Foot candles to EV units??

irvin wilson

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Jul 29, 1997, 3:00:00 AM7/29/97

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I have a couple of old Speed Graphics I like to shoot with. I've been
able to manage fine without a meter after reading an article on the
"Sunny 16" rule, bellows extension and all. Just came across a
Sekonic "Handi Lumi" whichs measures light in foot candles. Is there
a conversion formula for EV values??

Using meter in direct sunlight gets me a starting point as I already
know exposure. Would just like to know if there's easier way.

Thanks,

Irvin Wilson
iwi...@mindspring.com

Bruce Barrett

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Jul 31, 1997, 3:00:00 AM7/31/97

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Hi!

At the bottom of this nice page by Fred Parker:
<http://www.fredparker.com/ultexp1.htm>

we find the following...

> RELATIONSHIP
>
> Now for the fun part. Is there any intuitive relationship between EI,
> Footcandles and LUX? The answer is "sort of". EV and footcandles both
> start out at the same place. EV 1 = 1 FC. From there, EV progresses
> linearly (1,2,3,4.....) with each number representing twice the amount of
> light as the previous number. Footcandles start out at 1, but progress in
> a doubling manner (1,2,4,8,16,32.....) which better represents the
> doubling of light at each step.
>
> <useful table deleted>

So, footcandles = 2^(EV-1)
EV = LOG2(footcandle) - 1

-- Bruce Barrett You can reach me
-- Apple Computer, Inc. at: bbar...@best.com
_ : http://www.best.com/~bbarrett
[_]< View cameras, f22 and
/|\ be there...20 minutes early.

MPinker122

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Aug 3, 1997, 3:00:00 AM8/3/97

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There are a number of phot books that have conversion scales. One is The
Amature Photograper's Handbook by Aaron Sussman. My copy is the 8th
edition and the conversion tables are there.
With this table one may be able to use the "slate rule"

S+L=A+T

S stands for speed, L for light, A for Aperature and T for time. Best to
read pages 178-180 in this book. There are others. A nother edition would
not matter.

Mary Ann

Martin Knowles

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Aug 9, 1997, 3:00:00 AM8/9/97

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Yes, you can use S+L=A+T=E, but ONLY if your ISO/EI, shutter speed and
aperature have been converted to Sv, Lv, Av and Tv--so the formula is
actually Sv+Lv=Av+Tv=Ev. According to Compton/Current/Stroebel/Zakia's
_Photographic Materials and Processes_, the following conversion factors
can be used:

Av = Log2(f-stop)^2
Tv = Log2(1/shutter-speed)
Lv = Log2(footcandles/6)
Sv = Log2(ISO/3)

You can then plug them in to the S+L=A+T=E formula. Note that you only need
to use two terms of this formula at any given time--for instance, if you
know the footcandles and the ISO/EI of your film, you can either add the Lv
and Sv to find the EV, or you can find either shutter speed or aperature if
you know one but not the other.

This system in some books is known as "APEX" -- the Additive system for
Photographic EXposure.

Actually applying this process in the field either by hand or by
conventional scientific calculator is a royal pain in the butt. It's best
to program these formulae into a programmable calculator (I've done it on
my TI-82 and my TI-92). For those of you who actually remember anything
from high school algebra, you have to use the change-of-base law to convert
the base 2 logs into base 10 logs that your calculator can stomach.

If anyone is interested in the program for the TI-92, I'll be willing to
post it.

|| Martin Knowles / Lightning Systems
|| martin_...@halcyon.com
|| Software, Publishing and Multimedia Design Consulting

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