Converting EV to Lux to millilux-seconds

Chuck_P · Feb 20, 2023

Q #1:

I found an on-line calculator that supposedly converts EV to Lux. It tells me that EV 12.5 is 14,400 Lux, as the calculator says ....EV to Lux. It seems this number is already converted to millilux.....or does 14,400 need to be converted to millilux by multiplying by 1000? Which would then be 14,400,000 millilux?!

Q#2:

If that number is already converted to millilux, then to convert to milllilux-seconds, I would then multiply 14,400 by the exposure time to obtain the log exposure value?

Q#3:

In my mind I have it this way:

(14,400 × .25 sec) = 3,600 millilux-seconds.

Log 10 (3,600) = 3.56 log exposure.

Do I have any of this correct? Thank you.

Nodda Duma · Feb 20, 2023

The 14,400 number that you are sharing is close to correct for converting EV to Lux. Lux = 2.5 * (2^EV), so the actual number is 14,481.5 Lux.

To calculate milliLux, multiply by 1000.

Nicholas Lindan · Feb 20, 2023

EV is a measure of exposure - f-stop & shutter speed - not light intensity. So those values are at a reference film speed of ASA100.

bernard_L · Feb 20, 2023

Nicholas Lindan said:
EV is a measure of exposure - f-stop & shutter speed - not light intensity. So those values are at a reference film speed of ASA100.

+1
Replace "EV" with "LV" in previous posts and the usage will be correct.

Chuck_P · Feb 20, 2023

Nodda Duma said:
The 14,400 number that you are sharing is close to correct for converting EV to Lux. Lux = 2.5 * (2^EV), so the actual number is 14,481.5 Lux.

To calculate milliLux, multiply by 1000.

So.......14,481,500.5 milliLux,

Then multiplying by .25 sec = 3,620,375.125 milliLux-seconds. Am I getting this part right?

Then taking the Log 10 (3,620,375.125) = 6.56 log exposure? Am I getting the conversion to log exposure correct? If so that's way out there on the x-axis.

Chuck_P · Feb 20, 2023

Nicholas Lindan said:
EV is a measure of exposure - f-stop & shutter speed - not light intensity. So those values are at a reference film speed of ASA100.

Yes, I've seen the that reference to 100 speed film, thanks, I meant to indicate the film is TMX100.

Bill Burk · Feb 20, 2023

Are you putting EV 12.5 on the easel and exposing film to it? I would take the reading off the light meter scale for f/1.0

But at that rate I think you need to put some neutral density in the setup.

The light meter wants to put middle gray on the film -1.1 log mcs (close to a tenth) while you want to hit the film for testing with somewhere around 0 log mcs (close to one).

This way the step wedge density around 2.1 density (close to a hundredth) will bring the exposure down near the speed point while there’s still a couple steps darker on the Stouffer scale so you can graph the toe.

EV3.5 for a tenth second, EV6.5 for a hundredth would be a better easel illumination

Chuck_P · Feb 20, 2023

Bill Burk said:
Are you using f/1.0 for 1/5000 second?

If that question is to me.........no.

Nodda Duma · Feb 20, 2023

Chuck_P said:
So.......14,481,500.5 milliLux,

Then multiplying by .25 sec = 3,620,375.125 milliLux-seconds. Am I getting this part right?

Then taking the Log 10 (3,620,375.125) = 6.56 log exposure? Am I getting the conversion to log exposure correct? If so that's way out there on the x-axis.

Yes, what that is telling you is that a 1/4 second exposure for ISO 100 film would be way overexposed, i.e. above the shoulder of the characteristic curve. If you are doing sensitometry to measure the response of some test film, you want to either reduce your exposure time or illuminance to get good measurements across the characteristic curve.

aparat · Feb 20, 2023

Chuck_P said:
So.......14,481,500.5 milliLux,

Then multiplying by .25 sec = 3,620,375.125 milliLux-seconds. Am I getting this part right?

Then taking the Log 10 (3,620,375.125) = 6.56 log exposure? Am I getting the conversion to log exposure correct? If so that's way out there on the x-axis.

That would overexpose the ISO 100 film by a lot. In my experience, you need to aim for an exposure between 3 and 4 log millilux-seconds for an ISO 100 film. You will get there, just try to play with your variables. By the way, if you want to use a calculator or write a script, here's the formulas you need:

Code:

lux = (2 ^ EV100) * 2.5

to convert ev100 to EV at any ISO speed (EVS), you can use this formula, where S is ISO speed other than 100:

Code:

EVS = EV100 + log2(S / 100)

If you are wondering why you need to be around 3-4 log millilux-seconds, to get ISO 100, the speed point need to have the value of around 0.9 log millilux-seconds.

mshchem · Feb 20, 2023

How many candle power are in an Imperial gallon?

Bill Burk · Feb 20, 2023

Chuck_P said:
If that question is to me.........no.

I’m trying to figure out what you’re doing. You could use a lightmeter to see how much light you want to put on film. Look at f/1.0 if you’re hitting the film directly with that much light.

Chuck_P · Feb 20, 2023

Nodda Duma said:
Yes, what that is telling you is that a 1/4 second exposure for ISO 100 film would be way overexposed, i.e. above the shoulder of the characteristic curve. If you are doing sensitometry to measure the response of some test film, you want to either reduce your exposure time or illuminance to get good measurements across the characteristic curve.

Yes, that makes sense to me now because I exposed a step tablet contacted to my film in the holder. I placed EV12 and 2/3 on Zone X (Schafer's film test method). The exposure was 1/4 sec at f/5.6 and 2/3. The overexposure was intentional to comply with that method. I got curious as to how my spot meter's EV number translates to millilux-seconds.

Bill Burk · Feb 21, 2023

Chuck_P said:
Yes, that makes sense to me now because I exposed a step tablet contacted to my film in the holder. I placed EV12 and 2/3 on Zone X (Schafer's film test method). The exposure was 1/4 sec at f/5.6 and 2/3. The overexposure was intentional to comply with that method. I got curious as to how my spot meter's EV number translates to millilux-seconds.

http://beefalobill.com/images/sensitometrymarkvi-5.pdf

You put really close to as much light on the film as I do. Within maybe 0.05 so it’s super close.

This graph paper calibrated to my sensitometer, is almost calibrated to Schafer’s steps. I didn’t do it on purpose but it’s a good amount of light to hit film with. I would recommend that much light.

Here’s how what you did ties the meter to millilux seconds:

You placed 7 2/3 on Zone V

With this setting light meter aims to put: 100 speed film with EV 7 2/3 at -1.1 log mcs. This is ten times the speed point.

But the amount of actual light is greater by five stops. 5 x 0.3 log exposure change per stop means adding 1.5 log exposure.

This is positive 0.4 log mcs. Converting the log value to arithmetic value gives…

2.512 arithmetic meter candle seconds or 2512 millilux seconds.

That’s how much light you put on the film. You put the step wedge on top and it blocked light in steps of 0.15 from 0.05 on the right to 3.0 on the left.

The speed point for 100 speed film will fall under your Stouffer scale step having density 2.50, it will be very close to that.

Bill Burk · Feb 21, 2023

But EV12 2/3 was a measure of incident light, and the average reflected light is expected to be (insert favorite conspiracy gray value like 12%).

And a host of other factors go into the equation

We’d need Stephen Benskin’s help to get us from what I think is 2.512 meter candle seconds at the film plane, back though the lens to EV 12 2/3 light incident on the scene.

http://beefalobill.com/benskin/content/Re%20evaluation%20of%20Factors%20Affecting%20Camera%20Exposure.pdf

Chuck_P · Feb 21, 2023

Bill Burk said:
But EV12 2/3 was a measure of incident light

I don't understand this...............12 2/3 was a measure of the reflected light off the test target.

BTW, thanks for that graph paper, I had been looking for something like that. I may have questions later. And I have the same analog Pentax spot meter so that was very helpful in your previous post.....I have a couple of questions that I will send to you later today.

Bill Burk · Feb 21, 2023

Chuck_P said:
I don't understand this...............12 2/3 was a measure of the reflected light off the test target.

It was the meter’s evaluation of what the incident light is.

If you had used an incident meter, it would have used a corresponding set of assumptions to arrive at the same EV.

Anyway the graph paper shows you how much light you put on the film, I worked back from what the meter was trying to do.

Now to answer “What was the meter trying to do?”…

Stephen has done presentations on it. Look for one of his threads like “What is K”.

Chuck_P · Feb 21, 2023

Bill Burk said:
It was the meter’s evaluation of what the incident light is.

If you had used an incident meter, it would have used a corresponding set of assumptions to arrive at the same EV.

Anyway the graph paper shows you how much light you put on the film, I worked back from what the meter was trying to do.

Now to answer “What was the meter trying to do?”…

Stephen has done presentations on it. Look for one of his threads like “What is K”.

It dawned on me that maybe you thought I used a supposed "18% gray" as my target. I know that a reflected reading from an 18% gray target is supposed to be tantamount to an incident reading in the same light.......however, my target was white on the north side of my house: sunny, cloudless sky, in diffuse, uniform shade, no target glare, no shadows on the target. I view it as a reflected light reading from a non-18% gray value.

Bill Burk · Feb 21, 2023

I didn’t think that. A white target would certainly throw out my story point that an incident meter would agree with the spotmeter. But still the light meter was estimating the incident light according to what it wants to put on film.

I am sure interested in what took you down from 3,620,375.125 millilux seconds (your reading) to 2512 millilux seconds (my estimate).

Figuring it out involves K which we talked about but maybe never finished the class. I know I didn’t learn it well enough to be able to teach. All I can do is facilitate

p.s. The thread was “Is the K factor relevant to me or should I cancel it out?”

Doremus Scudder · Feb 21, 2023

I'm confused. A millimeter is 1/1000 of a meter (the prefix "milli" meaning 1/1000 of...), but a millilux is 1000x, not 1/1000 of a lux?...

Chuck_P · Feb 21, 2023

Bill Burk said:
. But still the light meter was estimating the incident light according to what it wants to put on film.

I'm hearing that my Pentax 1 degree spot meter is actually providing me with an incident reading.....another thing I'm going to have to try and wrap my head around. So there is light that is incident upon the target but the light entering the spot meter is reflected from the target. Is this an incorrect statement?

Bill Burk said:
I am sure interested in what took you down from 3,620,375.125 millilux seconds (your reading) to 2512 millilux seconds (my estimate).

The EV to Lux calculator I referenced earlier said that my 12 2/3 EV (that I placed on Zone X) was 14,400 Lux. So I wondered....... to get millilux, I multiplied by 1000? Then multiply millilux by the exposure time, which was 1/4 sec to get millilux seconds, which was 3,600,000? Then the log 10 (3,600,000) is 6.5? Then I said that's way out there on the x-axis........then someone said that's because I way over exposed. Well, I did, I did way over expose, by 5 stops, on purpose.......and so here we are.

I asked if I had those calculations wrong, don't believe anyone said no......I'm not saying it's right, I put it out there to ask if it was, so idk.

Bill Burk · Feb 21, 2023

It’s small. I multiplied by 1000 to get from a couple meter candle seconds to 2512 milli meter candle seconds.

It’s insane there’s no way a lens-camera is reducing the light to a thousandth

Maybe it’s supposed to be squared?

2,512,000 millimeter candle seconds?

If so, now we’re talking about 70%

Bill Burk · Feb 21, 2023

Chuck_P said:
I asked if I had those calculations wrong, don't believe anyone said no......I'm not saying it's right, I put it out there to ask if it was, so idk.

I think you’re right, but maybe we’re supposed to be squaring the 1,000 to get to milli since these are area-light

Sirius Glass · Feb 21, 2023

Chuck_P said:
I'm hearing that my Pentax 1 degree spot meter is actually providing me with an incident reading.....another thing I'm going to have to try and wrap my head around. So there is light that is incident upon the target but the light entering the spot meter is reflected from the target. Is this an incorrect statement?

The EV to Lux calculator I referenced earlier said that my 12 2/3 EV (that I placed on Zone X) was 14,400 Lux. So I wondered....... to get millilux, I multiplied by 1000? Then multiply millilux by the exposure time, which was 1/4 sec to get millilux seconds, which was 3,600,000? Then the log 10 (3,600,000) is 6.5? Then I said that's way out there on the x-axis........then someone said that's because I way over exposed. Well, I did, I did way over expose, by 5 stops, on purpose.......and so here we are.

I asked if I had those calculations wrong, don't believe anyone said no......I'm not saying it's right, I put it out there to ask if it was, so idk.

The Pentax spot meter is a reflectance meter, not an incident meter.

Chuck_P · Feb 21, 2023

Sirius Glass said:
The Pentax spot meter is a reflectance meter, not an incident meter.

Yeah, I know.

Converting EV to Lux to millilux-seconds

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