Lightning Storm Part 2

Following up with my previous tutorial on taking lightning photos, this article will cover the HDR and image stacking I did for the above image.

I use Nikon Capture NX 2 for my raw converter, but most of this would apply to Lightroom, Photoshop, or any other raw converter. I stick with Capture NX to get some camera specific functions that won’t work with 3rd party software, such as the Active D-Lighting feature in my Nikon D700 and automatic sensor dust removal. Running through my workflow, the first thing I did was override the automatic white balance I had set on the camera to daylight/cloudy, with a fine adjustment of about 4900 Kevlin and a slightly magenta tint. This got the sky to the beautiful purple I wanted. Next I set the picture control (profile in Lightroom) to “Vivid” to get more pop to the contrast and colors. I added a little sharpening since all RAW images require it and have no sharpening applied in camera unlike JPEGs. Active D-Lighting I set to low (unique to Capture NX 2 and Nikons). I bumped contrast up to 2, shadow protection up to 7 to get more details in the clouds, and saturation up to 18 to get better colors in the lightning bolts. There would be similar sliders under Tone and Presence in Lightroom. Play around until you like what you see. You would probably add a little noise reduction here too if your camera needed it. I typically don’t need any until ISO 2500 to 3200 on the D700.

After batching out my settings to all my images, I copied them into four more folders called -2, -1, +1, and +2 to get ready for HDR tonemapping from a single image. In each folder I took one image and adjusted the exposure compensation to -2, -1, +1, or +2 EV and batched that change out to the rest of the images in the folders. You could do this with virtual copies in Lightroom. Then I saved all my RAW files out as 16-bit TIFFs, so I had five exposures of each image, a different set of exposures in each folder. Depending on the dynamic range of your camera’s sensor and the editing software you use, you may be limited in how many stops of useful exposure you can save out from a single RAW image. Experimenting is the easiest way to find out. My D700 has about 10-12 stops of useful exposure latitude in a 14-bit raw file from ISO 200 to 1600, and drops off to 8 stops at 6400, so it’s pretty easy to save out a -2, -1, 0, +1, and +2 EV set for HDR work (each with 2 stops of range) from a single raw file, as long as your exposure was really good to begin with. If your exposure was off you will hit a wall in one direction or another, either noisy shadows or blown highlights, so taking a true bracketed set in camera is always preferable, but not always possible with moving objects or especially lightning bolts! If you shoot in JPEG, you might not have enough dynamic range to get more than two or three stops of useful exposure latitude, limiting the effectiveness of HDR.

A couple images I liked just the way they were with the lightning bolts that had been captured, and I skipped straight to HDR tonemapping. The rest I wanted more lightning bolts and so I stacked them first before doing any HDR work. For the image above I used 15 images taken very closely together that had the best lightning strikes in a 10 minute time period, where the camera had not moved on the tripod and I had not zoomed or changed the focal length. The images didn’t have to be aligned, which made things much easier. I selected my images (for one exposure) in Adobe Bridge, went to Tools, Photoshop, Load Files into Photoshop Layers. Once all the images were loaded in Photoshop as separate layers, I went to Select, All Layers, then Layer, Smart Objects, Convert to Smart Object. This might take a while if you have a lot of 16-bit TIFFs. Once your layers have been converted to a smart object, we have more options to explore. I went to Layer, Smart Objects, Stack Mode, and Maximum. This will blend the brightest parts of all layers into one smooth image, blending the lightning bolts of every previous image without brightening the trees and dark skies. You could try some of the other stacking modes, but this one did what I wanted. Lastly I flattened the image under Layer, Flatten Image to keep file sizes smaller and saved it as a new 16-bit TIFF. I repeated the same process for each set of exposures. I recorded a macro and played it back on each set to make life easier for me. 🙂 In the end I had five files with all my lightning bolts stacked of different exposures, -2, -1, 0, +1, and +2 EV, ready to process with my favorite HDR software.

The final step was bringing the different exposures into Oloneo to create my HDR image. There are many HDR programs you can use, from Photoshop to Photomatix Pro. The pros and cons of each are beyond the scope of this article. I get great results with Oloneo though that look more natural that most HDR programs I have tried, and I own half a dozen or so! I played around with the sliders until I got a look I liked, without generating too much noise or overexposing the lightning. The sky really popped after combining the clouds from the brighter images with the lightning bolts from the darker images. Once I was happy with my results I saved the HDR images as 16-bit TIFFs with zip compression and cleaned up all the temporary files I’d created, leaving only the final compressed HDR TIFFs and original RAW files that had not had their exposure adjusted (deleting the temporary folders I’d created at the beginning of the article). This freed up a lot of disk space that didn’t need to be archived or backed up. The only thing I did afterward was remove some hot pixels from the long exposures and rain drops from the lens in Photoshop via the healing brush, and export everything out with Lightroom to my website to get my logo.

I hope the image stacking technique gives you some creative ideas; post your results here if you it inspired you to do something interesting! 🙂

These images can be purchased through my gallery here:
http://galleries.aaronpriestphoto.com/Night/2011-08-01/

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