EV - Exposure values

marossity · Jan 10, 2011

Needless to say, I am just getting started in this hobby,and trying to understand a few things.
I am having trouble attempting to understand the value of "EV" or "exposure value".
Not in how it is calculated. That is pretty straight forward. But what is the value of this number? How is it used when taking a picture? The aperture and the shutter speed are set, either automatically or manually. These determine the EV.
O.K. so I have now determined the EV. So what? What good does this do for me, the photographer?

o hey tyler · Jan 10, 2011

The EV is a number that represents whether your shot will be over or underexposed. It's determined by the camera first metering the scene with whatever metering mode you have selected (metering is essentially "measuring the light"). If your camera is set to manual, it will tell you on a little line that looks like -2_ _-1_ _0_ _1_ _ 2 whether your current camera settings will properly expose the scene. Having the pin on 0 is what the light meter sees as the "optimal exposure", you may want to over or under expose a scene depending on the situation however.

marossity · Jan 10, 2011

Thanks o hey tyler -
My problem, (probably my brain) is this. I have my camera set up for manual adjustments. I set the aperature and shutter speed. From this I can calculate the exposure value. There is no indicator on my film camera that reflects the exposure value.
So I am sorry to say, I just don't see how I am suppose to use this value when I am out taking photos.

o hey tyler · Jan 10, 2011

What film camera are you using?

Derrel · Jan 10, 2011

Exposure value - Wikipedia, the free encyclopedia

EV or Exposure Value is also a system of relative levels of scene brightness...it's an old system that's pretty useful...from a practical standpoint, knowing the EV value of a scene gives you a rough idea of where your sensor will perform better, rather than worse....for example, at dim EV values, like EV 3, a lot of sensors really offer kind of sucky performance...and they are much better at higher EV values, like EV 14 to 17.

marossity · Jan 10, 2011

Minolta Maxxum 7000

Dao · Jan 10, 2011

Although most of the digital or modern film camera photographers do not need to use or understand EV because of the build-in camera light meter. But I found that it is good to know or understand the EV. (of course, no need to remember those math formula) especially when you are in a condition where light meter may give you a unexpected readings.

From the Wiki EV page link listed from Derrel, you will find a EV table as well as exposure values of different light conditions.

For example, I am planning to shoot full moon with my telephoto lens (i.e.300mm). And I found that the sweet spot (aperture where the lens perform the best) of the lens is f/8 at 300mm.

So I look up the EV table. So EV15 (row) and f/8 (column), I got shutter speed of 1/500 with ISO based on ISO100. What that means is in order to obtain a correctly exposed photo, the page in wiki suggest using

ISO100
f/8
1/500

to caputre fulll moon.

Of course, if your lens's sweet spot is f/11, the shutter speed is going to be 1/250.

Since you are using a film camera and cannot review the photo right away, I suggest you print out the EV table as well as the light condition chart and put it in your camera bag. Of course, update the chart as you go.

marossity · Jan 10, 2011

Big thanks for all the replies.
Dao - Your reply finally put some sense into this old head.
I have just printing the EV table and the copy is tucked away in my camera bag.
Have a nice everything, people.

clanthar · Jan 10, 2011

EV is especially valuable in simplifying the process of determining a correct camera exposure. Back in the day when not all cameras had built in light meters it was commonly possible to set an EV value directly on the camera. I still have a number of cameras that allow me to set and lock an EV value.

The real key was that the EV value locked on the camera. The process of taking a photo worked like this: Meter the scene (hand meter) and read the EV value from the meter -- say 14. You only had to read 1 number. Set and lock 14 on the camera -- still only dealing with 1 number. This then locks the shutter and aperture controls on the camera so that changing one forces the other to change and so holds the exposure constant. My Rolleiflex works this way. A mechanical lock went a long way in helping beginners learn the relationship between shutter, f/stop and correct exposure.

The next big advantage to using EV values is that it permits you to assign a single number to a reciprocal combination of shutter speeds and f/stops. This makes it possible to learn to get good exposures outdoors without a light meter. Instead of having to learn shutter/aperture combinations you only had to learn 1 number. For example correct exposure for Kodachrome on a sunny day was EV 14. You can then learn: hazy sun = -1 EV, light overcast = -2 EV, overcast = -3 EV, heavy overcast = -4 EV, open shade = -3 EV, etc.

When I go out in a canoe I take a camera that I wouldn't mind (too much) watching sink. Right now that camera is an old mechanical 35mm camera without a meter. I can easily get good exposures outdoors without a meter. ISO 400 film is EV 17 in full sun -- I know what to do from there.

Joe