Sometimes it’s difficult to know the values for a correct exposure. For example if we want to photograph the lights of the city at night the reading will be inaccurate because those distant sources of light are not intense enough to affect the exposimeter; Or maybe when we want to figure out if the 400 ISO film we have at home will allow us to take perfect photos with our Kodak Brownie during a sunny morning with the fixed settings of this camera (f/16 and 1/30).
There is a solution, but is a bit complicated!
What we’re going to know and use is the “Exposure Value” (EV) which is a unique number that indicates the brightness of a scene.
It is obtained by using a mathematical calculation referred to a sensitivity of 100 ISO:
Obviously the same exposure value can be obtained with different combinations of the three parameters (ISO/A/T) which is also the concept at the foundation of exposure and photography.
Leaving aside all the mathematical part, we can simply use one of the many charts that we can find on the net, or the one that I propose below (referred to 100 ISO):
Now to take advantage of the EV you have to connect it to the Sunny 16 rule (on a sunny day, with sharp shadows, to get a correct exposure you have to use f/16 and a shutter speed as closest as possible to the ISO of the film you are using). The EV is a calculation based on 100 ISO then, comparing the values of the Sunny 16 for the same value (f16 and 1/125) with those on the EV chart above, we derive that 15 EV is the amount of light that is present on a sunny day with sharp shadows!
Below I suggest an exemplary data table of how to exploit the EV value in combination with the Sunny 16 rule based on different iso settings but keeping 1/125 as shutter speed:
So I took the EV, I’ve mixed it with the Sunny 16 and I’ve got a handy data table to “guess” the perfect parameters for a good exposition: now you have to simply rearrange the table with your favourite shutter speeds, ISOs and apertures. I know that is a difficult mathematical exercise so I’ve done it for you 😉
A practical use of this value, as I mentioned, is to help us decide which film we can use referring to the amount of available light there will be in the scene that we are going to shoot. Or maybe it will help to understand the shutter speeds and apertures needed in conditions where the meter will not function correctly.
Transforming this knowledge into a practical example we can say that if we have to shoot at an indoor sport event, which we know have an EV of 7, we cannot take with us a 400 ISO film, because the camera settings will be f/2.8 1/60: too slow for almost every action.
At least with 800 ISO we can have a shutter speed of 1/125, but may be f/2.8 may result in a too narrow depth of field.
The best decision will be to use a roll of film that tolerate being pushed to 3200 ISO without losing too much quality so we can select an array of settings between f/2.8 + 1/500 or f/4 + 1/250 or even f/5.6 + 1/125. That will be enough to let us choose between a faster shutter speed or a deeper DoF.