EV and the EV table
An EV table is principally a large chart that allows the relationships between ISO, EV, shutter speed and aperture settings to be explored.
EV stands for Exposure Value, and the difference of 1EV is the same as 1 stop. To understand stops and their use in a simple way see Exposure required and Sunny 16 Rule.
EV is for a sensitivity or specific ISO value, the most common standard being an ISO of 100, and sometimes written as EV100.
So EV is a way of measuring the brightness of a scene. Some exposure meters give a direct reading in EV values but usually it is deduced from an EV table.
We know the EV value of many situations, and on some EV tables you have the ability on the reverse to look up some of these.
So for example looking at EV100 a typical sunny scene has an EV value of 15, while traffic vehicle lights at night have an EV of 5, and a floodlit circus of value of 8.
Other uses for the EV Table
We can use the EV table as an easy way to look at the effect of different things on exposure, for example using Neutral density or graduated neutral density filters (grads), Teleconvertors, and other items.
Let us suppose that we are looking at a very bright sky, against a dark ground surface, perhaps dark coloured steam engine in the shadows, that will puff white steam into a bright sky with clouds. We realise that the exposure tolerance from dark to light areas that will record in our camera well, is not great enough for the tonal range in front of us by (3 stops), 3 EV then we can overcome the problem by using a 3 stop graduated neutral density filter (grads), if we use a graduated filter marked as ND0.9, which has a 3 stop difference from top to bottom then we can cope with the exposure tolerance required, and seeing where this all plays out and the choice of ISO, Aperture, and Shutter Speed becomes easier to understand when we look at the EV table.
We are considering using a telephoto lens extending the range by adding a x2 teleconverter and using a polarising filter to cut some of the haze. We have a tripod. We know that the polarising filter will use 1 stop of light and the teleconverter two stops, a little more. A quick check through the telephoto lens alone at ISO 800 is giving us a reading or f5.6 (its widest aperture) at 1/125. We can see on the EV table that loosing 3 stops (3 EV), would drop the shutter speed to only 1/15th second. We can now see the problem too slow a shutter speed won't allow anything to move, so if we drop the polarising filter, and increase the ISO to 1600, we gain 2 stops (2 EV) and we can see that would allow us to use 1/60th, not ideal but if we wait until nothing is moving much we can just about catch it. This example is not about the precise problems but about being able to understand both the problems and options.
Expanded EV Table
Camera Images created an expanded EV table, this has the above table at its heart, but includes EV's higher and lower, apertures larger and smaller and has a secondary linked table on the right that lists EV at different ISO values. On the opposite side are EV values for a variety of conditions, and small tables showing full, half and third stops in each of the variations, allowing minor changes to be allowed for. It also shows the Sunny 16 Rule with the effect shown also on the EV table. It can be printed on to 2 A4 sheets laminate them back to back and then fold it into three, to make it easy to carry and use. We have a copy that you can print yourself by clicking here (PDF).
for details on exposure article route see the Exposure page