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Blended Exposures


— Achieving an 8-10 Stop Dynamic Range — 

This article from 1999 has now been superceeded by a newer one that details
three different blending techniques that are more appropriate for digital photographers.

The Problem

Colour transparency film has a useful dynamic range of about 4-5 stops between the lightest area with any detail and the darkest. Beyond that is found featureless black or transparent white. Colour negative film has a couple of additional stops and B&W film another stop or so. But, nature can present the landscape photographers with scenes having a dynamic range of 10 stops or more — double that which slide film can handle.

Most landscape photographers, myself included, use split neutral density filters to bring the dynamic range of a scene down to the range that film can handle. Occasionally I've found myself without a set of split NDs, faced with a challenging exposure. Below is one such example and the technique that I used to overcome it.

Matkatamiba Canyon Ñ Grand Canyon, May 2000
Photographed with a Canon EOS-1V and 24mm f/3.5L T/S lens on Fuji Provia 100F

On a Spring 2000 8-day rafting trip on the Colorado River through the Grand Canyon I took the above photograph after a long, wet and arduous climb. Because of the difficulty of the hike I was traveling light and had left my set of split neutral density filters, along with a lot of other gear, back in the raft a long way below. (You can read about the making of this photograph here).

Using the Canon 1V's spot meter I found that the scene had a range of at least 10 stops from the rocks in the upper right to the shadow areas under the foreground rocks. What to do?

The exposure above was a compromise, set by splitting the difference between the two spot readings. While this photograph captures the scene it satisfies neither the need for highlight nor shadow detail. A 2 stop split ND set across the middle would have been the solution. The image above is a simply a poor compromise because the filters weren't available, but it is the best that I could do.

The Solution

I realized at the time though that if I took two additional frames, one exposing for the highlights and another for the shadows (in addition to the compromise exposure above), I would be able to combine them digitally. Thus I could create an image that would render what I saw, but could not capture at the time on a single frame of film. 

    

Light (shadow) Exposure                                 Dark (highlight) Exposure


Blended in PhotoShop

The frames above tell the story. The first two are the raw separate exposures for the shadows and highlights. The larger center one below them is the blended image created in PhotoShop, using the technique described below.

The Technique

This technique is explained using PhotoShop 5.5. It probably can be done with most image processing software that utilizes Layers. (See Note #1 below regarding another approach.)

Here's how it's done. 

You can experiment and learn this technique using the example light and dark pictures above by right clicking your mouse and using Save Picture As... Then, skip to Step 4 below. (They are already aligned). Otherwise, this series of steps is described generally for you to use with your own two exposures.

  1. Scan the two exposures as you normally would except with the following alterations to your technique....

  2. Scan each frame with identical scanner settings. Don't let your scanning software alter the exposure between scans.

  3. Include a black border around the frame. You'll need it to ensure correct alignment, as seen below.

    At this point you want to load the scans into PhotoShop. But, since you'll be doing a lot of experimentation, it can be tedious and time consuming to do so at full resolution. Resave each of the scans at about 7"X9" size and 72 DPI. Use these low resolution files to experiment with and then when you have the technique well in hand return to the full-sized files and redo what you've learned.

  4. Load the two files and using Window / Tile display them side-by-side on screen.


  5. Press TAB and ensure that all required palettes are displayed. Now, click on the Foreground / Background squares and ensure that they are Black and White, with Black on top.

  6. Turn on the rulers using View / Show Rulers and then using the Move Tool drag rulers onto each of the pictures. Using the black frame edges as your point of reference use Image / Rotate Canvas / Arbitrary to ensure that both pictures are straight. This is necessary because it's almost impossible with most scanners to align the film exactly the same for each frame. Another method of aligning the two images is to temporarily set the opacity of the top layer to a low amount and then zoom in. 

  7. Select the Dark frame and then use Select / All and Edit / Copy. (If you're working on a large file you can now delete the Dark frame to free up memory and make PhotoShop work faster).

  8. Select the Light frame and use Edit / Paste(If you are working with digital files you can skip ahead to step #12).

  9. Select the Layer Palette and set Opacity to between 40% and 50%. This will allow you to see both layers at the same time. (Layer 1, the dark layer should be highlighted).

  10. Now the hardest part. Using the Move Tool move the dark layer so that it exactly overlaps and matches up with the background, or light layer. Magnify the image so that you can clearly see what you're doing. If you find that it doesn't match exactly everywhere just ensure that the match is as close as possible at the point where the two images will be blended. In the example above that point is where the dark foreground rocks meet the brighter rocks in the background. (Remember, you can use the arrow keys to move the images in small increments).

  11. Return Opacity to 100%

  12. Now use Layer / Add Layer Mask / Reveal All

  13. Choose a fairly large brush and select the Paintbrush Tool. Start painting over the dark part of the image (the lower half in the example above). You are removing the dark layer and revealing the light layer underneath. Don't worry about overdoing it because once the upper (dark) later is removed the process stops. Be careful not to get too close to the light area with the large brush. Also, make sure that you don't miss any areas.

(NB: You can use Add Layer Mask / Hide All instead, combined with White foreground & Black background squares. This allows you to erase the light areas rather than the dark. Depending on which area is larger this may be a more convenient way of working. 

  1. Change to a smaller brush and increase the magnification to actual pixels. Very carefully erase the dark layer along the edge of where the light area meets it. If you make a mistake, use the History Palette to  go back.

  2. When done doing the above, use Layer / Flatten Image and save the file as .PSD PhotoShop format file.

  3. Now, proceed to use all of the normal image processing techniques that you would normally apply, including Unsharp Masking, spotting using the Cloning Tool, Levels using an Adjustment Layer, and so forth. You'll also want to crop the black edges. 

  4. Don't forget that you can do colour adjustments and the like on each separate layer before flattening, further adding to your ability to create an image that matches your perception. (You may also be interested in having a look at my tutorial titled Instant PhotoShop for some basic yet powerful techniques.)

   

Compromise Single Exposure                         Blended Exposure

I find this technique to be very helpful when the use of a split neutral density filter in the field isn't possible. This may be because one isn't available, or because the area between highlight and shadow is too irregular.

Of course you must take two frames that are identical, except for exposure. A tripod is therefore a must. Also, I find that exposures with a spread of about 2 stops yields very good results.

Finally, like most techniques using PhotoShop, there are usually a number of different ways of accomplishing the same thing. If you have an alternative method, or a suggestion on how to improve this one, please let me know.


Note #1

There is a comparable yet quite different technique for accomplishing this available for those who use Picture Windows 2.5 from Digital Light & Color. A full description can be found here.


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Concepts: Neutral density filter, Graduated neutral density filter, Photography, Image processing, Adobe Photoshop, , Light, Grand Canyon

Entities: Canon, USA, PhotoShop, image processing, Grand Canyon, Matkatamiba Canyon, Colorado River, Windows, Michael Reichmann

Tags: image, split neutral density filter, , dark layer, PhotoShop, dynamic range, light area, scans, picture, split nd, light layer, foreground rocks, landscape photographers, , Move Tool, background squares, image processing, Dark frame, exposures, large brush, light layer underneath, raw separate exposures, dark foreground rocks, Colour negative film, Most landscape photographers, Colour transparency film, black frame edges, identical scanner settings, useful dynamic range, image processing software, Move Tool drag, PhotoShop format file, Layer Palette, normal image processing, separate layer, low resolution files, Black background squares, Layer Mask, Adjustment Layer, additional stops