Understanding EV stops and what is actually happening in the camera


TPF Noob!
Nov 7, 2011
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I have a Sony NEX-5N and I am trying to understand what is happening when I wind back the EV values.

Most of the modes have an option to change the EV except the Manual Mode.

So IN Program, Aperture Priority and Shutter Priority I can reduce the EV by +/-0.3 at a time through a simple click of a wheel.

But what is actually happening?

So lets say in Aperture Priority Mode - The shutter speed "floats around" based on what I am pointing at, as I reduce the EV the shutter speed increases. How does -0.3 EV translate to a decrease in shutter speed? I usually shoot in ISO100 all the time, however I note that when I set to ISO Auto and play with the EV, the same shot takes on a different ISO value, from ISO100 up to a maximum of ISO1600 and the shutter speed also moves but not nearly as much....

There are only 3 things that affect exposure in the camera - shutter speed, aperture and ISO - nothing else.

So when you are in the priority modes, such as aperture priority, you have control over two of those settings; the aperture and the ISO (in the case of aperture priority mode). The camera, meanwhile, is controlling the shutter speed and sets its value based upon the settings you select and the light that the meter detects.
This is no different to you setting the aperture and ISO in manual mode, half pressing the shutter to meter and then adjusting the shutter speed yourself to have the meter needle point at 0.

EV (exposure compensation) in the priority modes is a measure by which you tell the camera to under (-) or over (+) expose the scene based on the meter reading using the setting it has control over. So if you tell it to over expose by one stop (+1) the camera will select a slower shutter speed by one stops worth - and vis versa if you told it to under expose.

You can do this yourself in manual mode again, just be selecting the settings so that the meter needle is pointing at the value either side of the 0 that you want.
How does -0.3 EV translate to a decrease in shutter speed?

A few quick simplified definitions.
LV (Light Value): A measure of how bright something is. Bright daylight is around LV 13-15.
EV (Exposure Value): A measure of exposure depending on your aperture and shutter settings. EV=LV at ISO 100.

These numbers a useful shorthand that are set up in stops. LV 14 is twice as bright as LV 13 (+1 stop). EV 12 is half the exposure of EV 11 (-1 stop). In AP mode, you are telling the camera to keep the aperture constant, so -0.3 EV (reduce the exposure by 1/3 stop) results in a 1/3 increase in shutter speed (1/3 less light). -0.3 EV in SP results in a 1/3 decrease in aperture. In ISO Auto Iḿ guessing that in your camera the ISO only changes in full or half stops, leaving the shutter or aperture to pick up the smaller fractions of a stop, hence the smaller changes.
How does -0.3 EV translate to a decrease in shutter speed?
-0.3 EV is 1/3 of a STOP. -.07 EV is 2/3 of a stop.

1 EV is a full stop and a full stop is a doubeling or a halving, be it shutter speed, ISO, or lens aperture.

A shutter speed of 1/100 leaves the shutter open 2 x longer than 1/200 does.
Ideally a 1/3 stop change would be a shutter speed of 1/133 for a shorter shutter open time of 1/66 for a longer shutter open time, but you won't find those exact values available on your camera.

For shutter speed and ISO we can use x2 or x1/2 to calculate a full stop change (1 EV), but because lens aperture is about the area of the lens opens opening and not it's diameter, we have to use the square root of 2 - 1.4142 - instead to make our calculations. That's why full stop steps of aperture are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, etc.

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