Budget Solar Lens
Solar photography is a special breed of photography.
On one hand, it has the difficulties of astrophotography, like atmospherically distortion and extremely high dynamic range coverage among things in the frame; on the other hand, it also faces a problem that come with the sun and the sun only:
it’s too bright and too hot.
While some might think of reducing the ISO and using the smallest aperture available on the lens such as f/32, this will not work and will likely cause some disastrous results.
The extremely high EV of the sun cannot be offset by lessen the exposure setting; most modern lenses also have their diaphragm blades (the aperture thing) made of plastic, they will melt after several seconds when the lens is pointed at the sun, making a front neutral density filter a must.
But have you seen the price of these?
If you are using a 150-600 made by Sigma or Tamron, its front filter thread is 95mm, other Canon/Nikon/Sony prime telephoto lenses can have even bigger diameters, leading to some prohibitive pricing. Due to the size and power of the filter, they honestly don’t and can’t do much aside from being used on these telephoto lenses for solar photography, collecting dirt for the rest of the time.
Besides, telephoto lens is a big commitment in itself and few people are willing to buy one.
Is there a way to construct a setup for solar photography without bleeding at least two thousand dollars?
How about an old manual mirror lens?
Take the Tamron AdaptAll 500mm f/8 for example. It happens to have several traits:
Cheap. You can get one for mere hundred bucks with a teleconverter.
Small body. Barrel is less than 9cm long (about 3 inches for those without a STEM degree).
Common filter size. The front filter thread is 82mm, which is very common among the trinities (the f/2.8 zooms). It can be made even smaller with a step down ring and then mount a casual 77mm filter.
Accept rear filter. The last element of the lens is housed in a barrel that accepts filters as well, and it’s a tiny filter.
Metallic build and thus more resistant to weather and temperature.
Fixed aperture and has no diaphragm.
Since the subject of solar photography does not move (relatively speaking; during the eclipse; using the photographer as the frame of reference), manual focus is also not a problem… this seems to be the prefect choice for solar photography?
And that was precisely what I did.
Above is the set up, a Tilta matte box that holds a ND filter, then the mirror lens, then the Tamron 2x teleconverter, then Tamron to FD adaptor, then FD to RF adaptor, all further secured on rails.
There were some problems with filtering. Since I do not have filters small enough to fit the rear filter thread nor do I intend to buy one, I ended up using a 6-stop square ND filter typically for film. For safety I did inserted the Tilta VND into it but that did not see too much use honestly.
Combined with the teleconverter, the resulting system is a full-frame 1000mm f/16.
For those who have had experience with mirror lenses, you might know that 1st order aberration and spherical aberration are especially common with mirror lenses, so how does this one perform optically?
As you can see, quite amazing, honestly.
But there are also some caveats I found while using it:
Focusing the sun. The sun seen through this lens is not at infinity, but rather, a degree or two closer. For all intends and purposes the sun should be in clear focus when the lens is set to infinity, so this is either caused by heat expansion altering the refractive power of some element, or the myriad of adaptors introduced too many flange distance errors.
Barrel creep. Thanks to the fixed aperture, the only moving part on this lens is the focus ring. And almost the entire body of this Tamron lens is the focus ring, it creeps in and out easily due to gravity. Combined with the focus thing mentioned, the lens might lose focus if nor careful.
Lose connection. There are 3 adaptors between the lens and the camera: the 2x teleconverter, the Tamron-FD adaptor, and the FD to RF adaptor, so the connection of the lens is far from secure and steady. In fact it bends visibly without support, transforming itself to a lesser tilt lens.
Luckily, all these problems were (accidentally) solved by the Tilta matte box and the rails. The matte box has a tightening screw originally designed for it to firmly attach to the lens, and in this case, if the matte box is fixed on the 15mm rod, tightening the screw will lock the focus and secure the lens.
… that was easy.
