HDR imaging using a simple LPF type luminance mask.

Luminance masks are a powerful channel chop that you can use to create HDR images from two bracketed frames.

With HDR Luminance Masked Shadows

Without HDR Luminance Masked Shadows

Loading the composite channel’s selection in the Channels window creates a luminance mask. This causes all values in the image above middle gray to become selected as a continuous tone image rather then a regional selection. This simple mask splits the tonal range in the middle allowing you to take the highlights from one image and the shadows from another and have a great deal of logical control over the blending. Because of the continuous tone nature of the mask there are no masking lines. The highlight mask is then inverted to create a tonal Low Pass Filter with a bandwidth of 128 and a band center of 64. The main difference between this and a standard LPF response is that there is only roll off on the high end of the scale creating a soft transition between the two exposures. There is no roll off on the low end.

This can often produce a superior result to using Photomatix or other HDR compositors as you have a Photoshop workflow which has familiar controls and does not mangle the colors and basic tonalities.  Photomatix can be good but has its foibles. I have been working on HDR methods for use in the context of Photoshop. I have yet to hear a report of the Photoshop compositor functioning so that is not part of any of my solutions.

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I have a simple two layer example that demonstrates the concept. The frames in this case are 3 stops apart. One frame exposed for the sky and one that is three stops over, N and N+3. The exposure is changed via the shutter speed to prevent differences in depth of field. The example has the luminance mask on the shadows frame. The Shadows frame is very light but since all values above 128 are masked (discarded) it can be considered to be ETR’ed or Exposed To The Right. This allowed me to drop the brightness significantly and gain better middle tone separations.

I burned the right side of the sky some in the RAW converter on the way in to Photoshop. When I had the layers imported into Photoshop I knocked out some of the darker sky in the Shadows layer with a paint brush. This allowed me to take the whole sky from the Highlights layer.

Here is the Step by Step:

1. Select files in Bridge
2. Adjust in the RAW converter and hit Done button
3. In Bridge select files and use Tools->Photoshop->Load Files Into Photoshop Layers
4. Put the lighter layer on top and name it Shadows with the darker layer below it named Highlights.
5. In Photoshop I selected both layers and used Edit->Auto Align Layers - Collage
6. Select the lighter (Shadows) layer, go to the channels window and hit the Load Channel as Selection Button (dotted circle) at the bottom of the window.
7. This selected all values above 128
8. Invert the selection, Selection->Inverse
9. In the layers window select the Shadows frame
10. Press the Add Vector Mask (dot in dark rectangle) button at the bottom of the Layers window.
11. A continuous tone transparency mask will appear on your shadows layer that will expose the values below 128.
12. You have now created a HDR image using a luminance mask.

Once you have reached this point you are ready to adjust the balance. Use the Clip to Layer button to add a levels layer to the shadow layer and adjust. You can also achieve a similar effect by using the transparency mask from the Shadows layer on a Levels layer. After some levels adjustment of the shadows you can mellow the effect by using the Layer Blending dialog for the Shadows layer and setting the Blend Mode to Normal and the Opacity to less then 100%.

My goal in dealing with excessive contrast is to make an image where it is not obvious that the image is HDR other then the fact that a skilled photographer will recognize that it is a high contrast scene with full detail in shadows and highlights.

Here is a zip file with a small layered example file:

http://www.cookseytalbottgallery.com/shared/HDR-LPF-Luminance-Masking.zip

In the example I have not done any of the burning and dodging that I would usually do to keep it simple. Consider this example a first step in creating the beginning image ready to have the normal work done to accentuate the separations and colors.

Have Fun!!!

Cooksey
cooksey@cookseytalbottgallery.com
http://www.cookseytalbottgallery.com/
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