Determining Lux-seconds to exposure

[drazak]

I'm attempting to determine how many lux-seconds are required to get middle grey on an ISO 100 film(or 125, for plus-x). I'm not entirely certain how to calculate lux-seconds needed from film speed.

Thanks,
Ben

 

[Kirk Keyes]

What kind of meter do you have? An incident meter would make this easier.

 

[ic-racer]

Your speed point (Zone I) will be ISO = 0.8/(LUX-SECONDS), so add 4 stops of LUX-SECONDS to that (to get to Zone V). Since the ISO is arithmetic rather than logarithmic, I believe you would multiply by 4, perhaps someone else can confirm that.

There is another rule of thumb that might get you there quicker. I'll just quote from Ansel:

Zone V exposure is the reciprocal of its luminance (in foot candles) representing the shutter speed in fractions of a second, at an aperture number equal to the square root of the film speed. So, with a film speed of ASA 64, f8 would be the aperture, a luminance of 30 footcandles placed on zone V would be 1/30th of a second.

(One footcandle is equal to 10.76 Lux, also realize Adam's rule of thumb uses Lux, not Lux-Seconds.)

So, with a sensitometer you need .008 Lux-Seconds of exposure on your ISO 100 film to make a Zone I exposure and a .128 Lux-Second exposure for Zone V (per the first equation).
For in-camera considerations, the second equation tells us at F=10 with a 1/100th of a second exposure you need a subject of 100 foot candles (or 1076 Lux) to make the Zone V exposure on the film. So, in SI units that would be 1076 Lux times .01 seconds to give 10.76 Lux-Seconds. Realize that in this second case the light is passing through an aperture of F10 before it hits the film.

 

[RJS]

My recollection is that Lux and/or foot candles is a measure of light falling on a subject. This you would measure with an incident light meter. An EV of 9 measured with such would require an exposure of 1/8 at f8 with a film speed of ISO 100 to render a grey card approximately 12% to 18% grey, depending on the K factor used in calibrating your meter. I think this exposure would get you close, but variables such as developer, gamma to which your film is developed, (usually 'normal' meaning a gamma of approximately .56), inaccuracies of shutter speed and aperture markings (not to mention the phase of the moon, stage of the tide and additional unknowables) will cause some variation. I do not believe anyone can do more than recommend an exposure that will get you close. I would be very interested in hearing if my recommendation is close, as I am not able at present to test this myself. You are the recipient of my best effort, such as it is. Of course the paper you print on, the paper developer and contrast grade are additional variables. Your question is not a simple one, or at least the answer is not simple. I am taking Lux-fc equivalencies from the table on the back of my Luna Pro meter wherein 55 Lux equals 5 fc. equals EV 6. Good Luck!

 

[drazak]

Kirk;

I have a minolta IIIf meter.

I'm going to be running some experiments on the components of developers and how temperature affects the results of the developer. I guess maybe I should just measure the exposure of the film in lux, seems much easier. I had however, read a paper where it was measured in lux seconds, and I'm not sure why it was.

Ben

 

[RJS]

Oops! I left out the decimal: 5.5 Lux equals .5 fc. Sorry.

 

[Kirk Keyes]

Check the owners manual for your meter and it should have a table - my Minolta Flashmeter IV and VI both have an EX/EV to lux conversion. Then you multiply by the shutter speed to get lux-seconds.

10 lux * 1 sec = 10 lux-seconds and so on.

 

[Maris]

A good approximation for Zone V film exposure in Lux.seconds is to take the reciprocal of the film ISO speed and multiply by 10.

For example a 100 ISO speed film requires 0.1Lux.seconds while a 400 ISO film needs 0.025Lux.seconds of light for a middling exposure.

Remember, Lux is a unit of intensity but the quantity of light, Lux.seconds, is intensity multiplied by time. Actually measuring the intensity of light at the film plane requires you to put a Lux-meter there; not always an easy thing to do particularly with a miniature camera.

An easy work-around is to remember that placing the ordinary reading from a reflected exposure meter on Zone V means that the subject will send the film 10/ISO Lux.seconds worth of light.

 

[PascalMiele]

Hello,

in HD curves the "Qty of light" in in lux.s.
It's not a measurement of the light of the subject but ON THE FILM.

1) you can measure the light on the film plane, with a luxmeter (it's not easy) and you multiply by the time ==> lux.s (CQFD)

2) a second solution not so bas and easier
measure the luminance of the subject in cd/m2, you need a posemeter or better a spotmeter and a conversion table EV to cd/m2
apply the "equation de transfert simplifiée" (the french version, in english "simplified transfert equation" I suppose !)
H (lux.s) = 24 * Time (seconds) * L (cd/m2) / Aperture

3) The most important part
Ask : is it really necessary to have lux.s ?
Curves are not allways the best tool for analysing results !

Pascal

Excuse my poor english !

 

[ic-racer]

I believe you would multiply by 4,

I mean 16.