Sunday, June 10, 2007

Introduction to HDR photography. First posting!!!

An introduction to HDR photography

All cameras are limited in their ability to capture detail across a wide range of brightness values. The range of brightness values that a camera captures detail in is its dynamic range. In general, most digital cameras have about an 8 stop dynamic range. This is debatable though as it is really governed by how much noise you are willing to put up with. The darkest tones in a digital image are always the noisiest. So, your measure of dynamic range depends on whether or not you want to include the noisiest values recorded. Any case, there are innumerable situations in which the camera is not able to capture detail across the entire range of tones that we would like it to.
A classic example would be trying to take a photo of your living room in which you retain full detail in the scene through the windows and inside the room. Unless you are shooting on a very overcast day and your house is extremely well lit, the contrast of the scene is simply too great for the camera to capture detail throughout. This leads to compromises. You could expose for the windows and lose detail in the room, expose for the room and lose detail in the windows, or split the difference and lose some detail in both. None will result in the image that you really want.
In the past, I would suggest that you take two photos; one exposed for the windows and one exposed for room. You could then mask the two together. Depending on the image, this could take a great deal of time. However, you can get really great results doing this. Another problem with this technique is light spill. I you have some light source creating a gradient of light across the floor or wall, it is very hard to mask that in such a way that it will look natural.
Enter HDR. HDR is an acronym for High Dynamic Range. An HDR file is made by combining the detail from multiple exposures into one 32bit image containing far more information than a single shot could. Basically, you take the same photo multiple times while varying the exposure between each shot. This insures that you have data over a larger range than you could capture with a single shot.
This expanded range of tones contained in an HDR file is then mapped down to a range that you can work with in your image editor. I would suggest you tone map it down to a 16 bit per channel image.

How to shoot a scene to be combined into an HDR image
Let me begin by saying that this technique is only applicable to a scene of high contrast such as your living room during the day. Other examples would be a scene with containing deep shadows and open sunlight. HDR imaging also lends itself to the night photography very well as it allows you to retain detail in those areas that are not directly lit.
How do you know if the scene is high enough in contrast to warrant HDR techniques? If you have significant data climbing the left side of the histogram when you properly place your highlights as far to the right wall as possible without touching, then you should consider HDR techniques.
I you should begin by determining your initial exposure for the highlights that you wish to maintain detail in. Then change your shutter speed to allow in one stop more light. Repeat this until you are certain that you have captured all the detail in the shadows that you wish to maintain. Basically, the last histogram should show a ton of data climbing the right wall and nothing should be on the left wall. In fact, you should have a sizeable gap between the left wall and where the data begins. This will insure that the darkest areas of the image are recorded with ample light and will have little or no noise in the resulting combined image. I would suggest at least 5 shots. I think I have done up to 10. It can't hurt you to take too many. You can always clip some unwanted data when you tone map the HDR image down.
HDR photography is an advanced technique and accordingly requires good technique.
Things to keep in mind when shooting your images:

●You need to use a tripod. I have taken images without a tripod and then tried to combine them into a HDR image using multiple different programs with no success.
●You should set your white balance manually so that it doesn’t change between shots. I suggest you use the daylight setting when shooting at night. This insures that the funky colors of lights are recording as funky colors.
●Focus once and then don’t refocus. If you don’t have the best vision, you can always allow auto focus to determine the focus for the first image and then turn if off. If you are using a compact digital camera that doesn’t allow you to manual focus, then just be sure that it achieves focus at the same distance each time.
●Capture RAW files if your camera supports them. If not, capture using the highest quality JPEG option.
●Use a remote release or self timer to trigger your camera. If you shake your camera when pressing the shutter release button, then you risk blurry images and images that don’t register correctly.
●Use a lenshood to reduce the chance of lens flare
●Turn on your camera long exposure noise reduction feature. You are likely going to end up with some longer exposure times. This feature helps keep your images as noise free as possible
●Use manual exposure mode. You will need to change your shutter speed rather than your aperture for each shot. Manual mode will allow you to do this. If you want to try this with a compact camera that lacks manual exposure capability, then you have to use exposure compensation to vary your exposures. This should still work fine even if it changes the aperture as compact cameras always have great depth of field due to their small sensor and optical system.
●Use your lowest ISO. Since this technique requires that you take multiple exposures, it really lends itself to non-moving subject. Thus, the long shutter speeds necessitated by using a low ISO isn’t an issue. This will keep noise levels down even further.

Creating an HDR image and tone mapping

There are a number of programs available to create an HDR image from your multiple exposures. I suggest you start with Photoshop CS2 or CS3 if you already have it. If you don’t, try one (or all) of the free programs. They do a great job without all the bells and whistles that you may not need or want initially (or ever for that matter). Each program uses its own algorithms for tone mapping. It has been my experience that some types of images work better with some than others. I have categorized them by cost:

Free programs:
Freeware. This a full featured HDR generator with tone mapping capabilities. The homepage is in Austrian, so I can’t tell you much about the author. Most featured of the free programs.

●FDR Tools Basic. Freeware. This a free version of the extremely full featured FDR Tools Advanced. As such, it is very basic. Just enough to let you see the possibilities. No options to speak of in terms of tone mapping. Very easy to use though.
●EasyHDR Basic. This a free version of the extremely full featured EasyHDR Pro. Offers some tone mapping controls. Only supports JPEGs.

For cost programs:
●Adobe Photoshop CS2 and CS3. I begin with Photoshop as many of you already have it. Navigate to File > Automate > Merge to HDR. Once you generate the HDR image, four different methods of tone mapping become available when you opt to go down to 16 bit or 8 bits per channel. The last one offers the most control. Don’t be put off by the initially terrible preview. Enable the advanced options and you are presented with a curve. Begin by setting your black and white point. It will already look much better. Generally, your image will benefit from bringing down the three quarter tones. Then edit the rest of the curve as desired. I have read the the algorithms used for tone mapping have been improved in CS3, but I can’t say that I have noticed.

●EasyHDR Pro. 25 English Pounds. (around $40 dollars). Full featured program with a nice user interface. Trial version is available.

●FDR Tools Advanced. USD 57.69 Full featured program. It is supposed to support masking which would allow it to be used with scenes with moving subjects. Haven’t had a need for that yet, so I don’t know how well it works.

●PHotomatix. USD $99. Well established and full featured application. Good website. Some good resources. Feature rich. Also features the ability to tone map just two images together.

Happy shooting!

Seder Burns

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