Actually, they probably aren't noisy but if you clicked on this link, I bet they are suffering from grain. What's the difference between noise and grain? Let's talk. This post is an excerpt from my book “Improve Your HDR Photography.” If you'd like to buy the book, you can buy a digital copy for $5.99 direct from this website and then download the book to read on your computer. It's also available on the Kindle store, iBooks app (for iPad, iTouch, iPhone), Nook store, etc. Just search the name of the book in those stores and it'll pop right up.
Noise and grain are not the same thing. Noise is captured in the camera when stray electric signals are misconstrued by the camera as being light. The same thing happens when you use an old-school audio cassette. If the sound is quiet, you turn up the volume on the cassette player, and when you do so, you hear fuzz along with the sound of the voice on the recorded audio cassette. This occurs because the player confuses stray electric signals as being the source sound.
Sometimes you can capture perfectly noise-free and clean digital source files by using ISO 100 and short shutter speeds, but when you process the HDR, the result is a very grainy image. Is the grain caused by the camera confusing stray light signals as light? No. This grain didn’t exist on the source images at all. This grain is caused by Photomatix when you tonemap the HDR file improperly.
There are many sliders which, if used to too great an extent, can cause grain on the image even if the source files were perfectly noise-free. Your secret weapon for combating excessive grain in your HDR images is the 100% preview. Simply click on any area in the large preview of your image while in the tonemapping window and you will see a loupe appear which shows a zoomed-in view of a small portion of the image. That 100% view will always have some amount of grain when tonemapping, but the key is to keep the grain to a manageable level.
The first and foremost setting to manage is the smoothing setting. Choosing Max or High will produce significantly less noise than Min or Low.
The second most important setting is the strength setting. While a high value produces that stunning HDR look that we are searching for, it also adds to the amount of noise in the photo. Lowering the strength also lowers the amount of compression in the photo, which affects the amount of grain.
Luminosity and gamma are both tools which affect the brightness of the image. As was discussed above, it is best to leave these settings where they are or, if necessary, only adjust them slightly, to reduce the grain. Generally, it is best not to touch these settings at all and adjust the brightness in Photoshop.
The last setting which significantly affects the grain is the micro-contrast. The micro-contrast accentuates fine details, but it also adds to the amount of grain in the image. Like most of the settings in Photomatix, this setting is a trade-off between detail and grain.
There are other settings which can affect the grain in an image, but they have less impact on the amount of grain than the settings mentioned here. To know which settings can also affect the grain in the image, refer to the tip above about using the loupe view to carefully monitor the amount of grain as you tonemap.