Saturday, April 13, 2024

Partial solar eclipse

Other than watching it occur, I had no plans for the partial solar eclipse. Two things occurred that changed that.

In order to try out a roll of Rollei RPX25 superslow film, I had been refurbishing the Variable Cuboid and it's fronts the week before the eclipse occurred, reinforcing everything with a few layers of Mod Podge. About Friday, working on the 200mm front, I remembered how the 2017 eclipse was kind of an inspiration to play with narrow angles of view, known in the lensed community as telephoto, almost unheard of among pinholers. I had already loaded the RPX25, but I had a second back for the Variable Cuboid. On that one I had not upgraded the light trap to overlap the sides of the front. I did add a little more cardboard to the barrier the front presses against, but didn't make it overlap. How could any light get down that joint? 

I loaded it with Lomography 400 because color film tends to record detail in overexposed areas that can be revealed if you really burn in to them, whereas black and white film has a tendency to block up highlights. The weather forecast had been improving for Wisconsin for the orbital event. I planned and wrote down a list of exposures I wanted to make and the changes of the front of the Variable Cuboid so I could do it within a few minutes of maximum eclipse.

The 200mm front had a .50mm pinhole, smaller than the .596mm the equations predict for minimal diffraction. Recently some .60mm Gilder Electron Microscope apertures had turned up in my stash, so I upgraded to the perfectly round, smooth, thin, mathematically correct hole for the special occasion.

The second influence was that Monday dawned with a completely clear sky and the best weather forecast so far. On the spur of the moment, I put out the 4x5 Pinhole Lab Camera with the film plane at the 90° angle of view, 60mm slot with a .45mm pinhole on a sheet of Ilford Multigrade Paper, for a one day solargraph to see if the eclipse left a trace.

The camera was out from 7am until after dark that evening. The solar track is probably from about 11am until about 5pm. 

The raw scan
Since it was partly cloudy from late morning on, I expected to see random variations in the track which wouldn't be visibly different from the eclipse. Except for what must have been a few thick clouds about 12:30 and one brief pass just after maximum eclipse, the clouds, which looked fluffy and white across the sky, were thin enough the emulsion didn't distinguish whether they were in front of the sun or not. The clouds never dimmed it enough that you could look directly at the sun and see a shape. I did have a solar filter to look through, and couldn't tell whether or not a cloud was in front of the sun through that.

The eclipse is clearly visible in the curve where the track gets thinner from the bottom recording the remaining crescent at the top.

The day before the eclipse XKCD did a cartoon lowering the expectations of those who won't experience totality. The pinhole, camera and film must have seen that and tried to make a more psychedelic experience to what was a pretty deep 89% partial eclipse.

One of my objectives was to see if I could resolve the crescent of the sun. The sun is half a degree wide and the 200mm front is 17 degrees. On a 60mm wide piece of film, that crescent ought to be about a millimeter and half, big enough to see at full resolution, about 100 pixels in my normal scans. I kind of confirmed this visually with a pinhole and card during the event. 

I didn't just want to get the bare crescent in a completely dark field. I had planned the night before roughly where I could place the camera during mid-eclipse and get some foreground objects in the scene. I couldn't look at the sun and tell whether it was behind a cloud, but the shadows could tell if the sun was shining on the pinhole and used them, a protractor app and a compass app to make sure I was pointing in the right place. Exposure reading of a sunlit scene (without the sun in it) was about 6 seconds. This is at f330. Flipping a card away from the pinhole and back for probably about two seconds.

Quickly back to the arbor where I knew I would get rose vines in and maybe succeeded in a bit shorter exposure.

These were scanned at almost maximum darkness. At full resolution in Photoshop with the brightness as low as it gets, there is no indication of the crescent shape of the sun. Total over exposure. Not the kind of totality one hopes for, but nifty flare though!

I changed to the 35mm front. Confident that the tripod mount would tightly clamp the front and back together, I did not put rubber bands around the camera. What looks to me like a darn tight joint apparently leaked some and the left sides of the next four images are fogged. What I think happened is that as I twisted and pulled the camera to point it on the handleless tripod head, it flexed enough on one side to allow some light past. Here they are presented severely cropped and edited as best I could.

When the sun is eclipsed 80 percent or so it has changed from a half degree circular source to a 20 x 10 arcminute crescent. That gives shadows less penumbra where the light is partially blocked and results in slightly sharper edges. Along with a several stop reduction in brightness it makes for a strange and memorable quality to the light (OK, I know, nothing like totality). I was hoping to capture that, but edited to compensate for the light leak and some overexposure, this is probably more of simulation than a recording, but it's sort of what it looked like.

Out in the middle of Central Street during mid-eclipse. 

With the camera wrestled in place among the hydrangea and rose bushes, the shadows on the side of the garage show significant effect of the light leak. I had been looking for natural apertures projecting the crescent. The dried hydrangea blossoms were doing that but the wind was too frisky to catch any of it with a pinhole (see below). 

I changed to the 60mm front. I had arranged some stationary objects that would project the crescent sun. The macrame grid on the chair provided a variety of shapes.

In the past, I had seen one or two people use objects full of holes to make crescent patterns in eclipses. In 2017 they really caught on and there were hundreds on social media in recent occurrences. We often speak of the size of a pinhole, the distance to the image plane and optimal diffraction. With these colanders sitting right on the ground the images were simple circles, but moved out to a meter or so away they sharpened into accurate depictions of the sun.

The rest of the film I put to use checking out the performance of that Gilder aperture on the 200mm.  At the corner of Church and Main, I spent quite a while moving the camera to different positions and compositions. It looks like I flexed it enough for the sun to again breech the camera's defenses.

The camera decided to behave from there on. The left lion on the Library steps.

An architectural jumble behind Main Street.

Other than just getting closer, compression of space is the optical effect most associated with narrow angles of view. It makes Ames Point and Calumet County look a little closer to this side of Miller's Bay than it usually looks in my photos.

This is going to require some investigation, but it seems to me the calculated better hole isn't giving me as good an image as my too small hand-drilled pinhole.

Postscript: Pinhole phenomena recorded with a lens.

Because direct observation of an eclipse will destroy your vision without proper filtration, which is not always available, it is common to advise using a pinhole to project the image of the crescent sun. This is the only experience many people get of this simple physical principle.

I took some pictures with my iPhone of some of that experience.

The most common source of naturally occurring small apertures are the small gaps between overlapping leaves. Not much of that available in early April in Wisconsin.

The dried hydrangea blossoms in the garage picture had the little curvature in the shadows.

I was surprised to see the pine trees projecting crescents.

Using hand made optics.

My colander arrangement a half hour before mid eclipse.

Twenty minutes after peak occultation with the moon on the other side.

Since both eclipses and pinhole are phenomena based on light traveling in straight lines (except right near the mass of the sun), it's instructive to remember the scale of the sizes of the two objects and their distance. Pretty neat trick to drop that shadow right on the earth.

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