Understanding HDR part I – Exposure Value Step or EVS

Monaco is very known for its money and Formula 1. However, Monaco is a very beautiful city too. It is the second smallest country in the world. Only the Vatican is smaller. You find all sorts of beautiful old buildings. This flowerist on the corner I particularly liked. Photo by: Jacob Surland, www.caughtinpixels.com

Exposure Value Steps or EVS I find very important and practical to understand. Not only when I talk about HDR, but also when I look at new cameras and camera features. For an HDR photographer, understanding EVS is very important.

If you change your exposure 1 EVS, is the same as if you change your exposure one stop. These two expressions are equivalent. And it means that you either double or cut in half the shutter speed and this way you get either a brighter image (doubling the shutter speed) or a darker image (cutting the shutter speed in half).

You basically have three different settings you can change, to change the exposure. The shutter speed, the aperture and the ISO. The ISO and the Shutter speed are easy to grasp, you either double it or cut it in half to change the exposure 1 EVS. The aperture is slightly more complicated, because it is not linear.

This table shows how how the shutter speed and the f/stop’s are related. Any of these columns represent the same exposure. combinations give the same exposure, if you have the ISO fixed.

F-stop chart table

f/stop Shutter Speeds in Seconds
EVS +4 +3 +2 +1 0 -1 -2 -3 -4
f/1.4 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000 1/4000 1/8000
f/2 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000 1/4000
f/2.8 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000 1/2000
f/4 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000
f/5.6 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500
f/8 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250
f/11 2 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125
f/16 4 2 1 1/2 1/4 1/8 1/15 1/30 1/60
f/22 8 4 2 1 1/2 1/4 1/8 1/15 1/30

Have a look at the column having EVS = 0. If you have a fast lens and shoot at f/1.4 at 1/500 seconds that is the same exposure, as if you shoot at f/22 at 1/2 second. By the same exposure, I mean that the highlights and the shadows will be equally dark and the light. The two photos will seam identical. What has changed is the depth of field, because the aperture is changed.

When you shoot HDR photos you setup your camera to auto bracket exposure (AEB) mode, and shoot e.g. a -2, 0 and +2 exposure. And if you are shooting in aperture mode (as you should) shooting at f/8, would be three exposures of:

f/stop Shutter Speeds in Seconds
EVS +4 +3 +2 +1 0 -1 -2 -3 -4
f/8 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250

If you are shooting at a fixed shutter speed, the aperture will change:

f/stop Shutter Speeds in Seconds
EVS +4 +3 +2 +1 0 -1 -2 -3 -4
f/4 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000
f/5.6 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500
f/8 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250
f/11 2 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125
f/16 4 2 1 1/2 1/4 1/8 1/15 1/30 1/60

However, if you are shooting for HDR, you will be in trouble if you shoot in different apertures. The aperture changes the depth of field and consequently also changes image. Things get out of focus, because the depth of fields get’s more narrow the more open the lens is (smaller f-number). So you should always shoot in aperture mode (or manual if you want a little more control), when you shoot for HDR.

CONTINUE TO PART II about the Dynamic Range here.

Further readings

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You might want to read my detailed HDR tutorial here.

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