Beginners Guide to EV
The amount of light used in an image is the product of the lens aperture, the shutter speed and the ISO setting. This is the exposure value. Though the absolute value is rarely if ever used to plan a photograph, exposure value, or EV, is adjustable from a starting point of the camera's meter reading of the scene.
Camera meters are set to assume that every photo is a blank sheet of grey colour - 18% grey as it is called. One accurate way to use the camera meter therefore is to half press the shutter as you point at an 18% grey panel or card. They can be bought:
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So have EV compensation set to zero, point a digital camera at the gray card (held in front of your subject) and half press the shutter, and you have the correct meter reading. Just remove the card and press the shutter the rest of the way.
But. You will likely never see anyone with a gray card. So what do you do?
The casual way is to imagine your composition as a black and white image. Imagine what shade of gray it would be if averaged out to one smooth shade (a charcoal print that you then rubbed until all image detail was gone). How dark or light would it be and what do you do to tell the camera this?
The simple answer is that the darker the grey you estimate the more you lower the EV. So the lighter the gray the more you raise the EV. By setting the EV compensation you are setting the camera meter for a scene that is darker or lighter than the 18% gray. An extreme example of each:
A polar bear in the snow:- Everything is white (very light grey indeed) so set the EV to a high plus value. Up to +2 is available on most cameras. The camera meter is set for near white and keeps the shutter open until that is pretty much the resultant image. Had you not done this your polar bear and snow would have turned out pretty close to 18% gray. Complaints of 'dirty' snow are common amongst new photographers, and camera 'scenes' were invented so novice users could select 'snow' and the camera's EV would be automatically raised without the user needing to know or understand.
Batman at night:- A fictional character dressed all in black at night. Everything is very dark gray indeed so set the EV to its lowest (-2 normally). If you don't the shutter will stay open a very long time, until finally enough light enters the camera to render the image an average of 18% gray all over. Novices notice this problem less, since they are actually impressed that the camera has found all the detail in the dark. However they then declare it a shame that the image is all very blurry. After all the shutter had been open for ages and the operator could not have held the camera still for that time.
So in between the two extremes above you estimate the EV compensation, and with only a little experience you will quickly get the hang of it. You can review a shot and adjust again for a second. And some cameras offer exposure bracketing, which automatically takes a lighter and darker shot either side of your estimated EV compensation setting.
Estimating exposure compensation is complicated by back light; light shining into the camera. This might be the Sun behind the subject outside, or a bright window behind the subject indoors, or a street lamp behind the subject at night. When you take the picture the light from these is metered as a bright scene, making the shutter close far too quickly. You will recognise this in some photographs as the production of silhouettes. To compensate for this, if the silhouette was your intended subject, you either use the camera's flash to 'fill in' the subject, or you raise EV quite a bit to keep the shutter open long enough do detail the silhouette. In this latter choice you produce pure white areas near at and around your light source called highlights.
Across your photograph the amount of light can be plotted on a diagram. To the left is black, the right white, and in between various shades in between. This is called a histogram, and can be displayed on many digital cameras. Your task when setting EV compensation is to collect the light where you want it by adjusting the EV compensation. If it's night time, you don't want the histogram to show you light gathered up at the white end; for example. Though frankly you can see something is wrong with the image.
That gray card is a handy device, though no-one really carries one around much. But what might be useful to know is that 18% gray is available to meter on many occasions. Lawn grass is pretty close to 18% gray if correctly exposed in a black and white image. A test photo just pointing at grass (with exposure compensation set to zero) will inform you of the approximate camera settings you should be using.
The exposure value in any given camera can, in absolute terms, be off a little. A camera might take photographs a little lighter or darker than it should, so adjust as you see appropriate. Also small sensor cameras tend to produce slightly better photos on average if kept at -1/3 EV compensation. That's just from the experience of many people here on the forums.
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