May 12 2006

Simple Guide To HDR

Published by at 5:53 pm under Photography

What Is HDR

In computer graphics and cinematography, high dynamic range imaging (HDRI for short) is a set of techniques that allow a far greater dynamic range of exposures than normal digital imaging techniques. The intention is to accurately represent the wide range of intensity levels found in real scenes, ranging from direct sunlight to the deepest shadows. From: Wikipedia

HDR images are generated by taking multiple pictures of the same scene at different exposure settings and combining them digitally on a computer. Generally, the more pictures that are taken at different exposure settings the better the resulting HDR image will be.

Required Equipment

The very basic requirements for producing HDR images are a digital camera and a computer (which I’ll discuss later). However, it is strongly advisable to get a good tripod, the sturdier the better.

The Camera Surprising as it may seem all of my HDR images have been produced from pictures taken on a Canon IXUS500 digital compact camera. So long as a digital camera has exposure compensation it is suitable for producing HDR images. The list below is a random selection of cameras I found on Dabs.com that all support exposure compensation:

  • Sony DSC-S90 Cyber-shot £128.07 inc vat
  • Samsung Digimax V700 Red £194.27 inc vat
  • Casio Exilim Pro EX-P600 £275.98 inc vat
  • Canon Powershot S70 £239.35 inc vat
  • Samsung Digimax V700 Silver £187.81 inc vat

The Tripod One of the most import issues when taking pictures for a HDR image is that the camera must not move between shots. To reduce camera movement a good quality tripod should be used. However, when purchasing a tripod you should consider the camera the it will be used for. For example, I wouldn’t go out and purchase a £150 Benbo tripod for a compact camera and like-wise, you shouldn’t use a cheap compact camera tripod for digital SLR. I’ve personally gone with a Jessops Tripod 315 at £18.99 (inc VAT), it’s probably not the best tripod for compact cameras but it does the job I require just fine.

Taking The Pictures

The Subject When taking pictures for a HDR image the best place to start is where you have got strong contrast between light and dark areas. For example, inside looking towards a window. Assuming the window is in the middle of the picture and the metering mode is set to centre-weighted, a normal picture will show the window being correctly exposed but the surround detail will be very dark. The result of producing a HDR image will be to brighten up the areas surround the window without over-exposing the window itself.

The best time I have found to try out HDR is when there is bright sunshine as this tends to casts very strong shadows. If you start by tacking pictures on an overcast day you will find the light a lot more diffused and so the results wont be so dramatic.

Night time offers a lot of opportunities for interesting HDR images, but beware that the exposures will be a lot longer and will therefore require the camera to be held a lot steadier.

Taking the pictures The most important thing to consider whilst taking the picture is movement of the camera between pictures. If possible set the camera to a mode where you are able to change the exposure compensation without having to navigate through menus, this is one of the strong points of the Canon IXUS500. Every movement of the camera will result in the image becoming more blurred, whilst some software is able to align images before combining them, there is only so much movement that can be corrected before the images become distorted.

It is practically impossible to hold a camera in your hand whilst take the pictures, which is where the tripod come into play. Make sure the tripod is standing on a solid base or, if you are not using a tripod, make sure the camera is not on a surface that it is likely to slip on. If your pictures aren’t fairly well aligned when you take them the HDR results are going to be disappointing.

Movement In The Picture Whilst it is not desirable for the camera to move whilst the picture is being taken it is possible to achieve interesting effects if there is movement within the scene being pictured. An object the is moving slowly will become blurred in the final HDR, where as objects that move a lot will become ghosts. I haven’t tried it yet, but I have seen interesting effect produced by taking long exposures at night of car light trails.

How Many Exposures Whilst it is possible to generate a HDR from two images the result will not be terribly satisfying. My personal preference is to take five images at 1 stop intervals from -2 to +2 (the full range of the Canon IXUS500). The general rule is that the more exposures are taken the better the results will be.

Also, the greater the exposure compensation range the better the results. If you’re taking three pictures it is better to take them at -2, 0 and +2 rather than -1, 0 and +1. Those that are lucky enough to own a Canon EOS-1Ds Mark II will benefit from its -3 to +3 compensation range.

Combining The Pictures

The final part of the process is combining the images. For this you will need a computer and some HDR generation software.

Hardware Requirements Combining pictures into a HDR image requires a fair bit of number crunching so the more powerful your computer the quicker you’ll get the results. Also, if you’re working with large images you need to make sure you’ve got plenty of RAM. My home machine is an Athlon XP1800+ and 1.5Gb RAM but you could probably cope fine with less. If you’ve got got too much RAM make sure you close any un-necessary applications, i.e. clear out the apps that sit in the system tray.

Software Requirements There are several software titles out there that help you generate HDR images. The most popular being Adobe Photoshop CS2 9.0, Photomatix and Artizen HDR. I personally tend to use Photomatix as it is easy to use, has the ability to automatically align images and has easy to use tone mapping functionality. The rest of this article will be based on Photomatix.

Generating The HDR

  • From within Photomatix select the HDRI menu and then select the Generate HDR option, this will open a new window.
  • Browse for and select the component pictures you took for the HDR, click OK.
  • You now have the option of Standard response curve or Calculate response curve. Again I normally go for the standard option.
  • You also have the choice of whether or not the automatically align the images, its probably a good idea to select this option but if it doesn’t work you’ll need to re-run this wizard.
  • Clicking will run the HDR generator and the result will be displayed in the main Photomatix window.

Do not panic if what you see is not what you are expecting from a HDR, we haven’t quite finished yet. Next we need to adjust the tone mapping, this option is accessible from the HDRI menu. The options on the window that opens up are as follows:

  • Luminosity: Adjusts the brightness of the shadows and the amount of local contrast enhancement. Moving the slider to the right has the effect of boosting shadow details and brightening the image. Moving it to the left gives a more natural look to the tone mapped image. The optimal value depends on the image and the effect you want to achieve.
  • Strength: Controls the strength of local contrast enhancements. A value of 100% gives the maximum increase in local contrast. The optimal value depends on the image and the effect you want to achieve.
  • Color Saturation: Controls the saturation of the RGB color channels. The greater the saturation, the more intense the color. The value affects each color channel equally. The optimal value depends on the image.
  • White Clip – Black Clip: Both sliders control how the minimum and maximum values of the output image are set. Moving the sliders to the left increases global contrast. Moving it to the left reduces the clipping at the extremes. The White Clip slider sets the value for the maximum (pure white or level 255). The Black Clip slider sets the value for the minimum (pure black or level 0). The optimal values depends on the image.
  • Smoothing: Controls the amount of smoothing of luminance variations. A higher value tends to give a more natural look to the image. A lower value increases sharpness. In most cases, the optimal value is either “Medium” or “High”.
  • Microcontrast: Controls the accentuation of local details. The default value (0) is the optimal value in most cases. However, this control may be useful in the case of a noisy image or when the accentuation of local details is not desirable (e.g. seams of a stitched pano in a uniform area may become visible when local details are too much enhanced).
  • Pixel depth of output image: Specifies the pixel depth of the resulting image. A value of 24-bit compresses the dynamic range down to an 24-bit RGB image that can be saved as a JPEG or 8-bit TIFF file. A value of 48-bit compresses the dynamic range down to a 48-bit RGB image that can be saved as a 16-bit TIFF file. The default value is set to 24-bit. You can change the default in the Preferences panel under the Photomatix menu.
  • 360º image: This option needs to be checked when the image processed is an equirectangular image set to be viewed as a 360º panorama. Given that the tone mapping takes into account local contrast, the 360º seams of an equirectangular image will be assigned different tonal values, which will result in a visible seam once the tone mapped image is rendered in a panorama viewer. Checking this option will correct for this. This option should only be checked for equirectangular 360º images. Checking it in other cases may produce less optimal results. Note: Checking this option increases the amount of memory necessary to process the image by about 50%. Processing times will also be increased.

Once you have adjusted the image to your liking just click OK and save the image.

There are several points I like to take into consideration:

  • Firstly, I’m not keen on images that have bright edges around dark objects. I find the point of HDR is to make a scene seem realistic whilst giving it a hyper-real appearance. When was the last time you saw a halo around a tree?
  • Secondly, look for a scene that lends itself to HDR usage. I have come to learn that there is no point in sacrificing a perfectly good image in the hopes of getting a mediocre HDR image.
  • Thirdly, be critical about you work. Look at what other people are producing and try to get your images to be of a better quality. Even if the subject isn’t as good, try to make the focusing better and the image alignment better

All too often, and I too am guilty of this, I have seen images that look like the photographer hasn’t put any thought into the image, they’d just heard about this fashionable new technique and decided they’d try and bash out a copy in five minutes. Don’t worry if your first versions aren’t anything to shout about, everybody needs to learn from their mistakes… just make sure you do learn and don’t keep repeating the same mistakes.

HDR images can be very rewarding, good luck!

7 responses so far