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What is HDR?

by · February 21, 2012

HDR “High Dynamic Range”

  1. A set of techniques that allows a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging techniques or photographic methods.

What is HDR?

HDR, or High Dynamic Range is simply a technique used to increase the range between the lightest and darkest part of an image, aiming to either match what our own eyes can see, or further enhance the effect.

Our eyes have a much higher dynamic range than any camera to date. This means that on a sunny day you can look at the bright sky and see details in the clouds AND look at a shadow on the ground and see detail within that too. You take this for granted, it’s what you were born with!

A camera on the other hand has a much more limited dynamic range, only capable of viewing one section of the possible range of brightness available in a scene.  Don’t take it out on the camera, it’s all it’s capable of doing at this current moment in time. Even the high end SLR shooters such as the Canon 1D MkV or Nikon D4 still have a much more limited dynamic range to that of your eye.

Human eye dynamic range vs digital camera sensor

The above diagram shows that a digital camera CMOS/CCD sensor has a smaller range available than that of a human eye.

How a camera exposes a scene

When you see a scene which you would like to photograph, your eyes are seeing the scene with a much higher range of light levels.  As mentioned above, you can see detail in the brightest clouds down to the darkest shadow.

Example scene as seen by human eye

Example scene as seen by human eye

When you set your camera for a basic exposure, the camera is setting the brightness level to 18% grey, or “middle grey”.  It is working this exposure value out with it’s available (limited) range – which is less than that of your eye. Using the available range it produces an image which will crop off the lightest and darkest detail, setting what it cannot detect as blown out white, or un-exposed black.

Example scene as seen by camera at 18% grey exposure

Example scene as seen by camera at 18% grey exposure

Now you can adjust your camera to expose away from the default “average” to over expose or under expose a scene, which will give your final image either a brighter look, or darker look.  This is usually called exposure compensation on your camera. By doing this you are shifting your camera’s range away from one side of the available brightness, and allowing it to capture detail previously missed, see below:

Example scene as seen by camera at positive exposure

Example scene as seen by camera at positive exposure

Example scene as seen by camera at negative exposure

Example scene as seen by camera at negative exposure

Seeing what the eye sees

You now have three images, one exposed to how the camera see’s the scene (at 50% average), one image over exposed which shows more detail in the shadows, and one image under exposed which shows more detail in the highlights. Now, what if there was some way to merge or mix the three images to show all the detail from the highlights, down through the mid tones to the shadows?  That technique is known as HDR!

Exposure blend of all three camera images into one shot to increase dynamic range

Exposure blend of all three camera images into one shot to increase dynamic range

Original image from Steve Daggar, Flickr

Shoot to create an HDR image

So now you know the background to HDR images, and why you may want to create them, to capture them you need to shoot with the final post production in mind.  To generate a final HDR image you need a set of images of the same scene – exposed at different values to capture the smallest detail in the shadows right through to the brightest highlights.  A good rule of thumb to do this is to take 5 images:

  • Shot 1 – Correct exposure for scene
  • Shot 2 – Exposure compensation +1 stop
  • Shot 3 - Exposure compensation +2 stop
  • Shot 4 - Exposure compensation -1 stop
  • Shot 5 - Exposure compensation -2 stop

Your camera may have this built in, and it often called bracketing.  You can set up a default exposure and usually 2/4 other points +/- exposure and hit the shutter button to get your 3/5 consecutive shots without having to go into the camera settings and adjust per shot.  Check out your camera instructions for “exposure bracketing”.

Canon AEB Auto Exposure Bracketing

Once you have your 5 shots you can them transfer them onto your PC/MAC for manipulation.

You can also shoot a minimum of three bracketed shots, but this will not give you as much range as with 5, so it’s up to you what you choose, depending on your scene and camera capabilities/skill level.

The more shots you take at different levels of exposure, the more control you will have later on to extract as much information as possible.

Pseudo HDR and RAW

If you shoot in RAW format, you naturally get more dynamic range available than in an jpg, and the RAW file is more forgiving when adjusting exposure in post production.  You can generate your bracketed shots by editing and saving the RAW file as separate images.  This does however tend to increase the noise within your final image as you increase the noise in the over exposed shots, and the final HDR image will not have as much natural range as a true bracketed sequence of shots.

Software needed to create an HDR image

Once you have your bracketed shots, you need to add them to a software package to be generated into an HDR image.  Below are two of the main software packages which I would recommend.

Photoshop

Photoshop CS5 has a custom HDR module now, which allows you to import your image and create a final picture.  To get to this you go to File>Automate>Merge to HDR.  Photoshop is now very good at generating HDR images, and if you already have the package, you may have not known that is was capable of doing this!

Photomatrix

Photomatrix by HDRsoft is considered the daddy of HDR images, and can be purchased as a stand alone product or as a plugin for Lightroom, Photoshop and Aperture. The software is very simple to use and the tutorials available are very easy to follow.  There are a host of options available, including the ability to remove ghosted people which may occur if you are shooting scenes which may change between exposure (such as public areas, stations etc). You can get a trial copy before purchasing to see if you like it.

Photomatrix would be our recommendation.

Do it yourself

The technique of HDR is nothing new, in fact the first use of such a technique to blend exposures was back in 1850 by a chap called Gustave Le Gray.  If you have your images set at layers in Photoshop or whatever image manipulation program, you can use layer masks and layer display modes to manually blend the layers together – masking out areas of complete darkness and over exposed areas.  A simple example would be to take two images of a seascape, one exposure for the sky and one for the sea.  In post production, mask out the sky and sea on different layers to blend the two images as one.

LeGrey brick

Going beyond the limits

There are two sides to the HDR “scene”.  People who are looking to increase their dynamic range to generate a more “true to life” image which matched what they saw with their eyes. And those who like it’s surreal and unique look, over-processing the technique to create unusual and interesting images. See the below image of seized valves with an overprocessed look…

Valves

Either way, HDR is a great technique which should be tried by all amateur and professional photography alike.

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