HDR – Part 2
Ah yes, on to Part 2 of the HDR Chronicles. In this part I would like to cover Tone Mapping and explain the image controls and what they do.
Part 1 has brought up some questions from posters and some thoughts I might like to clarify. First off, HDR is used to enhance or expand the dynamic range of an image beyond that of what the camera can naturally capture in a single frame. Thus you can get more “true to the eye” look to photos, such as you would see if you were viewing it with your own eyes. Of course this is if you choose to process your images for a realistic look. Sometimes you want hyper-realistic and that is a perfectly valid use for HDR as well.
True HDR images are made from a mixture of MORE than one exposure taken with the camera. Ideally the exposures used should cover the entire dynamic range of the image you are trying to produce. If that takes +/- 2 stops on top of the main exposure, go with that. If you need 5 or even 9 stops extra to get a very dynamic environment, by all means go for it. If you want to manually bracket 32 exposures for some odd reason, give it a try, you might get interesting results. Using extra frames may not help, but it should not hurt either. And the more data the software has to work with, the more likely the end result will process with smoother gradients, less noise and more detail. I call HDR using a single frame either reprocessed to multiple exposures or imported directly as a RAW file a “Psudo-HDR”. The Psudo-HDR can give you similar results as a true HDR but generally they contain more noise and have less accurate color and detail. I will be covering this in another part so hang on for that if you are interested.
Please note, Photomatix suggests using a minimum of 2 stop EV range for your brackets unless you wish to take extra frames. This is not possible with all cameras so check your setup before shooting. I personally use a 1 stop bracket with my Nikon DSLR but often shoot 5 to 7 frames for outdoor scenes and 7 to 9 frames indoors.
Anyhow, back to Tone Mapping.
The last thing we have seen in Part 1 was a dark, contrasty looking image which is the representation of the 32bpp HDR file on-screen. To make that into a useable, printable image we use Tone Mapping which simply takes that data and uses the parts of it that you want based on the settings you tell the software to use. Of course that can vary from mild to wild.
Here is the Tone Mapping Settings control panel in Photomatix 3.1.3. These may vary slightly in PC versions and for users of older versions of Photomatix but the settings should still function similarly.
First, the Details Enhancer tab. From the Photomatix 3.1 User Manual:
“The Details Enhancer method is based on a local operator, which means that it takes into account the local brightness context for tone mapping. Pixels are processed differently depending on whether they are located in a bright or dark area of the image. This method offers settings that let you process the image in creative ways.”
This is the tab you want to use for really psychedelic looking HDR images. But don’t let that fool you, the HDR shown in Part 1 was done using the Details Enhancer mode so it is perfectly good for doing realistic images depending on your settings.
Strength: This controls the strength of contrast enhancement. The higher the setting, the more contrasty the image. For my “W Bath” HDR I am using a setting of 37.
Color Saturation: Works just like you would expect. Higher values result in more saturated color. A setting of 0 results in gray. I stay between 50 and 70 for most of my images.
Luminosity: “Controls the compression of the tonal range.” A higher setting boosts the brightness but also increases shadow detail. Lower settings tend to look more natural. I tend to use between -2 and +2 most of the time. Higher and lower settings can be used for more intense looking HDR images.
Light Smoothing: This controls the smoothing of contrast variations over the image. Higher settings are good for more natural looking images while low settings will result in a fake and “over processed” look. I tend to use the Medium or High setting for most images. A setting of Very High gives almost a dreamy look while a setting of Low or Very Low looks “artistic”. (Like a typical HDR most people brag about online. Besides, who wants “normal” looking when you can process it “fake” looking right?)
Microcontrast: This controls local details. I rarely make much of an adjustment to this. Default is 0 which is where mine stays. Low settings will soften the image overall and high settings will give a little more “grungy” looking contrast.
The next section of controls is the Tone Settings. This is where you can adjust the white point, black point and gamma.
The White Point and Black Point settings control the maximum values for the brightness and shadows of the tone mapped image. These are directly responsible for the contrast of the tone mapped image. You can think of these as the first and last sliders on the Levels control in Photoshop. A high white point will use more of the brightest pixels and have brighter highlights while a lower setting will have lower contrast and “dull” highlights. A high black point will result in darker shadows and of course a low setting will have greater shadow detail.
Gamma: This adjust the mid-tones of the tone mapped image. Higher settings (smaller number) will result in a brighter image. Think of this as the middle slider on the Levels control in Photoshop.
The Color Settings controls are fairly simple. I don’t use these often as I do any color tweaking later in Photoshop or in a prior step using Lightroom.
Temperature: This adjusts the color temperature of the image. This should be self explanatory.
Saturation Highlights: This controls the color saturation of highlights relative to the Color Saturation slider above.
Saturation Shadows: As above except for shadows.
Now to the Smoothing Settings which are somewhat tricky as the controls don’t really show much of a change in the preview window.
Micro-Smoothing: This smooths “local detail enhancements”. For example, this can be used to reduce the noise in the sky in the final tone mapped image. *Photomatix states that the Loupe view may not properly show the effect of Micro-Smoothing. I usually only keep this at the default value of +2 but higher settings may be more pleasing if images don’t need to contain a lot of fine detail. (Think of it as a “reverse” Unsharp Mask that only softens very fine detail.)
Highlights Smoothing: Reduces the contrast enhancements in highlights. This control is good for reducing “halos” around objects on bright backgrounds. Default is +2 but I do sometimes go as high as +10. Using a higher setting will result in softer details so if you are trying to retain a great deal of textures, a lower setting may work better for you.
Shadows Smoothing: Reduces the contrast enhancements in shadows. Like the Highlights Smoothing slider, higher settings will “clean up” noise and unwanted details however setting this too high may result in overly softened shadow details. I find a value between 0 and 20 to be good depending on the image. Settings over 40 or 50 will greatly soften shadows if you are not worried about shadow details.
Shadow Clipping: Use this slider to control how much of the shadow range is clipped. This is useful if the shadows contain a great deal of noise. This sets anything below the sliders setting to maximum black effectively leaving no detail behind at all.
Lastly, the 360° Image setting is only for panoramic images that wrap in a complete circle.
At the bottom of the controls is a menu for Reset, Undo, Redo, Load, Save and some other recently used options. When I get a setting a way I really like the results, I try to save those settings with some details so I know what I used them for last. This way I can load those settings again if I am processing another image of the same type that may work well with those settings. Of course the very last button is “Process”. As you can guess, that simply processes the image using the tone mapping settings.
Whew…. So far so good. (Someone get me a drink, we’re not done yet.) Now that I have more or less quoted half of the user manual that nobody bothered to read, lets put all that to use.
More photos and examples in HDR – Part 3 coming soon. I will have some “play along” samples for download so users can try it out on their own in Part 3.
Missed HDR – Part 1?