General exposure question - probably zone system related

[removed account4]

thanks brian ...

 

[Bill Burk]

I spent hours at a large Ansel Adams exhibit at San Francisco MOMA.

If there was any inconsistency in printing quality, I appreciated it as a sign that he grew in ability to make more sensitive prints over the years. It's reassuring to look at the very old vintage prints, because as a student or printer, those are the ones with lesser quality - not in a derogatory sense, but in a capability sense - that I (or you) are going to be able to achieve in our own darkrooms.

 

[Stephen Benskin]

I personally like the prints from the late 60s and early 70s. They were a good balance. His eyesight was going after that and he tended to over tone the prints. They had a distinctive purple tone. Mary Street Alinder talks about it in her Adam's biography. There was a large museum show in the late 80s or early 90s. I saw it at LACMA. One wall had examples of how Adams interpreted Moonrise over time. I believe there may have been a contact print, and a number of 16x20s, but dominating the wall was a mural print from the forties where the sky wasn't black, you could see cirrus clouds above the moon, and the foreground was grey and the crosses definitely didn't pop. I thought it was a great way to educate the general public.

 

[Stephen Benskin]

OK, so the let's assume you take a reading from a 18% gray card, and lets assume the subject is being illuminated by an external source and not generating light itself and furthermore that the reflectance of the subject is not specular. So, 18% reflectance is zone V, 36% reflectance is zone VI, 76% reflectance is zone VII, and 152% reflectance is zone VIII. However, 152% reflectance is a physical impossibility, so you should never need anything much above zone VII.

Do I have this right?

Reflectance is based on the equivalence of a Lambertian surface or perfect diffusing surface. A 100% reflectance is not the same as 100% reflection. To calculate the value of the luminance from a Lambertian surface, you need to divide by pi. Semi-specular surfaces can easily exceed 100% reflectance. Specular surfaces, light sources, the sky, and the sun can easily exceed 200% reflectance.

 

[alanrockwood]

Reflectance is based on the equivalence of a Lambertian surface or perfect diffusing surface. A 100% reflectance is not the same as 100% reflection. To calculate the value of the luminance from a Lambertian surface, you need to divide by pi. Semi-specular surfaces can easily exceed 100% reflectance. Specular surfaces, light sources, the sky, and the sun can easily exceed 200% reflectance.

Good point. That's why I in my post I specified a system where the reflection is non-specular. With specular surfaces all bets are off. I believe it is even possible to have curved specular surfaces that would do some light concentration in a limited part of the area.

 

[Stephen Benskin]

I put this together awhile ago.

 

[alanrockwood]

I put this together awhile ago.

View attachment 106421

What is the meaning of "q" in the equations?

 

[Stephen Benskin]

What is the meaning of "q" in the equations?

q is a light loss constant and is part of the exposure equation. q represents a larger equation with variables for lens transmittance, lens vignetting, flare, off axis factor, and distance from lens to object.

With TTL cameras, the camera meter measures the actual value of the image illuminance and doesn't need q. Hand held meters need to estimate an average value of light loss. Currently it's considered to be 0.65.

The attachment explains q in more detail. BTW, don't be mislead by the value 1.03 for the camera flare correction factor. It represents the flare at the metered exposure point and not the shadow where flare factor is calculated.



The exposure equation for average conditions creates another constant P, which represents the value of Eg (average image illumance or metered exposure value). All meters are designed to expose for the same value of P. Multiply P by 1/ISO and you have the value for Hg for any film speed. P = 8 (sightly rounded from 8.11).



In the example 8.11 produces a value of 0.065 of Hg. 8 would produce a value of 0.064. P * 1/ISO can be reduce to P/ISO or 8/ISO.

 

[Bill Burk]

Thanks Stephen!

I think you've told me that before but my eyes were glossed over back then. This makes sense to me now...

So q includes flare.

Nice to know.

 

[RobC]

This is what I believe to be the calculation for exposure with my spot meter.

The meter is designed on the assumption that the midpoint of the film characteristic curve is receiving an exposure level of 0.1 lux.seconds.

The formula used to calculate exposure is :

2^EV= (B*S)/K where

B= luminance in cd/m2
S=ISO sensitivity of film
K=calibration constant (whatever it is, 12.5 or 18 etc depending on the meter)

 

[Stephen Benskin]

This is what I believe to be the calculation for exposure with my spot meter.

The meter is designed on the assumption that the midpoint of the film characteristic curve is receiving an exposure level of 0.1 lux.seconds.

The formula used to calculate exposure is :

2^EV= (B*S)/K where

B= luminance in cd/m2
S=ISO sensitivity of film
K=calibration constant (whatever it is, 12.5 or 18 etc depending on the meter)

That is part of the meter calibration equations. In that equation B is luminance. The exposure equation determines the camera illuminance and film plane exposure. B or L is part of the exposure equation.

Here's how they relate:



If you're interested in a more detail explanation, please refer to the Is the K factor relevant to me or should I cancel it out? thread. I also have it in pdf form but it's too large to upload here. I am willing to email it if anyone is interested.

Here is a page from the 1971 meter standard and a breakdown of the equation.

 

[Stephen Benskin]

And defining K

 

[RobC]

Stephen, I PM'd you about getting a copy of your PDF. Thanks.

 

[Stephen Benskin]

Taking the exposure equation and using different values for L (not just Lg), an range of exposures can be determined. The average scene luminance range is 2.20 logs from 100% reflectance to 0.63%. Illuminance is 7860 footcandles.

Notice where the value of B (297 cd/ft2) from the exposure meter standard falls? Between step 10 and 11 at 12% reflectance (reflection density of 0.92).