Exposure meter's result EV relative to the film ISO density threshold

[River Mantis]

Let's take a film with properly measured ISO speed. It means that there is some threshold exposure where it's resulting density (if developed to a specific standard gamma) raises just slightly (there is a standard number I don't recall) above the base fog. So the question is, if you take a reading of a properly calibrated exposure meter pointing to a grey card, at what distance it would be (expressed in EV stops) from that threshold? My naive answer is

From the ISO standard
S = 0.8 / Hm

Then
Δ𝐸𝑉 = log₂(𝐾/0.8) = log₂(12.5/0.8) ≈ 3.97

Correct me if I'm wrong please

 

[ic-racer]

Usually 4 stops.

In "Zone" terms, your threshold is Zone I and the middle calibration point is Zone 5; they are 4 stops apart.

 

[pentaxuser]

From the ISO standard
S = 0.8 / Hm

Then
Δ𝐸𝑉 = log₂(𝐾/0.8) = log₂(12.5/0.8) ≈ 3.97

Usually 4 stops.

In "Zone" terms, your threshold is Zone I and the middle calibration point is Zone 5; they are 4 stops apart.

So, ic-racer, is River Mantis' calculation at 3.97 correct or is the 3.97 just a coincidence?

Thanks

pentaxuser

 

[MattKing]

@Stephen Benskin can probably help with this.
But be aware - a Zone System approach will yield a different answer (by ~2/3 of a stop) from an ISO approach.

 

[Bill Burk]

The answer is 10x

That’s 1.0 on the LogE scale

3 1/3 stops

 

[Bill Burk]

Here’s the evidence. My test results fell within 1/6 stop of hypothesis. Stephen Benskin has confirmed for me that expected result by math is 10x

I do not worry about the sixth stop difference. I treat it as 10x

 

[pentaxuser]

So is River Mantis' calculation as follows wrong and where is/are the flaws in it

From the ISO standard
S = 0.8 / Hm

Then
Δ𝐸𝑉 = log₂(𝐾/0.8) = log₂(12.5/0.8) ≈ 3.97

I shouldn't speak for him but it just seems to me that he presented a formula and was asking if he had got it wrong and presumably why

Put it like this: if it had been I presenting a formula I'd want to know where and why it is wrong

Thanks

pentaxuser

 

[ic-racer]

Jeff Conrad wrote a paper and got mathematically 3.51 stops from the 0.1log d point. However, I commonly use 4 stops. Below is his explanation for the difference.

"ANSI/ISO 6-1993" specifies a higher contrast than usually is desired for practical photography. Consequently, photographers often use a different processing, and the effective film speed may be different from the ISO speed. However, if, except for the contras, the method used to determine the speed is similar, the above relation often can be used if the ISO film speed is replaced with the effective film speed. "

Exposure Metering
Relating Subject Lighting to Film Exposure
By Jeff Conrad

 

[Sirius Glass]

There seems to be a lot of agreement that 4 EV [f/stops] is the number you are looking for. How are you going use that information?

 

[Bill Burk]

Jeff Conrad’s 3.51 stops makes sense…

Divide that by 0.3 (LogE per stop) … leads you to 1.053 LogE

Could be rounded to one decimal place as 1.0 to agree with Stephen Benskin

While four stops is 1.2

We all agree Zone System just calls it four stops.

If you round Jeff Conrad to two places he’s a sixth stop greater exposure than Stephen Benskin and a third stop greater exposure than Yoshihisa Maitani (my OM-4 experimental results).

 

[Bill Burk]

Ah how to use it… create a feedback loop between your metering technique and your results. Take a shot exactly at the meter reading, read the density at your meter reading. Then find out if you hit your aim.

 

[River Mantis]

Jeff Conrad wrote a paper and got mathematically 3.51 stops from the 0.1log d point. However, I commonly use 4 stops. Below is his explanation for the difference.

View attachment 348431

"ANSI/ISO 6-1993" specifies a higher contrast than usually is desired for practical photography. Consequently, photographers often use a different processing, and the effective film speed may be different from the ISO speed. However, if, except for the contras, the method used to determine the speed is similar, the above relation often can be used if the ISO film speed is replaced with the effective film speed. "
View attachment 348432

Exposure Metering
Relating Subject Lighting to Film Exposure
By Jeff Conrad

Thank you. So it looks like I've missed 0.728 multiplier. I guess it's related to lux*s to EV conversion

 

[Stephen Benskin]

What ic-racer didn't show was Conrad's b is the same as what is known as q in the exposure equation. q represents the light loss from an optical system. With hand held exposure meters, the actual value cannot be known for certain and is averaged. q is part of K. K incorporates q and a number of variables from the exposure meter. q has been considered to be 0.65. Conrad is using 0.728. Not that it can't be that value depending on the situation and the value of 0.65 isn't guaranteed, but it is what is used in the modeling of exposure. The difference between the two is that the standard value of q assumes an off axis value of 12 degrees whereas Conrad is using on axis. I believe the 12 degree value comes from attempting to average the illumination from the lens and that Conrad is using its brightest point.

Here's an excerpt from D . Connelly's paper, Calibration Levels of Films and Exposure Devices, The Journal of Photographic Science, Vol 16, 1968. Link to the paper: Calibration Levels of Films and Exposure Devices

Hg/Hm = 10
P / 10 = 0.80

You might find this relationship interesting: P / q = K or K * q = P.
If you think about it, it's a simplification of the equation for K shown in the next post. q and P are found within the K equation. I like how each constant represents an aspect of the photographic process: meter, camera, and film plane exposure.
Conrad's equation is a variation of the second equation. K * q is the exposure constant for Hg. Dividing it by the speed constant produces the ratio between Hg and Hm.


Excerpt from Conrad's paper.

Also the paper that much of the appendix in ANSI PH3.49-1971 uses word for word is below. Scudder, J, Nelson, C.N., and Stimson, A, Re-evaluation of Factors Affecting Manual or Automatic Control of Camera Exposure, Journal of the SMPTE, Volume 77, January 1968.

From Jack Dunn's Exposure Manual:

As for the Zone System, it was developed when the method of determining ASA speeds produced lower speeds. The four stop difference between the metered exposure and speed point with the Zone System produced equivalent speeds as the ASA speeds. They no longer agreed after the change of the 1960 standard while the Zone System methodlogy remained the same. The relationship between the shadow and metered exposure also changed. From C.N. Nelson's Safety Factors in Camera Exposure.

 

[Stephen Benskin]

This might be getting into the weeds, but here it is from a slightly bigger perspective. The aperture and shutter speed theoretically should produce the exposure value based on luminance and film speed, but in reality other factors play a part. K represents those factors. A2 / T using the Sunny 16 rule equals 256 cd/ft2, but evaluation shows the actual luminance needs to be 297, on average, to compensate for the light loss factors of an optical system and physical aspects of the exposure meter. 297 / 256 = 1..16 cd/ft2 or 12.5 cd/m2. The equation for K at the bottom defines the variable. The variable K1 is the target exposure at the film plane (minus shutter speed). It is the value of P in Connelly's paper. 8.11 * 1/100 (shutter speed) = 0.08. Based on the B&W film speed equation of 0.80 / 100 = 0.0080 or a difference of 10x, 3 1/3 stops, or Δ1.0 log-H.

The equation for q:

And how it relates to camera exposure. Eg becomes Hg when the shutter speed is applied.

0.064 / 10 = 0.0064 and plugging it into the speed equation: 0.80/0.0064 = 125

Almost forgot, @River Mantus, the point you were describing in the your first sentence is the fractional gradient speed point which is defined as the point where the gradient is .3x the average gradient. It is the same point as the modern Delta-X point. While the Delta-X criterion is calculated differently using the 0.10 fixed density point to determine the Delta-X's location (ΔX), it is not the same as the 0.10 fixed density point. The 0.10 fixed density point is only where the speed is determined, not a limiting gradient or density. Nor does it actually define where the exposure should fall. When the ISO processing conditions are followed, the Delta-X speed point is 0.296 log-H to the left of the 0.10 fixed density point. That is effectively the minimum point (depending on the degree of flare).

 

[Bill Burk]

Thanks Stephen!

I fixed a glitch on my website where your papers are hosted so …

https://beefalobill.com/benskin/content should be working again.

 

[Stephen Benskin]

Thanks Bill.

That site has all the papers mentioned in my above 2 posts and more. It's a good resource and Bill has been very generous in sponsoring it.