Terra Galleria Photography

Reducing depth of field by focus stacking in Almaden Quicksilver

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With its live oak and chaparral-covered foothills, Almaden Quicksilver County Park, located minutes from the suburbs of San Jose, at first resembles the other nature preserves ringing the Silicon Valley. However, at their height, mercury mining operations that took place there (quicksilver is another name for mercury) made the site the second-most productive mercury mine in the world, yielding nearly 84 million pounds. Starting in 1847 and for three decades, it may have been the nation’s most significant mineral resource, as it was essential in producing explosives for the Civil War and in amalgamating gold during the California Gold Rush. Carleton Watkins, arguably the most important landscape photographer in history, documented the site quite extensively.

Back then, 1,800 miners and their families lived in the area, but almost nothing remains of the structures that housed them. A number of mining structures still stand, of which the Almaden Quicksilver Chimney is one of the most prominent. Starting from the Hacienda Trailhead where a rusted collection of mining equipment is on display, sitting on a hill and surrounded by trees, the chimney quickly comes to view. Built in the 1870’s, it was used to release dangerous sulfur fumes from the Hacienda reduction works below.

Despite the fact that much of the region is now covered with thicker vegetation than in the 19th century, there are still numerous traces of the once-active mining hub. The most impressive is the rotary furnace that was built in 1940 to provide mercury for munitions during World War II and remained active until 1976, when mining operations ceased at the site. More than 100 abandoned mine entrances can be found in the park, together with sporadic pieces of machinery, foundations, and deteriorating roads that have left a lasting impression on the landscape. For safety, all of them have been sealed, with the exception of the San Cristobal Mine. I read that it was possible to enter the tunnel for a short distance, but was disappointed to see the entrance closed with a grid. Perhaps if I couldn’t enter the dark passage to experience what I imagined to be heavy and humid air, I could at least take a picture to remember this quick peek into the past?

While it was possible to insert a phone lens in the interstices, the grid pattern was only about half an inch, much smaller than any full camera lens front element. I knew right away that I would put to good use the automated focus bracketing of my new Sony A7R5 camera. Not only focusing manually a stack would have been quite tedious, but also the camera support was less than rock solid. In order to blur the grid to the largest possible extent, I had placed the lens as close as possible by resting its front against the grid, with the two other support points provided by two tripod legs. Touching the camera to re-focus would have risked minute changes in the camera position or lens focal length.

To illustrate how I made it work, first here is a picture taken with a 35mm focal length at the f/11 aperture I would normally use to ensure front-to-back sharpness. One can see how to tight grid pattern strongly intrudes into the picture, which is not especially desirable because the grid is contemporary.

35mm, f/11

As the aperture is opened up, the grid gradually gets thrown out of the depth-of-field area of the lens and starts to fade, but even at the lens widest aperture of f/4, it still remained quite visible.

35mm, f/8

35mm, f/5.6

35mm, f/4

Depth of field is inversely proportional to the square of the focal length, so by framing the tunnel more tightly, with a 60mm focal length, the depth of field is much diminished.

60mm, f/11

60mm, f/8

60mm, f/5.6

60mm, f/4

By f/4, the grid is sufficiently out of the depth of field area that it has become almost invisible. However, the shallow depth of field also means that only a slice of the scene, the traverse planks with the words, appears in focus. The solution is to do a focus stack with each of the component images captured at f/4. Even though the closest element of the scene, the plank at the top of the photo, is not that close, getting everything in focus still required 36 frames. Stacking them with Helicon Focus led to the final image after a few quick processing steps in Lightroom.

Focus stacking is normally used to extend the depth of field. In this example, I used it to selectively reduce it, which had the effect of making the unwanted grid magically disappear. The same technique could be used to blur a background while keeping a subject with extended depth entirely sharp.

One Comment

  1. Almaden Quicksilver County Park is a local Bay Area gem. I am exploring this park for more than 15 years now and by my count have hiked there at least a thousand times.

    Back in 2016, it was indeed possible to enter San Cristobal Mine for a short distance. Next time I checked it out in early 2018, it was already sealed off. Here are two images from December 2016 when one could still get inside:
    * https://photos.app.goo.gl/rdeYNpRuk5DZcMP39
    * https://photos.app.goo.gl/qd2UMogvi46y7yHv5

    Other interesting places still having some remains of its past I can recommend to check out are: April Tunnel Trestle, Enriquita Mine, Buena Vista Shaft, El Senador Mine, English Camp, and Hidalgo Cemetry. There are also several old car wrecks hidden in the park. So far I have found three.

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