Converting conventional exposure values to LogE

[pete_caluori]

Greetings,

 

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

 

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Regards,

[Bill C]

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.

 

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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.

 

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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

 

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Will this scheme get you where you want to go?

[pete_andrews]

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.<br>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 <i>f</i> number squared times

0.2.<p>Without knowing what you want to arrive at, subject brightness,

film density or whatever, there's no point in expanding on this simple

relationship.<br>I have dug up some useful links that should give you

all the information you need to know. See:<br><a

href="http://www.quantel.com/digfilm/basfilm.htm">Exposure and subject

brightness</a><br><a

href="http://www.cctv-information.co.uk/constant/lightrans.html">Lens

efficiency</a><br>and <a

href="http://www.kodak.com/country/US/en/motion/programs/student/handb

ook/sensitometric2.shtml"> General sensitometry</a><br>These will

probably tell you all you need to know.

[pete_andrews]

Ooooops!<br>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 <i>f</i>

number squared. (This doesn't take into account the absorbtion and

reflection light losses in the lens)<br>Sorry about that. I hope this

has made things clearer.

[pete_caluori]

Thanks for the info guys!

 

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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.

 

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Thanks!

 

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Regards,

[Bill C]

>>B�but if I'm understanding what you guys are saying, I can use

arbitrary, relative units. <<

 

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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!

[pete_andrews]

Hmmmm! Looks like that rule of thumb formula I gave a bit earlier

might be a bit out.<br>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.<br>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.<br>I

now think the formula for film plane illumination should

be:<br>Subject brightness x 0.3, divided by <i>f</i>number

squared.<p>BTW if anyone knows the correct, rather than empirical way

to derive this, please let me know.

[alfred_robinson]

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

[andy_eads3]

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.

[andy_eads3]

Try this: Aperture Value (AV)series - AV(f1.0)=0, AV(f1.4)=1,

AV(f2)=2, AV(f2.8)=3, AV(f4)=4, AV(f5.6)=5, AV(f8)=6, AV(f11)=7...

Time Value (TV) series - TV(1sec)=0, TV(1/2sec)=2, TV(1/4sec)=3,

TV(1/8sec)=4, TV(1/15sec)=5, TV(1/30sec)=6....

Exposure Value (EV). EV = AV + TV. The camera settings 1/30sec @ f2

is AV + TV for the respective settings. EV = 6(TV)+2(AV) = 8. To keep

the same exposure but change the shutter speed, the formula is

AV=EV-TV. Let's change the shutter to 1/2sec. AV=8-2= 6; AV6=f8

This is the old Exposure Value system still found on many light

meters. Have fun!