HDR Photography: Photographic Stops/EV
A photographic stop is a principle that allows you to compare changes in exposure, which is an important concept for understanding high dynamic range photography. One stop doubles or halves the amount of light for a given photo, depending on whether you increase or reduce exposure.
This behavior applies to all the methods of controlling exposure, which means you can describe aperture, shutter speed, and ISOs in terms of stops.
Stopping down means you are reducing exposure.
Stopping up means you are increasing it.
Camera controls can change stops incrementally, which means you can change the shutter speed by a third of a stop.
Here’s where Exposure Value (EV) comes into play: One EV is the equivalent of a stop of exposure. EV often refers to the location of the current exposure on the camera’s exposure index or exposure display. It can also refer to the relative distance from one exposure to this mark or another exposure, expressed in EV.
This figure shows the LCD monitor of a Sony dSLR, which has an exposure index handily visible at all times. It is marked off from -2 to +2 EV, with ticks every third of an EV (which is normally one shutter speed or aperture increment one way or another).
This is your guide to measure the effects of altering your camera’s settings on exposure in manual mode. In this case, the combination of 1/5 second shutter speed, f/8 aperture, and ISO 100 result in an anticipated overexposure of +2.0 EV.
EV 0.0 is assumed to be the ideal exposure given the lighting conditions at hand. Only you can tell whether it is aesthetically the correct exposure or not.
If a camera in manual mode meters a scene and tells you the current exposure is +1.0 EV, you know the photo will be overexposed by a stop. You should either increase (make faster) the shutter speed or make the aperture smaller to bring the exposure down to 0.0 EV. Similarly, a bracketed photo taken at EV -2.0 is intentionally underexposed by two stops.
When setting auto exposure bracketing distance, you set the relative EV that each exposure differs from the ones around it. Common settings are +/-0.3, +/-0.7, +/-1.0, and +/-2.0. While the lower settings are not as effective for HDR, +/-1.0 and +/-2.0 are right in the ballpark.
Underexposed: A photo is underexposed if it is too dark. In HDR, you shoot underexposed photos on purpose to tone down highlights in a scene, which will retain their details.
Overexposed: An overexposed photo is too bright. In HDR, you shoot overexposed photos on purpose to reveal details in shadows.