Before the Coffee

October 13, 2008

The Circular Polarizer and HDR – don’t retire that polarizer yet

Filed under: have the book? Read more, HDR — Tags: , , — beforethecoffee @ 7:46 pm

We are all aware of the benefits of the circular polarizer in single image photography. It has the ability to darken the sky and remove reflections from shiny surfaces. I started wondering, could there be a benefit in using the circular polarizer when shooting HDR image sets or does the merging and tonemapping process nullify or eliminate the polarizing effect? I decided to create two HDR images of a scene, one taken with a polarizer and the other taken without the polarizer, then compare the results.

Some salient points about the polarizer.
1. Darkens the sky greatest at 90 degress to direction of light.
2. Removes reflections or glare (darkens water).
3. Degree of polarization varies with direction of light.
4. Images taken with wide angle lenses can have uneven blue skies (note: 24mm used below).

Single 0EV Images – Polarizer Left Image    No Polarizer Right Image

Compare these single images and you’ll notice the polarizer (left) has darkened the sky and has dramatically reduced the glare on the red chairs. Notice the Polarizer has also created a triangular shaped blue region in the top right. This is due to the 24mm focal length capturing a large region of sky and in this case the direction of light approaches 90 degress toward the top right.

HDR Images Polarizer Left Image    No Polarizer Right Image
(both tonemapped with default settings in Photomatix)

HDR processing of the polarized image set (left) and the unpolarized image set (right) has created similar tones in the blue sky. Using a polarizer doesn’t help the saturation of the blue sky when creating HDR images. The HDR merging process simply uses a less exposed image in the set to achieve saturation.

When it comes to glare reduction it’s a different story. The polarizer offers a noticeable benefit in the reduction of glare and more saturated colors when creating HDR images. Notice the red chairs and compare points 1, 2, and 3. I would still keep your polarizer in your camera bag and use it for glare reduction on those shiny surfaces.

May 4, 2008

Digging a little deeper into HDR

Books are constraining by nature, they go through a stage called editing and editors do what they love – “Edit.” It’s vitally important that a “How to” book not be too long and drawn out that the reader can’t get through it. As a new addition to my blog, I’ll put additional information on topics that were cut. If you have the book this will be an opportunity to learn more and if you don’t have the book it’s a taste of what the book is about.

The chapter on Software Comparison Page 74, shows the 0EV image and 5 images processed by the various programs: Photomatix Pro, FDRTools, Adobe Photoshop CS3, Dynamic PhotoHDR, Artizen HDR. Here is the 100% crop of each image and a short comment that didn’t make it to print.

Single 0EV Image – When comparing the single shot 0EV image with the HDR images as a whole, there are two salient points. First, the single 0EV image has greater global contrast and second, it has greater noise. Having greater global contrast is not a bad feature but the noise is bad as it destroys the finer details of the image. Those details are never recoverable. Notice the noise in the darker areas of the image.

EASY HDR the “Mask” Operator (default) has done a good job of rendering the scene and it’s hard to find any area to criticize. The wood design and dove on the flag are well rendered with low noise and good details. With the default settings, the image appears slightly flat but this is only a matter of fine tuning the black and white points in Levels.

 

Artizen – Lock06 (default) suffers from a loss of detail due to noise, most likely being taken from the –2EV exposure. The flag is not well reproduced; the white dove has a loss of detail and is near over saturation ie. blown pixels. Additionally, it appears the 0EV image has more details and less noise.

FDRTools Compressor (default) has done an exceptional job rendering the details of the scene. The image has low noise with excellent local detail enhancements. Local tonal variations (contrast) bring out the texture and 3-D feel of the wood. The white dove is accurately rendered with good detail in the wings. The FDRTools image is better than the single 0EV image in color, noise-free detail and dynamic range.

 

Photomatix – the 100% crop shows that Photomatix handles noise well with no apparent increase over FDRtools or PS. The dove is well rendered with details inside although it has a slight magenta/red cast. The Photomatix image is slightly softer than the 0EV image but it has noise-free detail and displays a higher dynamic range.

 

Photoshop CS3 – Local Adaptation has also done an excellent job capturing the detail in the wood in CS3. Take a moment and examine the wood figures – notice the waistline, arms etc. in each image. In my opinion, Photoshop CS3 is the best. The overall color is accurately captured giving the image a realistic look. [The book goes into more detail on the weaknesses of CS3 when the dynamic range of the scene is high. This example is a medium contrast scene and is not a problem for CS3.]

Dynamic Photo HDR – I used the “Eye Catching” tone mapping operator and as you can see the 100% crop shows good detail and local contrast. There is color shift similar to Photomatix.

 

 

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