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Photography Question 
Tim Cancila

EV vs. f-stop

What is the difference between an EV and an f/stop? I have a Canon A2E which allows me to make +/- EV adjustments after the initial meter reading, but how does this differ from an f/stop?

I've read in my A2E manual that the camera supports 0-20EV. This makes me think it is a scale of light values, but how does this (if at all) correspond to an f/stop?

Does opening up a stop = +1 EV?!?

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John A. Lind
BetterPhoto Member Since: 9/27/2001
The answer is YES.

Going to +1 EV increases the amount of light reaching the film by 1 EV and is the same (by definition of EV) as opening the lens up by a stop.

EV = Exposure Value

Your camera manual mentions it is capable of metering 0-20 EV, and probably mentions this is for ISO (or ASA) 100 film speed. It is the specification for the sensitivity range of your camera's meter.

The definition of EV:
EV is the "Additive Photographic Exposure System" with two equal definitions:

EV = Av + Tv
Av = Aperture Value (from the f-stop)
Tv = Time Value (from the shutter speed)
Aperture of f/1 has an Av=0, and an exposure time of 1 second has a Tv=0

EV = Sv + Bv
Sv = Speed Value (from film speed)
Bv = Brightness Value (how bright the lighting is)
ASA 3 has an Sv=0, and 1 foot-Lambert of light brightness has a Bv=0

Both definitions are "equal" to each other. It is how the brightness of the lighting and film speed translates to an aperture f-stop and shutter speed for a proper exposure. Change the aperture by one stop and you change the EV by one. Double or halve the shutter speed and you change the EV by one. Double or halve the film speed and you change the EV by one(for a proper exposure).

How to find the Av, Tv, Sv and Bv numbers:
They are calculated using base two logarithms.
Av = log2(A^2) where A = f-number
Tv = log2(1/t) where t = shutter speed in seconds
Sv = log2(ASA/3) where ASA = film speed
Bv = log2(B) where B = light brightness in foot-Lamberts

You can probably find everything except Bv with a calculator, unless you have a very sophisticated light meter that measures light brightness in foot-Lamberts.

Exposure setting example:
Find the EV of f/8 at 1/125th second.
Av = log2(8^2) = log2(64) = 6
Tv = log2(1/(1/125)) = log2(125) = 7
EV = 6 + 7 = 13

For the extreme ranges of your camera with highest speed Canon lens, a 50mm f/1, the EV exposure range you can handle are:
Av for f/1 = 0
Av for f/16 = 8
Tv for 30 seconds = -5
Tv for 1/8000 = 13

Wide open at slowest speed:
EV = 0 + (-5) = -5
Stopped down at fastest shutter speed:
EV = 8 + 13 = 21

Thus, if you put the absolute fastest lens you can (the 50/1) on your A2E, it has an EV range for setting exposures of -5 to 21 (spans 26 stops), which exceeds the 20 stop metering range! The most confusion about EV comes from thinking that EV is the level of lighting alone.

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