Ansel Adams Exposure Formula?

[Bill Burk]

Chan Tran,

I think you got it.

Luminance : candela/square foot

Illumination : foot-candle

So the old Weston meters do show candela/square foot. You really need a convenient way to get candela/square foot to really enjoy the Ansel Adams Exposure Formula. Old Weston meters would be a way to go.

The L-758C can't be programmed to display candela/square foot. It can display foot-lamberts, but then you would have to convert to candela/square foot which would take some of the fun out of it.

 

[2F/2F]

It's an $800+ light meter. You shouldn't have to do any $#!+ remotely like that to find an exposure or a luminance range.

Foot candles are no better or worse for figuring out luminance range than are f stops. Both are symbolic/arbitrary when used for this purpose anyhow. You don't need to know how much light there is when measuring a luminance range. You just need to know how far apart lights are from each other. One full step on a meter with either an f number scale or a foot candle scale means the same thing: either a doubling or a halving of light intensity read by the meter. You don't even need numbers. All you need are hash marks showing the full steps.

What does make visualizing luminance range much, much easier is having an analog scale with a full ring-around of equivalent exposures. If this is how you plan on using your light meter, sell the $800+ one while it still has value, get a Studio Deluxe used for under $100, and enjoy $700 worth of hamburgers or something.

 

[Chan Tran]

It's an $800+ light meter. You shouldn't have to do any $#!+ remotely like that to find an exposure or a luminance range.

Foot candles are no better or worse for figuring out luminance range than are f stops. Both are symbolic/arbitrary when used for this purpose anyhow. You don't need to know how much light there is when measuring a luminance range. You just need to know how far apart lights are from each other. One full step on a meter with either an f number scale or a foot candle scale means the same thing: either a doubling or a halving of light intensity read by the meter. You don't even need numbers. All you need are hash marks showing the full steps.

What does make visualizing luminance range much, much easier is having an analog scale with a full ring-around of equivalent exposures. If this is how you plan on using your light meter, sell the $800+ one while it still has value, get a Studio Deluxe used for under $100, and enjoy $700 worth of hamburgers or something.

Well, if you are truly interested in measuring luminance then there are luminance meter like that of Konica Minolta (KM still makes and sell them unlike the exposure meters) LS100 or LS110 for only around $3,500 each. Your choice.
What I meant is that to accurately measure luminance requires a much more expensive meter than an exposure meter.

 

[2F/2F]

What I meant is that to accurately measure luminance requires a much more expensive meter than an exposure meter.

I was responding to the OP's stated reasons for wanting to read in foot candles because he thought it was better for determining a luminance range. It was not in response to anything you wrote.

The best tool on that meter for determining the brightness range of a composition is the spot meter, set at it's narrowest setting. As long as you aren't also using it to determine exposure, the actual units used are irrelevant. Only the number of steps between the measurements are meaningful.

 

[Bill Burk]

Foot candles are no better or worse...

I know you're point is about measuring the range. That's fine.

I just want to reiterate what Chan Tran pointed out to me today that what messed me up was confusing Candela per square foot versus foot candles.

Reflected light is measured differently than incident light. You can't convert between the two.

(If you arrive at a difference around 4 stops you probably confused reflected vs incident measures.)

But this thread has also revolved around the question... Why bother?

I would say Ansel Adams' Exposure Formula was meant to be a practical way to get to the point where you can predict your f/stop and shutter speed before you pull out the meter because you instinctively know what the meter will read anyway.

EV or LV numbers aren't as practical for that game. I would recommend picking up a Weston Master II III or IV meter if you want to experiment with Ansel Adams' Exposure Formula.

 

[Diapositivo]

I suppose in Adams times the candle/square foot reading from the light meter was converted into and exposure value through the use of same table, disk, etc. Probably the formula, more than a way to avoid use of the light meter, was a way to avoid the use of a disc calculator, making the calculation mentally.

 

[markbarendt]

The stated intent of the formula was that it "facilitates rapid exposure calculation mentally" but still, one must take a reading and that reading gets dropped into a formula.

The problem Adams was trying to avoid was "setting dials on meters".

The meters he was working with (the tools available when the book was written) didn't spit out "proper" camera settings like modern electronic meters.

 

[Steve Smith]

If you search 'light meter' on ebay you will see a few meters which are not for photographic use but give a light level reading for industrial/business use such as installing office lighting. This formula could be used with one of those meters if it measures in the right units.


Steve.

 

[Bill Burk]

... but still, one must take a reading ...

The Exposure Formula is associated with the quote "Chance favors the prepared mind," so I take it to mean that knowing the formula and using it can help in a pinch when a meter fails or is unavailable.

... light level reading for industrial/business use such as installing office lighting. This formula could be used with one of those meters ....

No because these meters return incident candlepower measurement units and the formula is tied to reflected candlepower measurement units.

I don't assert that the four stop difference is a constant. But can anybody say? Maybe it is just that.

 

[Arvee]

Just happened to be playing with this too. I took my Weston Master III outside with the Invercone installed and took an incident measurement. Here in UT sunny 16 is really sunny 16.66 and the Weston measured 350. So, with an ISO of 400, the square root is 20 (which translates into f16.66 conveniently enough!) and the shutter speed would be 1/350. Normalized, that would be f16 at about 1/600 shutter speed, which is absolutely right on the money! So, the magic number in UT is 350 on the Weston III. I also have a foot candle illuminometer which tells me that a bright sunny day is around 12-13K foot candles, depending on atmospheric conditions.

I don't know what the units are on the Weston (I have heard it is foot candles and my Sekonic 398A has a scale in foot candles) but both the Weston and the 398A say a bright sunny day in UT will be very close to 350 'units' (actually the Sekonic measured about 400). I am sure AA was using a Weston to derive his formula and it works perfectly on my Sekonic and Weston.

The Sekonic is dead on (at 400) and I think that the Weston (mfd. in the 50s) may be slightly tired. But, of course, we don't know if the units on the Weston are indeed the same units on the Sekonic, similar, yes.

HTH,

PS. I have gone to the Sekonic website and downloaded their EV/Lux/FC conversion chart and was able to correlate EV numbers to their FC numbers on my Sekonic 398A and the Sekonic Illuminometer.

 

[markbarendt]

The Exposure Formula is associated with the quote "Chance favors the prepared mind," so I take it to mean that knowing the formula and using it can help in a pinch when a meter fails or is unavailable.

Ansel's formula requires a reading. No reading, no answer.

This isn't a rule of thumb thing like sunny 16, or an exposure table.

 

[Steve Smith]

I don't know what the units are on the Weston (I have heard it is foot candles and my Sekonic 398A has a scale in foot candles) but both the Weston and the 398A say a bright sunny day in UT will be very close to 350 'units' (actually the Sekonic measured about 400).

If you set the Weston to ISO 100 (or Weston 80) and set it for a reading of 400 it will give the standard sunny 16 combination of f16 and 1/125 so perhaps the Weston is scaled in foot candles.


Steve.

 

[Bill Burk]

Ansel's formula requires a reading. No reading, no answer.

You are right. I gave Ansel's formula another reading. He only gave one example where the moon was 250 candela per square foot. Now that is a reasonable constant brightness that could be worth memorizing.

And he reminds that estimating exposures from memory like this and Sunny 16 are emergency measures unlikely to result in a fine print.

So taking meter readings is important.

 

[Terry Freeman]

So in p.66 of Book II, Ansel Adams outlines his exposure formula:

Square root of your ASA give you the key stop
Take your meter reading in c/ft^2 (foot-candles) and the inverse is the shutter speed at the key stop.

So if shooting ASA100, the key stop would be f/10. And if luminance value is 100FC, then the exposure is 1/100.

I'm trying to get this to work with my Sekonic 758C. I'm not coming in close. At f/10 and 100FC with ASA 100, my exposure reading is 1/8 second. Even taking into account a K factor, this is orders of magnitudes off.
What am i doing wrong?

***
Resurrecting a dead old thread because I think I've figured this out - I was on the bus to work reading the self-same section of "The Negative" - the whole point of AA's Exposure Formula means that if you know the Luminance in candela/ft^2 (experience, exposure tables, guesstimation) and you know your Key Stop, you can estimate exposure, which is how AA got any shot at all of "Moonrise, Hernandez NM" - his meter had gone missing but he just happened to know the luminance of the moon in cd/ft^2.

So, first thing - everyone I've seen who tried to replicate this was using their meter in INCIDENT mode - but this is a luminance calculation, so it should be a reflective reading. So, I put the Lumigrid on my Sekonic L-398, which measures in cd/ft^2 and took a reading. When you use the lumigrid, you don't use the High Slide, but you take your reading off of the red H indicator per Sekonic's instructions. When I took the reading like that and plugged it into the Exposure Formula, my calculated exposure was within 1/3 of a stop of the reading on the meter once I spun the dial to the H indicator.

It was a quick and dirty test and I'll do more later to make sure it works, but I think I've solved it.

Oh yeah - hi! I'm a long-time lurker and this is my first post...

If anyone tries this and it turns out I'm wrong, please respond - I don't see a big need for this EXCEPT for one: if my light is changing fast and I can save time by not spinning my meter's calc dial and interpreting it, I can just look at the Incident reading directly off my dial in cd/ft^2 and I know my shutter speed at my key stop, so I can make faster adaptations to changing luminance.

Yeah, I'll probably never use it but it'll be nice if I can just say I honestly understand what Ansel was saying in that chapter of his book. I've seen a lot of posts about the Exposure Formula and it always peters out into "it's useless anyway, just read your mete!" and "I guess old meters just worked differently back then."

No, you just need a reflected reading in the appropriate units. I may be able to use it for pinhole photography as well...

Have a good day!

Terry Freeman
Portland, OR

 

[juan]

40-years ago, I had a meter that read candle/square-foot and the formula worked very well. I believe that at one time there were calculations that allowed you to interpret candles/square-foot from other meters, I believe the Pentax analog spotmeter, but since graduating to modern incident meters, I haven't thought about this in years. All I can say for certain is that the formula worked very well with a meter that read candles/square-foot. You are right, Adams formula applies to reflected light.

 

[Jim Jones]

Fred is right in post 35. After losing the dial from my Weston III long ago, I continued to use the meter by the simple calculations that Fred posted. In a 1957 TV documentary, Adams is shown loading his car with a SEI meter and two Westons (for backup), among much other gear.

 

[Alan Johnson]

This is just my version, IDK if it is correct:
My Weston Master II was produced before nominal film speeds were doubled so for 100 ASA film I set it to 50 Weston.
Then square root of 100 ASA = f10.
As a check I set sunny f16 = 1/100 second on the meter.
The scale reads then reads 400 so sunny f16 is given as 1/400 at f10, which is close.
This would be for a reflective reading.
Mr Butkus provides instructions for an invited donation:
http://www.cameramanuals.org/flashes_meters/weston_foot_candles.pdf

 

[Chan Tran]

I learned a lo from Adams book but his exposure formula isn't what I care for. It involves having to do square root which for me quite difficult. Instead of remember the moon is 250 foot.candle (using Adams term) I much rather remember that it's 14.3LV. That is EV14.3 @ ISO 100. From then on I only need to do add and subtract.

 

[Alan Johnson]

It seems likely that Ansel Adams may have used a Weston Master III exposure meter as this was the first to be calibrated in ASA. IDK about the Weston Master IV.
The Weston Master V had a scale calibrated in EV and not the candela/sq ft of the Master III.
http://www.westonmeter.org.uk/Chronology.html

 

[Alan Johnson]

I found by experiment that if the Weston Master II is set to film speed Weston 40, which would be the equivalent of ASA 100 before nominal fim speeds were doubled, the scale does indeed read the correct shutter speed for f10 (f9.5 actually) for the reflective metering for 100 ISO film according to my Canon SLR meter.

 

[Bill Burk]

I learned a lo from Adams book but his exposure formula isn't what I care for. It involves having to do square root which for me quite difficult. Instead of remember the moon is 250 foot.candle (using Adams term) I much rather remember that it's 14.3LV. That is EV14.3 @ ISO 100. From then on I only need to do add and subtract.

You only need to Square Root your EI... and the series relates directly to f/stops so ... you should know a few members of the set.

f/5.6 -> EI 32
f/8 -> EI 64
f/11 -> EI 125
f/16 -> EI 250

 

[Bill Burk]

I found by experiment that if the Weston Master II is set to film speed Weston 40, which would be the equivalent of ASA 100 before nominal fim speeds were doubled, the scale does indeed read the correct shutter speed for f10 (f9.5 actually) for the reflective metering for 100 ISO film according to my Canon SLR meter.

I think this is a non-sequitur... "before nominal speeds were doubled" affected the film speed ratings, not the computation of f/stop and shutter speed.

I'm looking at my Weston Master II and Weston Master III side by side. They only differ by 1/3 stop in their markings. This difference is a "method" difference.

I find the Weston Master III follows the Ansel Adams Exposure Formula exactly, and if you just remember the key stop for your EI, you set your shutter speed to the number the needle points at... that's as simple as it gets.

 

[Bill Burk]

And... after a while working with a Weston Master, you can (in daylight scenarios), get pretty good at guessing what number the needle will point at. The immediate feedback... increases your confidence.

That's when you get into the happy place where you can guess your exposures pretty well.

 

[Bill Burk]

Of course now I have to come to grips with my new discovery that the Weston II and III are only 1/3 stop different from each other. I had written 2/3 stops different in many posts and on my meter itself... Wonder where I got that from, will have to hunt for my old posts.

 

[Chan Tran]

You only need to Square Root your EI... and the series relates directly to f/stops so ... you should know a few members of the set.

f/5.6 -> EI 32
f/8 -> EI 64
f/11 -> EI 125
f/16 -> EI 250

This is my approach if I know the moon is LV14 1/3
subtract 2/3 from the LV to get EV for ISO 64 so I get EV 13 2/3
subtract 2 EV from the value because he placed it on Zone VII and not Zone V so I get 11 2/3
subtract 1 2/3 EV from the value for the 3x filter so I get 10
subtract the value for f/32 which is 10 then I get 0
0 stands for a shutter speed of 1 sec.