Exposure Bracketing 101

An introduction to a powerful feature of your DSLR

By ccrookston in Articles and Tips on Jun 15, 2014

Just about every DSLR camera on the market today comes with a handy, powerful and often overlooked feature: Auto-Bracketing.

What is auto-bracketing?  In its simplest terms, auto-bracketing is a camera function that allows you to take multiple, distinct photographs of the same scene (or subject) with one click of the shutter release button, all at different exposures.

How can this feature help improve your photographs?  And why use it in today's digital world?  There are at least three solid reasons why using auto bracketing is a good idea. But before we discuss them, it's vital to first understand the concept upon which auto bracketing is based: Exposure Value and Exposure Compensation.

Understanding Exposure Value and Exposure Compensation

A photograph’s Exposure Value (or EV) is a number corresponding to the combination of shutter speed, aperture and ISO required to capture the most natural exposure based on the amount of light hitting the sensor.

Sometimes, however, the lighting in a scene can be tricky, and you might want to play around with overexposing or underexposing your image.  To do this, you can use your DSLR's Exposure Compensation EV+/- abilities. Your camera has the ability to shift your EV.  For example, an Exposure Compensation setting of EV+1 will allow more light to reach the sensor, which will overexpose your image.  In the other direction, an Exposure Compensation setting of EV -1 will allow less light to reach the sensor, making your image more dark.

Exposure Compensation is measured in stops, and is usually written as follows:

EV +2 , EV +1, EV +/-0 , EV -1 , EV -2, etc.

So, for example, EV -2 is two stops below the correct exposure setting. But don't let the term "stop" confuse you.  We're not talking about f-stops. Remember, the Exposure Compensation stop is a combination of three settings: f-stop, shutter speed, and ISO.

Below are three images of a white rose, all taken at 1 stop apart.  You can see the subtle but noticeable difference between each of exposure.

What Is Clipping, And What Can Be Done About It?

When your camera is in full auto mode, it figures out the the Exposure Value on its own. And although it does a pretty good job most of the time, it doesn’t always get it exactly right.  It makes a mathematical decision, but it has no ability to judge aesthetics.  Oftentimes our camera will leave the highlights blown out (too bright), or the shadows underexposed (too dark).  

This underexposing of shadows and overexposing of highlights is known as “clipping”, and when it happens, your final photograph will be missing detail in the clipped areas.  Our own eyes are often more forgiving of clipped shadows, but we tend to notice when highlights are washed out and missing detail. Either way, we sometimes want those details back.

While it's possible to partially restore details in Photoshop, the best way to capture them properly is in-camera.  And to do this, we can shift our Exposure Value.   Shooting in RAW mode reduces the risk of clipping, but sometimes even this isn't a replacement for correct exposure up-front.

If you want to restore details lost in highlights, then you’ll need to underexpose your image, or drop the EV shift to a negative number.  And if you want to restore details lost in shadows, you would boost your EV shift to a positive number.

Bracketing: The Solution to Your Light Range Problems

While adjusting for clipping in either the highlights or the shadows is fairly simple, what about those extra tricky shots where the range of light is so vast that you have clipping in both the highlights and in the shadows?  It’s simply not possible to adjust for both with one shudder click.  

This is where bracketing comes into play.

Bracketing simply means taking a series of photos of the same scene or subject in rapid sequence, all at different exposures. The smallest bracket is typically a set of three photos, with one being above, one below, one directly at the midpoint.

It's not hard to manually create a set of bracketed photos.  Using a tripod (or resting your camera on a steady surface), set up your camera and frame your shot.  Take one photo at the normal exposure.  Then adjust your shutter speed up and take a second photo.  Then adjust your shutter speed down, below the original point, and take a third photo.

And that’s it!  You now have a set of three bracketed photos!

Why do we adjust the shutter speed and not the f-stop?  Good question. If you plan to merge your photos later using software, you’ll want to make sure they all have the same depth of field, so the same points are in focus in all your bracketed photos.  To do this, leave your f-stop constant.

Auto-Bracketing: Why Do It Manually when Your DSLR Will Do It For You?

Depending on your DSLR model, your camera will almost certainly do all the work of bracketing for you. Just about every model on the market today allows you take a series of 3 bracketed photos. Many will let you take 5, and some even 7. Check your user’s manual to find out the specifics for your make and model.

When you set up auto-bracketing, your camera will ask you how far apart you want your stops. A typical range most cameras will allow is from 0.3 to 3.0. If you set your stops to 2.0 with a set of three bracketed photos, when you press your shutter release button one time, your camera will take three photos in rapid succession:

• One at EV -2
• One EV 0
• One at EV +2.

Be sure to set your camera to aperture priority mode so that the f-stop, and thus the depth-of-field, stay constant across all of the exposures.

Here’s an example of three bracketed photos of a sunrise over Lake Tahoe in Nevada.  They were shot on a tripod with the stops set to 2.0 apart, with the intention of eventually merging them into an HDR image.  (These are the three images straight out of the camera, with no adjustments in Photoshop.)

Click an image to view a larger version...

EV +2.  Detail in the foreground trees, but the sky is totally blown out.

EV 0.  Starting to lose detail in the trees, but seeing some in the sky.

EV -2.  Detail in the sky, but the trees are totally clipped.

History of Bracketing

Back in the days of film, well before Photoshop, bracketing was used as an insurance policy, especially by professional photographers on assignment when they were shooting in situations with challenging light.  They could not afford to come back without the best possible photograph, so they would bracket their images, taking two or three shots above EV, two or three below, and a couple at zero.

In the post-film, digital era, bracketing may seem pointless, but it does still have its uses.

Why Use Auto Bracketing Today?

There are three good reasons why setting your DSLR to use auto bracketing might be a good idea.

1. Exposure: Sometimes the perfect image is slightly under or over exposed.
   You may find that while the shot taken at EV 0 is a pretty good photograph, one of the bracketed photos above or below EV 0 gives a more artistic rendition of the same scene, and slightly alters the mood and feel of the picture.  A shot that is over-exposed may give a feeling of intense light, or even heat.  While a shot that is underexposed can offer colors that are more rich and deep.
   
   It’s true that you can play with the exposure after-the-fact in Photoshop.  However, these adjustments simply can’t have the same level of quality and detail as adjusting the amount of actual light that reaches the sensor in-camera. 

   Below are three images taken with the camera hand-held bracketed at 1.5 stops apart, with no intention of ever merging them into an HDR image. In this scene, the sun had just risen over the horizon and was throwing all kinds of light into the morning mist and off the surface of the lake. 

   Click an image to view a larger version...

   EV +1.5

   EV 0

   EV -1.5

   The first image is too bright and not all that interesting to look at.  The difference between the middle and last picture is subtle, but still pronounced.  The third image at EV -1.5 is more striking, and almost haunting.  In this case, the most interesting image was captured below EV 0.

2. Speed: Sometimes you don’t have time to manually adjust the exposure.
   In any situation where your subject is moving or changing rapidly, there is a good possibility that you won’t have time to snap a shot, check it on the LCD, adjust the exposure, and shoot again. Setting your camera to auto bracket can give you a wide range of exposures all with one click of the button.

3. Merging:  Sometimes the perfect photo is a blend of the bracketed series
   In cases where both the highlights and shadows are clipped at EV 0, you can use Photoshop to mask and blend the bracketed photos to fill in the missing details.  Or, you can use Photoshop or Photomatix to merge and tone map the bracketed images, creating an HDR image.  (While it is possible to create in-camera HDR images with most DSLR models today, the in-camera result will usually not be a good as what you can create on your own with software.)
   
   Earlier you saw three photos of a sunrise over Lake Tahoe, each at different stops. Below is the merged and tone mapped image that was created from the three bracketed images above.  The final result is more rich and inviting than any one of the three individual images, and it avoids having either the shadows or the highlights clipped.

   

It’s worth playing around with your auto-bracketing feature, if for no other reason than just to see what results you get.  With SD cards getting larger and less expensive, memory isn't an issue. Unlike the good-old-days of film, you can take as many photos as you want at no extra cost.  

Have fun!