I have a digital camera, 12 mega pixels, cheap, small. Well I'm not familar with the term "encircled exposure" but I do know what "EV" is and how to use it.
You adjust/change the EV when you want to keep a certain shutter speed but know that the exposure set by the camera is not going to give you the results you want. In other words, you use "EV" to "fool" the camera into doing what you want.
The following examples are hypothetical and are based on you using the camera in "A" or "aperture preferred mode". (You set the aperture, the camera selects a shutter speed.) It will, however, work if the camera is selecting both aperture and shutter speed - usually referred to as "P" or Programed mode. What you're doing, in effect, is "fooling" the camera.
Suppose you're shooting a photo of a wooded area and you're in "A" mode. At an ISO of 200 you set the aperture at f4. The camera selects a shutter speed of 1/125. You look at the results and think its too dark.
If you "open up" a stop, to f2.8, the camera will simply change the shutter speed to 1/250 - and the exposure will be the same as before.
So instead of changing the f-stop you have two options:
Full manual control so you can change the f-stop and leave the shutter speed where it was originally (1/125 at f4) and make your exposure at f2.8 at 1/125.
Or you can stay in "A" or "P" mode and change the EV to +1. The camera "thinks" it is making an exposure of 1/125 at f4 but the exposure is really at the equivalent of f2.8. Your photo will be lighter than the original.
If you thought the original exposure was too light you'd set the EV to -1 if you're in "A" or "P" mode. In full manual you'd leave the shutter speed at 1/125 and change the f-stop to f5.6
Here is why:
f2 admits twice as much light as f2.8; f2.8 admits half as much light as f2
f2.8 admits twice as much light as f4; f4 admits half as much light as f2.8
f4 admits twice as much light as f5.6; f5.6 admits half as much light as f4
When the lens admits less light, the light has to strike the sensor longer.
Suppose you are shooting in "A" mode (you select the f-stop, the camera selects the shutter speed) at an ISO of 200. Here is what happens as you stop down. Remember this is a hypothetical example. In bright sunlight you set the f-stop to:
f2 @ 1/1000
f2.8 @ 1/500
f4 @ 1/250
f5.6 @ 1/125
f8 @ 1/60
f11 @ 1/30
f16 @ 1/15
All 7 of the above exposures are identical. As you admit less and less light the shutter speed gets slower and slower. If you printed them out the only difference you'd notice under close examination is that the ones between f5.6 and f11 would be sharper. This is because lenses perform best within that aperture range. You would also notice an increase in Depth of Field (the area in front of and in back of your main subject that appears in focus.)
I know all of this sounds very complicated. If I've totally confused you I apologize. There is no other way I know of to explain EV than the way I've done here. |
