Sunday, August 11, 2013


High Dynamic Range is a method of merging photographs to capture all of the visible scene in one image. HDR is only needed when the dynamic range, the difference from light to dark, is greater then the camera’s image sensor can capture. Many of todays cameras have a dynamic range of around 11 stops, the human eye has a dynamic range of around 14 stops. For this reason it is common to see photos where the landscape has a good exposure but the sky is blown out and has lost its blue color and all of the detail in the clouds, or the sky is in perfect exposure but the landscape is dark, lost in shadow. Using HDR to merge photos of differing exposures allows a person to create a photo that has detail visible in both the sky and the landscape.

Many of todays cameras have a built in HDR setting which removes the need to have software capable of merging the images for you, though software gives much greater flexibility in exactly how the images are blended. For instance on my Panasonic DMC-ZS10 point and shoot the HDR setting is a Scene function of the cameras automatic setting, as such I have no control over the aperture or shutter speed of the camera. On this camera when I use the HDR setting there is a slight lag to the shutter, the camera holds the shutter open creating an image out of the data captured. In this manner an image is created with a wider dynamic range then the camera would otherwise be capable of.

On the EOS Canon 6D the HDR functions more like it would if the photographer were to preform the image merging on computer in post production. The 6D has an HDR function that operates as part of the manual settings on the camera and can operate in M, P, Av, and Tv modes. On this camera when HDR is set you meter for the mid-tones and then select an exposure compensation value of either 1, 2 or 3 stops when the shutter release is pressed the camera quickly takes three images one exposed for the mid-tones one under exposed for the highlights and one over exposed for the shadows. The camera then creates a single image out of these three files.

In both of these cases software in the camera is attempting to mimic what can be done on computer in order to capture a wider range then the sensor normally would. Before digital HDR did not exist but other methods were employed to capture a wider dynamic range, HDR is an extension of those techniques. In the days of film when a photographer needed to capture a wide dynamic range a gradient filter would commonly be used to stop down the sky so that it was closer in exposure value to the landscape. If further correction was needed the developer of the film could selectively dodge and burn the film to lighten some areas and darken others. Gradient filters can also be purchased in blue or orange to better match the tint of the sky or sunset.

As photoshop and digital manipulation started to advance as a tool it became possible to capture a wider range of information then had been possible on film. Most film has a dynamic range of 4 or maybe 5 stops, this was particularly important in studio lighting and is where the 3/2 lighting ratio for subject and a background 2 stops brighter then subject (when you wanted a white background) come from. When you have a total of 4 stops it became critical to keep the lighting in a narrow band. The early digital sensors also had a very narrow band of sensitivity, however they had one distinct advantage over film. With digital it is possible to take several images at different exposure values stack them up and merge them into an image with a dynamic range of 10, 20 or even 100 stops if that were necessary. The spectacular images from Hubble are examples of HDR taken to extreme limits.

The other option that is available to digital is similar to the dodging and burning of the negatives during development. This is the use of RAW files, I mentioned above that many modern cameras have a dynamic range of 11 but there effective range when generating 8 bit Jpegs tends to be closer to 8 stops. By shooting in RAW and exposing for the highlights (this is key as anything lost in the highlights is lost) a photographer can then go into the file on computer and use RAW processing to pull up information that is being lost in the shadows. From this file an 8 bit Jpeg can then be produced that fully utilizes the 11 stops the camera has.

Now that we have spent a lot of time on what HDR is and where it comes from I am going to give a brief run down on how to do it. If you are about to ask why I am not spending more time on the how too, that is because it is about to be surpassed by technology, I mentioned that many cameras are already capturing 11 stops, they will soon be capturing more stops and in greater detail then the eye can see, as the greater demand for high definition moves forward the 8 bit format for Jpeg images, computer displays and printers will be surpassed by 16 or even 32 bit formats and the need for image stacking will again slip back into the realm of astronomy where it is being used to capture massive amounts of data that is not available in visible light. I have some confidence that this prediction is correct, in that, 8 years ago to get a good HDR image it required stacking 5, 7, or even 9 images to get a wide enough range of exposures. Now three images will allow you to use the 11 the camera captures plus 2 stops (or three if you use a 3 stop interval) below and above giving a rang of 15 (or even 17) stops.

So now we have an idea of what HDR is, how does it all work? I brushed over that above in that you take pictures with different exposures. The setup up is really quite simple though the blending on computer (the post) takes a bit more practice. To take the images you will need a camera and a tripod. It is possible to make an HDR with the camera set to auto (in some cases) by using the cameras exposure compensation settings to get the ver and under images, but it is best if you are comfortable shooting in full manual mode. Once the camera is on the tripod the image is framed and the focus has been set, turn the auto focus off (if you did not focus in manual) take your first images metered for the mid-tones then use the shutter speed to adjust down 2 stops, so if your mid-tones were shot at 1/125th then your shadows will be shot at 1/30th and then adjust up two stops above the first image to 1/500th to shoot your highlights. 

If you are using an older camera and need a wider range you could also shoot down an additional 2 stops at 1/7th and up two more at 1/2000th, but if you are using a fairly modern camera (made in the last 3 years or so) then you really wont need to. Once you have the 3 (or 5) images you will merge them in Aperture, Lightroom or Photoshop - depending on your software you may need to find an additional plugin to do this.

When is HDR used? HDR really is best used in situations where the dynamic range is too great for the camera and modifying the light is not practical, several examples are sunrise/sunset photos, landscapes, cityscapes, architectural photography, things that are too big to effectively light AND the dynamic range of the scene is greater then the camera can capture. If you are looking at a scene and the camera can capture the image without an clipping occurring in the histogram HDR is not needed.HDR is not really practical for subjects that move, portraits, pet photography - anything like that it is much better to correct the lighting before the image is taken then it is to try and get 3 or more images where everything but the exposure is the same.

Image created with HDR
Image created with HDR
This first image is an HDR composite.

The three following images are the multiples used to create the HDR shot at 2 stop intervals 

2 stops underexposed to get highlights
2 stops underexposed to get highlights

Exposed for Mid-tones
Exposed for Mid-tones

2 stops over exposed to get shadow detail
2 stops over exposed to get shadow detail

This last image is the scene as the camera wanted to capture it using the auto setting

Comparing the first to the last it is possible to see how HDR both allows a photographer to capture a wider range and how close camera is getting to making HDR an unnecessary art. If you wish to explore HDR in more depth I suggest looking to the works of Colin Smith

Follow Us

No comments:

Post a Comment