### Converting conventional exposure values to LogE

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Greetings,

Does anyone know how to convert conventional exposure units (i.e. 1/60 @ f11) to Log Exposure units?

Regards,

-- Pete Caluori (pcaluori@hotmail.com), November 02, 2000

Pete, you don't have any units in here that will directly link together. Depending on what you're trying to accomplish, possibly the best (easiest) way to go is to use a relative log exposure scale. That is, just make up a value for a certain exposure. For example, say the log exposure scale is in "Pete Caluori units", where you define how many units at a certain exposure. From there, each doubling of exposure increases the log exposure Pete units by 0.301.

For example, maybe you would call that 1/60 at f/11 exposure 0 log E Pete units. In that case, 1/30 at f/11 gives 0.30 log E Pete units, 1/15 at f/11 gives 0.60 log E, etc. Decreasing exposure goes the other way to -0.30, -0.60, etc.

I can't think of any reason why you need absolute units for this except to actually measure an ISO film speed. In that case, you'd pretty well need a sensitometer with known power output. If you just wanted to compare speeds of different films, the relative method (using log E "Pete units") would work just fine. You can also plot sensitometric curves, etc

Will this scheme get you where you want to go?

-- Bill C (bcarriel@cpicorp.com), November 02, 2000.

I'd agree with the previous post that you're probably better off working with some arbitrary relative log exposure scale, if it's simply a question of doing your own sensitometry tests.
However, it should be perfectly possible to convert back from the light falling on the film, in Lux-seconds, to a real world subject brightness. The camera exposure is only one step in this process, and 1/60th @ f/11 doesn't mean a great deal on its own, except as an indication of the amount of attenuation that the subject brightness has suffered on its way to the film. A rough rule of thumb seems to be that the light falling on the film plane is equal to f number squared times 0.2.

Without knowing what you want to arrive at, subject brightness, film density or whatever, there's no point in expanding on this simple relationship.
I have dug up some useful links that should give you all the information you need to know. See:
Exposure and subject brightness
Lens efficiency
and General sensitometry
These will probably tell you all you need to know.

-- Pete Andrews (p.l.andrews@bham.ac.uk), November 03, 2000.

Ooooops!
The last sentence of that first paragraph should have read: A rough rule of thumb seems to be that the film brightness is equal to the subject brightness times 0.2, divided by the f number squared. (This doesn't take into account the absorbtion and reflection light losses in the lens)

-- Pete Andrews (p.l.andrews@bham.ac.uk), November 03, 2000.

Thanks for the info guys!

My purpose for wanting to convert exposure to logE is for plotting film curves. I can read the density of the film so I just need to relate it to the LogE, but if I'm understanding what you guys are saying, I can use arbitrary, relative units.

Thanks!

Regards,

-- Pete Caluori (pcaluori@hotmail.com), November 03, 2000.

>>Bbut if I'm understanding what you guys are saying, I can use arbitrary, relative units. <<

Exactly! You can even compare films against each other as to the speed difference factor, etc, as long as you exposed both under the same condition. And you're welcome!

-- Bill C (bcarriel@cpicorp.com), November 03, 2000.

Hmmmm! Looks like that rule of thumb formula I gave a bit earlier might be a bit out.
I did some tests last night, taking readings from a lightbox surface using a photometer, and then from the film plane of a Nikon fitted with an f/1.2 lens, pointed at the same self-luminous surface.
It looks like the 20% figure given is a bit low. I was getting readings closer to 30%, and I tend to believe me, and my photometer, rather than some spurious figure off the web.
I now think the formula for film plane illumination should be:
Subject brightness x 0.3, divided by fnumber squared.

BTW if anyone knows the correct, rather than empirical way to derive this, please let me know.

-- Pete Andrews (p.l.andrews@bham.ac.uk), November 06, 2000.

#### Converting conventional exposure values to Log E

I think the tie-in that you are looking for is to plot the densities against the Exposure Index values which were used in making the exposures for which you are measuring the density. The EI is a direct measure of how much light is reaching the film. It should be in logarithmic units, like the old DIN film speed ratings. This is explained in more detail in my website www.vsta.com/~alrob

-- Al Robinson (alrob@vsta.com), November 19, 2000.

I wrote an article for PhotoTechniques magazine (Jan-Feb 2000) that details the relationships between all elements of exposure control in "logE" form. The advantage is you can do all your calculations by adding or subtracting. There's not enough room to explain it all here; besides, it's a great magazine. I believe they have back issues still available. If not, contact me.