Sunday, October 27, 2013

Confessions of Image Blending

Alice Lake, Sawtooth Mountains - Idaho
Created using exposure blending.

Confessions of Image Blending

For many years I have used my trusted Galen Rowell split neutral density filters for probably 60-70% of all my landscape shots. They are, essentially, the ‘dodge and burn’ of a traditional darkroom - reincarnated into graded optical resin filters that many of us use so often for reducing the brightness of a sky.
Graduated Neutral Density Filters. Screen shot from Singh Ray website.
The split filter simply allowed a photographer to reign in the latitude of an image (the amount of bright highlights and deep shadows) so that a digital sensor, and ultimately a paper print, would be as natural looking to a viewer as the photographer saw it with his or her own eyes.

Split density filter in use - Thailand.

Exposure ‘latitude’ is the range of tones that can be reproduced without over or under exposure, and still achieve acceptable results. Traditionally, black & white and color slide film could successfully capture 7 stops of exposure (a ‘stop’ is a doubling or halving of the amount of light let in when taking a photo, or one f-stop change on the camera settings), color print film captured 5 stops, and new digital technology can capture 9-11 stops. The human eye, however, can process around 20 stops of latitude; with a combination of adjusting it’s iris and altering the chemical receptors.

So, where is this all going? If I was shooting a mountain scene and the sky was too bright, I would slide in a split neutral density filter (half gray and half clear) and adjust it so that the sky was darker. The problem with this is that the gradation fades in a horizontal line across the image, so any peaks or trees sticking above the horizon will instantly become darker, and may even become black in the final image. This works seamlessly for ocean scenes as, as we all can see by standing on the beach, the earth is flat – and the gradation works perfectly. But for mountain scenes, we have, over the years, just gotten used to the darkened protrusions above the horizon line. Galen Rowell, a master of ‘found’ mountain photography, helped create this filter system, and to look back at any of his award winning images, you can clearly see the darkened details within the filtered area.

Enter Photoshop and the modern high megapixel camera.

For the past two years I have not used my split filters – even once. I love them, and cherish them, and take great care of them – but find that I can achieve much better results in the post processing session than I ever got with the filters. How could this be? Is this just satin speaking through my damned husk of a body? Have I no soul left at all?

For the holy sanctity of simplicity, lets start at the ocean – a nice stretch of Kona coastline at dawn - the pulsing heartbeat of the earth, where one's digital sins are washed away with the gentle flow of water and sand.
RAW image, exposed for foreground.

RAW image, exposed for sky.

There are billowy pink clouds on the horizon and the waves are breaking with a nice aquamarine light. Normally, I would have used perhaps a +3 neutral density filter and brought down the sky to a reasonable brightness – not now. I want to keep that silvery bright line on the horizon, and maintain the little highlights in the clouds. I adjust the exposure for the foreground and shoot a test. I check the histogram on the back of the camera to make sure I am not clipping the image in the shadows, and then do the same for the sky, making sure I am not clipping the highlights. I then go back to the foreground settings and take an image, quickly adjust for the sky, and take another. I now have two images, captured within a second of each other, that are identical except for exposure.

This method does require some practice, and a certain degree of expense. You must have a rigid tripod with a bombproof head. I use Benro carbon fiber legs and a heavy Manfrotto 468 hydraulic head. When set, this will not move unless struck by a small rhino. And yes, this is the set-up I carry with me at all times. One item to note for this scene – sand moves. Each time the waves surround your tripod legs they will shift and sink just a tiny bit. I suggest that when you find the spot you want to shoot from, plant that tripod with gusto, and then let the waves come and go. Eventually the tripod will find its place and settle to a stop.

Concerning the ocean scene in question, there will be movement in the water between your two images. The clouds, however, will not move perceptively. This will be managed later.

Once you are back home you will open both images in Lightroom or Camera Raw. You will simultaneously adjust both for lens correction (if desired), chromatic aberration, clarity, and color balance. Then, one at a time, you will adjust the exposure for whichever image you have displayed. In the sky version, don’t worry about what happens to the foreground, and in the foreground version, don’t worry about the sky. Remember that the sky, in nature, is almost always lighter, and to darken it too much will produce an unnatural photo, quite like an HDR image.

I then save them both, and open them both at the same time, in 16 bit, in Photoshop. Now it is time the blend the images.
Both images opened in Photoshop, after RAW processing.

Split the screen so that both images are displayed at the same time. Then grab one image, say the sky, and drop it on top of the foreground. They will find their own edges and will be overlapped perfectly. Now, if your tripod had absolutely no movement whatsoever, there will be no problems from here on. If there was, then there will be some work to do, or the image may just be unusable (another reason to take multiple photos while on location – flash memory is cheap).

Close the file that you will not be using and let the blended exposure go back to full screen.

This is what you see - the darker image (correct exposure for the sky) now covers the lighter image.
Now choose the eraser tool and make it really big, with fuzzy edges. Here is where a steady hand comes into play. I start with 100% erase, and totally ‘paint out’ the lower part of the image. As you do this, the lower, correctly exposed foreground shows through. As you get closer to the horizon, I back off to about 60% and shrink up the brush size. Don’t try to make a fine line between the two exposures – it will look artificial. The sky is always lighter at the horizon in nature - so let some of the erase rise above the horizon line.
Start erasing the top image to allow the correct foreground exposure to show through. The area in the lower right corner has been erased to show the layer below. 

Once you have a blended image that looks acceptable, and natural (remember that you are trying to reproduce what you saw that morning) merge the visible images into one and rename. Now you have the starting point for the rest of your workflow, one with wide latitude and yet well within the parameters of printing. Instead of the sky being taken down with one solid curtain of grey resin filter, you can paint, in or out, depth and contrast from the actual full-color scene.  Now adjust as normal: crop and straighten first, adjust color balance if needed, adjust levels or curves (with or without masks), saturation, contrast, etc.
The finished blend - still some work to be done.

This process my sound complex, but once you get the hang of it, it really goes quite quickly. And even though this example was for a flat horizon, the mountain scenes turn out dramatically because the points of peaks and trees can be allowed to show through an overexposed sky.
The finished image.

I recently sold my entire set of split neutral density filters and am not looking back. I loved them, but hey were just six more things to carry around, to keep clean, to worry about. Now, I just make damn sure my tripod is stable (rhino-proof) and shoot away. Some images need very little blending, and some benefit from complex blending at the extreme end of the artistic scale – but all, I feel, are superior to the filters.

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