10th at f45)
Why would the pure white poster board be only 2.33 stops from an incident reading, instead of 5?
Aren't you supposed to remove the done when metering reflectance ?
Reflectance three stops above the metered exposure point is about 95% reflectance. Reflectance refers to a Lambertian surface or perfect reflector. So 100% reflectance doesn't mean the same thing as reflecting 100% of the light back toward the viewer.
In order for any surface to be higher than around 91% reflectance it has to contain some specular reflections. 100% reflectance is considered to be a diffused highlight. Glossy white poster board might fall around 95% reflectance.
You need to ask yourself how confident can you be with the results using this method, and whether it's worth the time and effort. Without the use of a densitometer, you aren't even able plot the curves and determine the contrast. I personally don't see any advantages.
Incident reading is about like reading a gray card.
I don't know the reflectance of your white card but for discussion, assume the spotmeter is reading 95% reflection - in density terms this is 0.02
2 1/3 stops difference in readings is 0.70 density units.
Then adding the two density units 0.02 and 0.70 arrives at 0.72 density units.
This is 19% reflection
Here is a convenient cross-reference of density to reflectance/transmittance:
http://www.xrite.com/documents/apps/public/WhitePapers/Density_to_Percent_T_or_R.pdf
I worked out the other day the relationship between Incident reading and Reflected reading for Sekonic using the formula R = ( pi *K ) / C and the values Sekonic uses for K = 12.5 and C = 340 works out to be R = 0.115 or 11.5%
I wonder why you are at effective 19% instead of 11.5%
One possibility is that the lighting you arranged is giving readings equivalent to the flat diffuser. When you use a flat diffuser on the Sekonic, (retract the dome), K = 12.5 and C = 250, it works out to be R = 0.157 or 15.7%
The difference between 15.7% and 19% is 0.04 density units, that's getting close. This might be a good model of what's happening.
Finally, there's the developer. Is it a general purpose developer which produces speeds similar to the ISO speed, or is it a specialty developer which is most certainly not similar to the developer used in the ISO testing?
Thank you for this. It's reassuring me that I didn't do anything wrong so much as the procedure that I was following may be flawed.
Besides accounting for reflectance, one new thing to me is the notion of 12% reflectance. Does this have any association with the 18% of gray cards? I'm not trying to confuse the issue but trying to understand if they are related, and if so, why 18% instead of the 11.5/12% that appears to be ideal.
There are a number of really complex and interesting threads about meters and calibration and constants here on APUG.
Threads started by Stephen Benskin are particularly interesting.
Speaking simply, it appears that 18% may have been chosen merely because it is a useful, repeatable, mid-gray constant, not because it is a reflectance that matches an average scene.
The 12% figure comes from empiracle data about the average scene.
Thank you for this. It's reassuring me that I didn't do anything wrong so much as the procedure that I was following may be flawed.
Besides accounting for reflectance, one new thing to me is the notion of 12% reflectance. Does this have any association with the 18% of gray cards? I'm not trying to confuse the issue but trying to understand if they are related, and if so, why 18% instead of the 11.5/12% that appears to be ideal.
There are so many things that are perpetuated because "it's always been that way". Paper sizing is that way, too. Would you suggest I calibrate the spot meter to 12%?
No, because spot meters are never used to read an overall, average scene - just individual "spots" within those scenes.
You use a spot meter to place an exposure where you want it, and to measure where other parts of the scene fall as a result.
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