On most light meters, the reading comes in EV values ... from which one gets f stop and shutter combinations once the ISO value is set.

Does anyone know of a reference which explains the relationship between the parameters: EV, ISO, F stop, Shutter speed ?

There must be a relationship, or we would never have those dials or digital outputs.

-- Bruce Karnopp (karnopp@umich.edu), February 09, 2001

Answers

A change in 1 EV number doubles or halves the exposure. 1 EV up doubles; 1 EV down halves, thus a change of 1 EV is equivalent to a change of 1 stop or 1 shutter speed (modern shutters on 2x scale: 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, etc. Older shutters used 1/2, 1/5, 1/10, 1/25, 1/50, 1/100 which was not quite as close for the 1st & 3rd steps given here).

I think EV was originally designed to be on an absolute scale. Thus every meter should read the same lighting situation should give you the same EV number. I doubt, however, that you can count on this.

-- Charlie Strack (charlie_strack@sti.com), February 09, 2001.

When you say the "reading comes in EV values" do you mean that is the only way exposure info is expressed? The exposure value (EV) appears on most light meters, but it seems to me it is usually an adjunct to the more prominently displayed f-stop and/or shutter-speed readings. Examine your meter and see whether you get both forms of information, either at the same time or one or the other as you choose. The relationship among f stop, shutter, ISO (or EI), and EV are evident from manipulating the various settings on the meter and seeing how the others are affected. The EV is an arbitrary number identifying a particular light value and thereby a set of equivalent f-stop/shutter combinations. I have a Rolleiflex TLR on which EV is used to adjust how the shutter and aperture lock together so that changing one automatically changes the other. This is of limited value to me, however; so I've never found EV very useful.

-- Keith Nichlls (knichols@iopener.net), February 10, 2001.

System of EV's makes it easier to read the values between the highlights and shadows. The contrast range can be read using f-stops and shutter speed combinations but Ev values make it much easier. You can use a preselected f-stop and read off the shutter speeds for the high values and the low values but if you want to change anything it is slightly more complicated. Using EV's makes it easier. James

-- james (james_mickelson@hotmail.com), February 10, 2001.

Sometimes asking a question is enough to find the answer.

f (stops) icrease in stops, The same is true of EV values. THe film speed increases linearly as does the recipricol of the time (shutter speed) (e.g. 500 for 1/500th of a second)

With the above, the relation is (or seems to be)

(f^2)*T=ISO*EV^2

where f f stop T 1/time (100 for 1/100th) ISO film speed EV Exposure value

-- Bruce Karnopp (karnopp@umich.edu), February 10, 2001.

Sometimes asking a question is enough to find the answer.

f (stops) icrease in stops, The same is true of EV values. THe film speed increases linearly as does the recipricol of the time (shutter speed) (e.g. 500 for 1/500th of a second)

With the above, the relation is (or seems to be)

(f^2)*T=ISO*EV^2

where f f stop T 1/time (100 for 1/100th) ISO film speed EV Exposure value and f^2 is f*f or f square

-- Bruce Karnopp (karnopp@umich.edu), February 10, 2001.

Hmmm.... *scratching head*

ISO is the value of film speed. A certain amount of light is needed for a proper exposure. This is accomplished via apertures and shutter speeds.

"1 stop" is the same as: -> one change in f/stop -> halving or doubling shutter speed -> halving or doubling the film ISO speed (alternate EI) -> 1 EV.

Negative film has roughly 6 stops latitiude. That means it can see detail 3 stops over and 2 stops under average grey. Depending upon your camera/light meter, this could mean physical stops in shutter speed or aperture or EV.

That being said, frame a scene and spot meter what you would expect to be average grey. Read that EV value. Then take additional readings on objects in the frame that might be in the highlights or shadows in the scene. If they fall in the +3 EV / -2 EV range of your original average grey reading, you should be able to have detail in those highlights and shadows of your photograph. If you are not happy with what you have, you could adjust what you want to be average grey and move your range of film latitude accordingly (within reason, of course). For example, if you want more shadow detail, find something in your scene that would be one EV less than your original average grey. This will then boost your original shawdow detail one stop more (rendering more detail). Also you can go the other way if you want to render a highlight more (rather than have it blown out). Once you are satisfied with your range, your can set your f/stop and shutter speed to make your average grey come out exposed properly. Shadows and highlights will fall into place.

The same can be done with using shutter speeds and f/stops. Go into aperture or shutter priority (or fixate on one or the other on your full manual camera). Play the game of metering around and checking "stops". Again, +3 / -2 stops will be what you are looking for. Touching on what James said, it can be done with f/stops and shutter speeds. Using EVs is just a little easier. You would work with whole numbers vs. halving-and-doubling everything.

Oh and by the way, you have just touched on what the Zone System is all about. It can be used as a tool to inform you ahead of time of what could be rendered as shadow and higlight detail in a photograph.

-- Johnny Motown (johnny.motown@att.net), February 10, 2001.

Update:

At any combination of ISO and EV values, the quantity (f^2)*T is constant. In fact, the constant is:

(f^2)*T= [(2^EV)*ISO]/100

Once this combination is determined, either the f stop, f, can be obtained from inserting T (e.g. for 1/200 th sec, T= 200) (f)= square root{[(2^EV)*ISO]/100*T} or T= {[(2^EV)*ISO]/100*f^2}

The above may be of very little interest to anyone - except those who have respond to the question ("what is the relation between EV numbers and f stop and shutter combinations ?)

Now the final question is this: Presumably EV numbers can be desribed in terms of the nifty "candlepower" or the more hip "lumens"? And is the translation linear? That is, if EV = 1 corresponds to x lumens, does EV = 2 correspond to 2*x lumens.

Thanks for comments or corrections.

Bruce

-- Bruce Karnopp (karnopp@umich.edu), February 11, 2001.

From the dial of my venerable old Weston Ranger 9, at a film speed of 125, EV 14 equals 125 candles per square foot. The manual says to multiply that number by 4 to get footcandles. They also say the reading should be taken from a piece of white paper, so I'm not sure how accurate it is. It's also 1/125 at f/11, so you can figure out any other setting from that and the info posted above. The EV system is supposed to be absolute, and shouldn't vary from meter to meter.

-- Conrad Hoffman (choffman@rpa.net), February 12, 2001.