In HDR photography, the median exposure is the base image in our Exposure Bracketing Sequence. Most of the time, the correct median exposure is not what your camera would consider correct. However, having the correct median exposure is the most important part of HDR photography. An incorrect median exposure can ruin your final result as there will be a lot of detail lost in the final HDR image. Detail loss may occur because of adjustments in the exposure in post production or simply because there was no detail there to begin with. In other words, if your median exposure is too dark, the brighter exposure is not going to be bright enough to capture all of the detail in the shadows. On the other hand, if your median exposure is too bright, your darker exposure is not dark enough to capture all of the detail in the highlights. In this article, we will discuss 3 techniques that you can use to verify that you have set the correct median exposure when shooting HDR images.
1. TRIAL AND ERROR TECHNIQUE
The first technique is the simplest one: Trial and Error. With this technique, you will approach a scene and dial in the correct Auto Exposure Bracketing (AEB). For example, you could dial in a 3 frame, 2 stop bracketed sequence. Once you have dialed in your AEB, dial in what you think is the correct median exposure based on your metering mode and the overall scene. Then, simply take a sequence of images. After you have shot your first sequence of images, make sure that the Highlight Alert in your camera is turned on. Then, starting with the median exposure, flip through each image to make sure you can see all of the details in the shadows in your brightest exposure and all of the details in the highlights in your darkest exposure. The Highlight Alert will show you where in your images you have lost detail.
To turn on the Highlight Alert on the Nikon D800, select “Playback Display Options” from the Playback Menu.
Then, go down to “Highlights” and check the box to turn on the Highlight Alert, as shown below.
This Trial and Error Technique is more of a visual confirmation since you are using the Highlight Alert to help you determine whether or not your median exposure is correct. However, this is a great way to check your median exposure. If the median exposure is incorrect, then simply adjust the exposure accordingly until you reach an accurate median exposure.
2. HISTOGRAM TECHNIQUE
The next technique to use is the Histogram Technique, which can be a bit technical. However, the Histogram Technique will yield more consistent results. When you approach your scene, turn on the Live View feature in your camera. Most modern DSLRs should have this Live View feature. Then, turn on the Histogram inside of the Live View. With the Histogram enabled, you can dial in the exposure based on this Histogram.
The image below has a correct median exposure.
The Histogram should have a U-Shape, where the shadows are close to, or pushed along, the left side. The Highlights should also be close to, or pushed along, the right side. Below is the Histogram of the image above. Essentially, your Histogram for the median exposure should look like this Histogram.
With the Histogram up on your Live View, you can adjust the settings in your camera accordingly. Once you are done adjusting your settings, turn off the Live View and turn on your AEB. Then, adjust your focus and any other settings necessary. Once everything is set, start taking your shots at different exposures. Because you have already adjusted the median exposure based on the Histogram, both of your darkest and brightest exposures will be correct as well.
3. INTERNAL HDR TECHNIQUE
Depending on the kind of camera you have, this last technique may not be available for everyone. This technique involves using your camera’s Internal HDR Function. This function allows the camera to shoot a sequence of images and then process them in camera. Using this technique, you will shoot your sequence of images, look at your camera’s internally processed HDR image and see if that HDR image looks right. A correctly processed HDR file should have a Histogram that looks essentially like an upside-down U. You can also look at the image with the Highlight Alert enabled so that you can see all of the details in the highlights and shadows. This technique provides an on-site preview of what that HDR file might look like and we can choose whether or not we want to use the image in post production. Most likely, we will not end up using the camera’s internally processed HDR file because we will be doing our own processing in a separate HDR processing software. However, the camera’s Internal HDR Function does give you a good idea of whether or not you got the median exposure correct. From there, you would dial in the exposure accordingly. If the HDR image looks too dark, then brighten up the median exposure. Likewise, if the HDR image looks too bright, then darken the median exposure.
These are the 3 techniques that you can use to verify that you have dialed in the correct median exposure for your final HDR images. A correct median exposure will ensure that you are maximizing the detail of your HDR images. Overall, your final product will look a lot more professional as your images will look a lot cleaner and have better detail and color.