EV conversion to F stops

Exposure value (EV)is a range of f-stop, shutter speed combinations by a single setting that yield the same exposure. "For instance, the exposure value 13 gives a range of aperture/shutter speed combinations- e.g., 1/500 sec. at f4, 1/250 sec. at f5.6, or 1/125 at f8- each of which gives the same amount of exposure. A scale from 2-18 is sufficient for all normal photographic purposes, since it represents exposures from 1 second at f2 to 1/500 second at f22; in other words a range of over 1:60,000". (Focal Encyclopedia of Photography)

Check out http://users.aol.com./thombx19/ev.html for a useful table of aperture/shutter combinations for a given EV.

If the question means what I think it does, a *change* of 1 EV is equal to a change of exactly one f-stop or one halving or doubling of shutter speed. If you are at 1/500 and wish to add 1 EV of exposure, the new shutter speed would be 1/250.

Thanks Danny...that was what i meant...change i 1 EV equals to how many f stops...thanks for the rest for your answers as well

One thing that wasn't mentioned above: The specific f-stops/shutter speeds for a given EV will depend on the film speed.

Mike, you said "the specific f-stops/shutter speeds for a given EV will depend on the film speed". This is not correct. EV refers to a combination of equivalent f-stops / shutter speeds. Film speed is not involved here; e.g., EV 13 is always 1/250 at f/5.6 or equivalent, regardless which film you use.

Film speed comes in if you want to make a statement about a specific light intensity: If your light meter displays EV 13 for a 100 ASA film, there must be of course more light than if your meter gave this reading for a 400 ASA film...

Uh, not quite. It depends on the kind of meter you use. For my Pentax digital spotmeter, if I take a gray card reading in my office, it will always say EV6 (just as on a sunny day it'll always say EV15) regardless of what film speed is set. My Minolta IVF, on the other hand, will give different EV readings in a given lighting setting depending on the ISO speed set; e.g. in my office it will say it's EV7 if it's set at ISO 200, EV6 if it's at ISO 100, and EV5 at ISO 50....

The EV can also be read from a mechanical scale (like on the Hasselblad lenses), but an ISO reference is necessary as a starting point to determine the correct f-stop/shutter speeds combinations for proper exposure. Using a Sekonic DigiLite F, I can switch to the EV scale on the meter while changing my ISO readings and watch the EV increase or decrease as I move up and down the ISO scale. The point is that the EV reading is still a scale of f-stop/shutter speed combinations. If you want a proper exposure, you must first set the the ISO then read the combinations.

You are correct in answering your own question: 1 EV is equal to one stop. But EV (Exposure Value) is only a shortcut to expressing a shutter speed/F stop combination that, for a given proper exposure, changes with adjustments to ISO. Given a gray card properly exposed at 1/500th at f/8 (EV=15) using 400 speed film, if we were to use 100 speed film to photograph the same card under the same light, an EV of 13 (1/500 f/4, for example) would be indicated because the film is two stops less sensitive.

Danny, if you were to go from 1/500 to 1/250, then you're actually subtracting 1 EV because 1/250 represents one stop more exposure. The higher the EV, the less exposure indicated.

Bill, your Minolta IVF meter will certainly indicate a different EV if you change the ISO, since it is only indicating what would be a proper exposure. If you go from EV7 at ISO 200 (let's say to make that gray card "gray" again), and then you slap ISO100 speed film in your camera, you're naturally going to need one stop more exposure, thus EV=6, and so on.

Hope that helps!

Tony, Just to clarify, to go from 1/500 to 1/250, you are increasing the amount of time the film gets exposed, thus increasing the amount of light, by approximately 1EV. However, I believe you wanted to say that given a situation of 13EV and then if that changes to 12EV, that corresponds to a decrease from 1/500 to 1/250. I was a bit confused when I first read your post.

I think Tony & Wayne are both right. Wayne is just interpreting Tony's answer a bit differently, but essentially saying the same thing. The terminology gets a bit confusing along the way, but I thought I followed both ways of coming to the same conclusion.

Just a short remark: Bill, what your Pentax Digital Spotmeter displays in the finder is *not* the exposure value (EV) - it's simply an index number. You watch the finder display and then you transfer this value to the shutter speed / aperture rings on your Spotmeter's lens barrel. Here you can finally see the actual EV. Of course, different film speeds will result in different EV numbers (under the same light conditions), but that's consistent to what I said above: if you switch to a higher film speed you get a higher EV - just because you now use a different shutter speed / aperture - combination.

EV numbers are just shortcuts for equivalent shutter speed / aperture combinations, independent from the film speed used. If a 100 ASA film needs EV 13, a 400 ASA film will require EV 15. EV 13 is a shortcut for 1/250 @ f/5.6 (or equiv.), while EV 15 is a shortcut for 1/500 @ f/8 (or equiv.).

I just stumbled into this thread! Let me give you my interpretation.

To relate EVs to an absolute illumination level, you need to specify a film speed. Thus standard "sunny f16" corresponds to EV 14 @ ISO 64, or EV 15 @ ISO 125. The EV value is just a shutter-speed/aperture combination, the ISO is needed to relate it to a particular illumination level.

EV = AV + TV, where AV and TV are numbers related to aperture and shutter speed. f1=AV 0, f1.4=AV 1; f2=AV 2, and so on. For shutter speeds 1 sec = TV 0, 1/2 sec = TV 1, 1/4 sec = TV 2 and so on. So 1 second at f1.0 is EV 0, and each stop in shutter speed or aperture changes the EV by 1 unit.

There is also the little used BV scale, which is a measure of absolute illumination. BV 10 is "sunny f16". To convert BV to EV you add 3 for IS0 25, 4 for ISO 50, 5 for ISO 100 and so on.

Did I add to the confusion?

Interesting semantic tangle, Tony. Generally, when someone says he took a meter reading and added an EV to the exposure, it means he opened up a stop. It's true, thought, that this means the absolute EV level would be one stop lower. In the context of the question asked, I'll stick to my description as the correct one

Danny, yup, no argument there. Although, if one never knew what an EV was or what it was used for, it wouldn't make much of a difference to his or her photography; it's just not that important. If one knew how to set shutter speed and lens stop, and if one still produced lousey pictures, it wouldn't be because of some EV! :)

You meet the mysterious EV already when buying the camera and checking specifications. "Exposure metering 0/20" (usually given other conditions, like a fast lens and ISO100 film) means the manufacturer claims the meter is reliable within this interval.

Surprisingly, exposure values do have a meaning. (Which seems to have been set arbitrary.) EV 0 (ZERO) corresponds to exposing a frame for 1 (ONE) second at apreture value 1.0 (ONE DOT ZERO). Or for twice as much time (two seconds) at half as much light, i.e. AV one stop smaller, that is 1.4 (ONE DOT FOUR). Knowing this, you can derive a table of EV for all time and apreture values within a reasonable interval (should be at least from 1/8000s to 30s, and from 1.0 to 128). (Another example: you get EV 15 at 1/500s f8.0, or 1/250s f11, or 1/125s f16. - According to the f16 rule, the latter indicates that a 100ISO film at medium light needs EV=15. Or a half EV less, it depends.) It can't do any harm to have an idea about this and about the basic combinations of ISO and light conditions relevant to the particular EV's. (Like the mentioned EV=15 for a 100ISO film at normal light.) In fact it may be useful:-)

If you're interested in a comprehensive table of EV, I'll gladly send you mine. (I would as gladly post it here but I'm afraid it would end up "rearranged".)