Understanding EV

[lobalobo]

Having recently ruined a number of shots making EV adjustments on my digital camera, I realized that I needed to understand better how

EV adjustments work. I looked first for others who might have had the same questions and sure enough found a thread that was entirely on

point:

 

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00N2jR

 

The problem is that no one ever quite answered the original poster's question. The generous answers were loaded with information by

clearly knowledgeable people, but unless I'm mistaken (which is possible) either none of them understood what the OP was getting at or

some or all of them thought they had answered. At the risk of being redundancy I'm going to try again by posing a hypothetical:

 

Shooting in RAW and in Auto mode I point my camera at a scene and read the internal meter, which tells me, say, that it is about to

expose at ISO 200 and f/4 for 1/100th of a second. I am afraid that this will blow highlights so I move the EV adjustment to, say, -.7. The

question simply is what happens differently when I press the shutter after the EV adjustment as compared to what would have happened

had I pressed the shutter without making any EV adjustment. Here are three possibilities that come to mind along with some comments

about each:

 

a. A Hardware Adjustment. That is, by adjusting the EV, I cause the camera to change either the shutter speed or aperture. From reading

other posts, I'm guessing that this is not what happens as this would be an exposure compensation adjustment instead, or put another way

a biasing of the meter, which is different from an EV adjustment.

 

b. A Fundamental Software Adjustment. That is, by adjusting the EV, I cause the camera to change the way it records the image even in a

RAW file. An ISO adjustment would fit into this category, but I take it from other posts that this is not what occurs. Rather, if this category

is the correct one, I take it that making an EV adjustment is the equivalent of changing the tonal curves of the image, but I don't

understand how that is relevant in RAW, which brings me to final category.

 

c. Nothing. That is, by adjusting the EV, I have done nothing at all to a RAW file as a digital EV adjustment is relevant only to the

production of an in-camera JPEG (or perhaps to the default settings of a RAW converter). This is what I had thought to be the correct

answer before reading other posts; but now I'm not sure.

 

Is the answer in one of these? If not, what is the answer?

[phule]

[[shooting in RAW and in Auto mode I point my camera at a scene and read the internal meter, which tells me, say, that it is about to expose at ISO 200 and f/4 for 1/100th of a second. I am afraid that this will blow highlights so I move the EV adjustment to, say, -.7. The question simply is what happens differently when I press the shutter after the EV adjustment as compared to what would have happened had I pressed the shutter without making any EV adjustment.]]

 

The final image on any camera is decided by your choice of aperture, shutter speed, and ISO. When you choose a non-zero EV value, the camera will adjust one or more of those values (based on your camera's configuration).

 

This would be obvious if you were to test this directly. Meter a uniform scene with zero EV compensation and then meter again with -1. Depending on the mode you're in, the camera will either change the aperture, the shutter speed, or the ISO (if your camera supports that functionality) to compensate.

 

Let's assume you're in Av mode with a selected aperture value of f/2.8. The camera has chosen a shutter speed of 1/60th at ISO 100. If you then select EV -1 the camera will change the shutter speed to 1/125 at ISO 100 for the same aperture value f/2.8.

[lobalobo]

Rob Bernhard said: "The final image on any camera is decided by your choice of aperture, shutter speed, and ISO.

When you choose a non-zero EV value, the camera will adjust one or more of those values (based on your camera's

configuration). This would be obvious if you were to test this directly. Meter a uniform scene with zero EV compensation

and then meter again with -1. Depending on the mode you're in, the camera will either change the aperture, the shutter

speed, or the ISO (if your camera supports that functionality) to compensate."

 

Makes perfect sense. A number of posters in another thread, though, said that this did not occur, or so I interpreted

them. Of course you are right that the test is obvious, but given the posts that denied this result, I wasn't sure whether

the reported change in aperture or shutter speed (or ISO) is actual or virtual to reflect the adjusted EV. I take it from your

response, that this answer is "actual."

[SCL]

Most cameras would treat it as "A". EV=exposure value - a measure of light intensity coupled as related to aperture and shutter speeds. Exposure compensation=change in exposure by means of a change in aperture and/or shutter speed. This isn't really a difficult concept. There are EV scales scattered on the web....EV was a briefly popular concept in the 1950-60s...but was quickly eclipsed by easier to explain concepts and then everything became automated so for 99% of the population it became irrelevant. I still have one camera which lists EV values along with the related shutter and aperture settings. And frankly most people today use the concept of changing exposure by "f stops" rather than EV values.