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EV conversion to F stops

Peter Lo , Jun 09, 1998; 09:20 a.m.

How does one convert EVs to f stops...ie 1 EV is equal to how many F stops approximately?

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Barry Pehlman , Jun 09, 1998; 09:59 a.m.

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)

John G. Serbein , Jun 09, 1998; 11:49 a.m.

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

Danny Weber , Jun 09, 1998; 12:35 p.m.

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.

Peter Lo , Jun 09, 1998; 04:29 p.m.

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

Mike Dixon , Jun 09, 1998; 05:08 p.m.

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

Dieter Lefeling , Jun 09, 1998; 06:11 p.m.

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...

Bill -- , Jun 09, 1998; 09:22 p.m.

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....

Barry Pehlman , Jun 10, 1998; 12:30 a.m.

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.

Tony Rowlett , Jun 10, 1998; 02:05 a.m.

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!


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