Discussion in 'Exposure Discussion' started by davidave, Jun 7, 2011.

1. ### davidaveMember

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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?

2. ### Mustafa Umut SaracMember

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I have no idea but may be he writes in metric terms ? Did you check for it ?

3. ### davidaveMember

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Yes. Definitely foot candles, not meter candles.

4. ### MaximusM3Member

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Just out of curiosity, as I have no idea of how this formula work, what would be the advantage of this versus taking a simple incident meter reading? Of course, it sounds like it could be valuable if one doesn't happen to have a meter and, like Ansel, be able to guesstimate luminance values.

5. ### davidaveMember

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I find it useful to be able to work in a luminance value as it is easier to figure out contrasts then go think about exposure after that. And you would still need a meter for a luminance reading.

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6. ### Christopher WalrathMember

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Slight increase of aperture, slight decrease in duration. 1/125. E=i(t)

7. ### UsagiMember

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Do you mean EV values as luminance value?
EV is easy - but c/ft^2, sounds complicated. Does any modern exposure meter even give option for unit like c/ft^2?

I wonder what is it's standard (metric) equivalent. The thing that always bugged me when reading Adams' books - the use of units that are not used in science.

8. ### DiapositivoSubscriber

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I presume at the time of Adams there were light meters around which gave you only the candles per square foot values, and you had to convert them in photographic values. It's the same measurement, if I get it right, expressed with a different unit of measure.

My Minolta spot meter F comes with a conversion table between EV at ISO 100, candles per square meter, and foot-lamberts. E.g. EV 14 is equivalent to 2300 cd/m^2 or 670 fL.

If you have to use a light meter, and you use it for photography purposes, then it's better to use one which gives you straight EV values, or maybe exposure "couples". I see no advantage in having a measure in another unit and then converting it to your purposes.

9. ### davidaveMember

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@Chris,

I'm not getting anything close to that. Try metering something that is 100FC at f/11 at 100ASA, not anywhere near 1/125.

10. ### Christopher WalrathMember

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Addendum. That will be a slight increase in exposure, of course. But that can be accounted for in printing.

11. ### Christopher WalrathMember

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Do you read in cm or cf? If you read in cm, you reading give over exposure? The formula is for candles per square FOOT. He based this formula upon the use of the Weston Master light meters and others such as that which provided c/ft2 on the dial of the meter.

12. ### davidaveMember

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Yes modern light meters use foot candles. I use a Sekonic 758C and it uses foot candles.

I hope someone who actually understand the formula can offer some advice instead of people trying to say it's not useful; you're just making it for harder for other people to find out the answer. Just because you don't find it useful doesn't mean it's NOT useful.

13. ### davidaveMember

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@Chris: Yes. CF on the Sekonic 758C.

15. ### DiapositivoSubscriber

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Ufff, OK.

I used this converter:
http://www.unitconversion.org/illumination/foot-candles-to-meter-candles-conversion.html

EV 14 = 213.676992 foot-candle = 2300 meter candle.

EV 14 = 1/125 @ f/11 at ISO 100 as we all know.

Formula: with square root of ISO, you use f/10.
The formula you post suggests 1/214.

That's not that off the mark. 1/214 @ f/10 instead of 1/125 @ f/11. The formula gives you an exposure which is a bit closer than what we would expect as normal. I reckon about 1/3 of a stop closer.

I suppose your problem is in your light meter which does not really give you candles per square meter, but something else. (Or maybe you should check if it is set at ISO 100).

Fabrizio

Parenthesis without polemic.

I still don't get the practical reason for this exercise. In photographic units each EV corresponds to "twice" the luminance. In meter-candle or foot-candle "twice" the luminance is expressed in "twice" the number. If you want to know the contrast of illumination of the scene, in foot-candles you just divide the two numbers and then derive the stops of difference.

For instance, if two light sources are measured as EV 14 and EV 11, you know there is a three stop difference.
If the same sources are measured as 1600 fc and 200 fc, you divide 1600 by 200, obtain 8, know that 2^3 = 8 so there is a 3 stops difference.

EDIT the equivalence above between meter-candles and EV is taken from my light meter's instruction booklet, so that might be a bit approximated and the formula in foot-candle might be a bit closer to the "normal" exposure value.

EDIT maybe you are using your lightmeter with the dome in front of the cell? If I understand it right, this kind of measurements are reflected light measures.

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16. ### davidaveMember

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Thanks for the example. I will continue to do more tests under different conditions.

17. ### MaximusM3Member

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I'm sure it's useful in some strange way but at the end of the day, you're just making it harder on yourself to go out, take that picture by simpler means and turn it into a masterpiece in the darkroom. Of course this is just my humble opinion and not meant to offend or lecture anyone.
Again, I certainly don't have the answer for you but my question remains: aside from personal preference, endless thirst for knowledge, and the need for various forms of sadomasochism and torture, why would anyone want to complicate matters to achieve the same results, which depending on someone's overall skills can be utter crap or genius?
This is more of a general question but I do often wonder...

18. ### Mustafa Umut SaracMember

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I think answer is to try to walk in shoes of great master. If you want to learn painting , you might able to copy a masters painting. Of course you can buy a print but as Shaw says unreasonable people makes the progress.

19. ### MaximusM3Member

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There is of course validity to that but I don't believe it lies in emulating/replicating the strictly technical aspects of photography, but in studying the creative side of an artist's point of view, while nurturing and progressing one's own. I constantly see too many people getting wrapped in technicalities and getting lost in endless tests and quests for magic bullets. But I digress here . Sorry..

20. ### ic-racerMember

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I believe the S.E.I. has a scale that reads in foot-candles, thus accounting for the incorporation of that unit of measurement into Ansel's heuristic.

21. ### DiapositivoSubscriber

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I understand that in general, but in this case the great master might have substituted the figures on the dial of his light meter to read in EV instead of foot-candle. It really is only the painting of the dial that changes.

If the dial is painted in EV, you make a subtraction.
If the dial is painted in foot-candles, you make a division and then extract the base-2 logarithm.

The result is the same, but one of the two cases is more immediate

Adams should have bought a light meter with the proper dial or have it rewritten with the "proper" scale.

EDIT: I read now on Wikipedia the EV concept (I would say convention) was developed in the '50 in Germany. That would mean that before that epoch light meters did not indicate EVs. I find EV the best invention since the iron thread to cut eggs. I memorize and read on the instruments EV15, EV12 that's much easier to remember. EV 12.3 is easier to memorize than 1/125@f/5.6 + 0.3 stop.

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22. ### Steve SmithSubscriber

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Light meters have never indicated EV (or if they claim to, they were mistakenly marked). EV is Exposure Value and relates to a combination of aperture f No. and shutter speed. LV or Light Value is exactly that, the Value of the level of Light.

Any meter marked up as EV is actually measuring LV

With ISO 100 film, the EV required is the same number as the LV measured.

I suspect that at one time, Mr Adams had a meter which only measured in foot candles so this system was perfectly workable for him. Now there is not much point as you have to use a meter either way so it might as well be what we now consider to be a normal photographic light meter.

EDIT: Just to take issue with myself. When I wrote "Light meters have never indicated EV" that was not 100% correct. A lightmeter can indicate which EV to set the camera to if it has a means of entering the ISO value but it does not give an EV level as a measure of light.

Steve.

23. ### DiapositivoSubscriber

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Yes I agree, that's why in post #8 I specified "EV at ISO 100". In fact, the use of LV instead of "EV at ISO 100" as a measure of illuminance is not really standard as well:

http://en.wikipedia.org/wiki/Exposure_value
where is says:

Recently, articles on many web sites have used light value (LV) to denote EV at ISO 100. However, this term does not derive from a standards body, and has had several conflicting definitions.

I agree that LV is faster - and more sensible - than saying "Ev at ISO 100" but it's not widespread.

24. ### Bill BurkSubscriber

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I think I know the root of the problem.

Sekonic L-758C reads directly in foot candles (Incident with dome retracted, Advanced function 16 setting 2)

Weston Master scale is labeled with brightness units in a series similar to foot candles but I believe it is an arbitrary scale about 4 stops higher.

My Sekonic L-28C is a true incident meter with a scale that claims "foot candles" on the faceplate. Setting ASA 64, rotate dial L indicator to 64, the computer recommends f/8 between 1/2 and 1/4 second.

But on a Weston Master if you try to dial in the same number (for example 50 which is close to 64) you get a recommended combination of f/stop and shutter speed that is four-stops higher. For example setting Exposure Index 64, rotate arrow to 50, computer recommends f/8 at 1/50.

Ansel Adam's Exposure Formula is not valid for real foot candles. It is valid for the old Weston Master meter brightness units instead.

p.s. SEI has a scale for Log Foot Lamberts

25. ### Chan TranMember

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1. Many meters do readout in EV not LV.
2. Ansel Adams was talking about cd/ft^2 ( a luminance measurement ) not Foot-candle which is an Illuminance measurement.
3. 100 cd/ft^2 = 1076 cd/m^2 which is equal to LV 12.9 with a K factor of 14 or LV 13.1 with K factor of 12.
4. Ansel exposure setting of f/10 @ 1/100 which is EV 13 so his formula is right on the money.
5. His method is only applicable if your meter only measures in cd/ft^2 otherwise it's easier to do it with EV or direct aperture and shutter speed readout.

26. ### Chan TranMember

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The Sekonic 758c can make measurement in foot-candle but not candle / foot ^2.
the ft.cd is an illuminance measure or incident. the cd/ft^2 is a luminance value or reflective reading. It does however readout in cd/m^2 and foot lambert (FL).
I believe the confusion between foot candle and candle per foot squared led the original poster to come up with the exposure time of 1/8 second.