Evaluating Step Wedge

[MattKing]

This may be what has happened.

It’s odd to me but there’s 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:

An incident meter that uses a dome "integrates" all of the light that hits the dome - irrespective of what direction that light originates from.

The spot meter may read less flare and extraneous light.

And the poster board may be a lot closer to a 12% gray than you think.

 

[Stephen Benskin]

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.

You first need to know reliably where the exposure should fall. Ralph's suggestion is a good way to go. My advice is not to be so concerned with film speed and focus more on the contrast.

 

[nworth]

I've used essentially the same routine when testing developers, except that I use HP-5 (usually) and a lightbox. My results have generally been pretty good and reproducible. I find it is very useful in testing developers.

 

[Stephen Benskin]

This may be what has happened.

It’s odd to me but there’s only a 2.33 stop difference between spot metering the white poster board and taking an incident reading.

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.

Here's what you are seeing:



Find Zone V exposure. It's at 12% reflectance. Move upward on the table three stops and you get 95% reflectance which for argument sake could be around where your white board might fall. The angle of the board in relation to the camera and light source can make a big difference in the meter reading. There are probably other factors involved.

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.

 

[Bill Burk]

This may be what has happened.

It’s odd to me but there’s 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 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.

 

[bascom49]

Aren't you supposed to remove the done when metering reflectance ?

 

[Baisao]

Aren't you supposed to remove the done when metering reflectance ?

The dome is not removable and the spot meter is on the side (no dome).

 

[Baisao]

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.

While I don't understand the formulas, thanks to your chart and diagram, I feel that I understand what you are saying. As Matt said earlier, there may not be as big a difference between white poster board and 12% gray. I didn't understand how until you explained it.

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.

You very well may be right. I have several films I want to test, and each with different dilutions, and each with rotation vs semi stand. Having Fred Newman do these will cost an arm and a leg. I have heard GREAT things about his service but hoped I could get some value and knowledge from doing a less rigorous test on my own.

 

[Baisao]

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.

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.

 

[Baisao]

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?

I'm using Pyrocat-HD (1:1:100) @ 21° @ 8:30 @ 30 rpm, per Sandy King's suggested starting point for FP4+. I've been using Pyrocat-HD for roll film in semi-stand and receiving very pleasing results, including a modest boost in film speed for some films, going by visual inspection. I rate FP4+ at EI 160 and give it a semi-stand for 21 minutes with beautiful results. This, and Sandy King's published suggestions, are what made me start doubting the results of the step wedge test I performed.

 

[MattKing]

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.

 

[Baisao]

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.

Apparently so! I've recognized metering as an art for some time but the amount of science behind it is astonishing. Before there were Makers, I lived on a farm and we did everything ourselves: why buy it if you can make it. This value has stayed with me, for better or worse, and has presented itself in my photographic endeavors. I'm certain that's why I've been doing it the hard way and so willing to learn from others.

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.

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%?

 

[Bill Burk]

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.

MattKing gave a good explanation.

18% is the gray card you can buy.

All the lore around 12% etc... boils down to this:

If you had a special gray card that you could pull a tab or turn a dial and change its percentage reflectance infinitely... say from 10 to 20%

And you put it in a scene that you meter with an incident meter.

Pull the tab or turn the dial while reading with the reflected light meter.

The percentage your card would be when the readings match between incident and reflected...

Would most likely be around 12%.

At that point you can answer the question: What percentage reflectance would make a good gray card that I could take readings off and use directly for an exposure recommendation that would be the same as an exposure recommended by an incident meter.

 

[MattKing]

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.

 

[Baisao]

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.

Got it! Thanks, Matt.

I'm going to go ahead and have Fred run some tests. I read more about the BZTS methodology and it makes a lot of sense to me.

I am grateful for everyone's assistance this week! Thank you.

 

[davedm]

Why not check your meter reading for sunny 16/11 rule ?