Thursday, July 23, 2020

Why I Use Exposure Compensation

Most of the time modern digital cameras will do a pretty good job adjusting the shutter speed, aperture, and/or ISO to get a well-exposed photograph.  Other times the photo comes out too dark or too bright and can't be saved.  Why does this happen and what can be done to get a better exposure?

Digital camera light meters work by evaluating light reflected off objects in the frame.  Depending on if you are in aperture priority, shutter priority, or manual, the camera will attempt to adjust one or more settings to make the overall exposure close to a standard middle gray (also known as 18% gray).

If most of the things in the frame are very dark, the camera will attempt to brighten up the exposure. If the subjects are mostly bright it will darken the exposure so the overall scene is close to 18% gray.  As smart as modern cameras are, they can't tell what parts of the scene are important.  While the overall scene may be properly exposed, the important subject may be too dark or too bright.

Default exposure - 1/1000 sec., f/6.4, ISO 160

In the photo above, the bright sky behind the Bluebird caused the Bluebird to come out dark.   There are a few things that can be done to fix this problem.  I could (1) set my camera to spot metering so I only measured the brightness of the bird, (2) move so the background is not so bright, or (3) use Exposure Compensation to adjust the exposure.

In most cases, I don't change the metering mode when photographing wildlife. Switching metering modes on most cameras is cumbersome.  The bird would be gone by the time I made the changes.  

Exposure Compensation allows me to override the exposure settings that were determined by the camera, in order to darken or brighten images before they are captured. On my Fuji X-T1, X-T2, and X-T3 cameras there is a dial on the top of the camera that I can turn to quickly and easily adjust the exposure compensation. Unfortunately, exposure compensation is not as easy to change on all cameras.  You will need to learn how to change it on your camera.

Using the Bluebird as an example, I adjusted the exposure compensation to make the photo 1 2/3 stops (EV) brighter and took the photo again.

+1 2/3 EV exposure - 1/1000 sec., f/6.4, ISO 320

When photographing birds I have my shutter speed and aperture fixed and let the camera adjust the ISO.  When I told the camera to make the photo 1 2/3 stops brighter it did so by doubling the ISO. You can see the second photo is noticeably brighter than the first.

How the camera makes the adjustments depends on what mode you are using
  • Aperture Priority - the camera will leave the aperture where you set it and adjust the shutter speed.
  • Shutter Priority - the camera will leave the shutter speed where you set it and adjust the aperture.
  • Manual with Auto ISO - the camera will only adjust the ISO.  This is the mode I was using.
  • Exposure compensation may not be available if you are using Auto mode.
Being mirrorless cameras, my Fuji's show me the adjusted exposure and live histogram in the viewfinder as I am composing the shot.  With DSLR cameras you may have to make a shot, check out the exposure, adjust and shoot again. An alternative is to use exposure bracketing to automatically take 3 or more shots at different exposure compensation settings.

There is another alternative that can help get the right exposure.  The problem in this example is that sky is too bright compared to the bird.  Sometimes we can maneuver ourselves to change the background to something closer to the brightness of the subject.
1/1000 sec., f/6.4, ISO 1250

In this third shot, I moved so that there were trees behind the bird instead of the sky.  I didn't have to make any adjustments to the exposure.  Most of the time it's not practical to move but it does a great job when I can.

I hope these three examples and explanation gives you a feel for how Exposure Compensation works.  I suggest you figure out how to use it on your camera and give it a try.