Safety first: your retina has no pain sensors so the only thing that can prevent eye damage is your brain. Here’s what to know.
Filters must be OD 5 (Optical Density 5, 0.00001 transmittance or 16.6 stops), OD 6 (20 stops) or welders glass shade 14 if you are going to look through them. OD 4 (13 stop) filters can be used on cameras or camcorders ONLY if using live view and not looking through an optical finder and ONLY if they block both infrared (IR) and visible light. It is possible for the sun’s image to damage a shutter curtain camera sensor. One problem with commonly available welders glasses shade 14 is it does a great job blocking infrared but is so dark in the visible range that some folks will look around it to find the sun – very bad idea.
It is easy to find a large number of suggestions for DIY filters made from things like old floppy disks, developed film, aluminized Mylar balloons, stacked multiple ND camera filters and etc. The problem with all these is they often do not block IR light as well as they do visible leading to eye damage. Some modern OD 3 (10 stop) Big Stopper type filters do block IR but older versions didn’t so unless you can test for that or the manufacturer clearly claims the one you have will block all IR, don’t use them. Plus a 10-stop filter lets in too much light for both viewing and photography. Stick with materials made specifically for solar viewing and imaging. And make sure your filter cannot be easily knocked off during use.
Some long tele lenses are fitted with drop-in filter holders. The problem is the suns image is so concentrated at this location that it can shatter a glass filter. In addition, so much of the suns energy has passed through the lens before this point that the lens can be damaged or change focus due to heating. Use the correct filter at the very front of a lens or telescope.
You will see many ads for cardboard-framed eclipse viewing glasses. These are one-size fits none and you risk having light leaks. A better approach is a cardboard face shield with a window of solar safety film for viewing or a close fitting pair of black frame glasses. The other safe option is a projected image of the sun of some kind such as a pinhole projector onto white pater.
During totality, no filter is needed for either naked eye viewing or photography.
Eclipse Filter Options: The filter size(s) you’ll need depends on the lens(es) you plan to use. As we discussed a couple meetings ago, there are a couple types of photographs/videos and time lapses typically made during an eclipse:
- A wide field image with the sun overhead sometimes with a composite of the sun in various stages of the eclipse. During this eclipse the sun will be about 30 to 50º high so unless you do a focal length composite, you’ll need wide angle of view of 60-70º or more to include room for the foreground.
- A magnified view of the sun during the progression of the eclipse. At totality, several specular features of the sun become visible including the corona and prominences. Super telephoto lenses 500-600mm and short 1000mm telescopes are typically used for corona and prominence images. Shorter teles are often used for eclipse composites.
Now you know what size filter you’ll need but what density should you use? With an OD 5 (16.6 stop) filter the exposure needed to maintain detail in the sun like sun spots and granulations will typically be ISO 200, f8, 1/1000 sec. Filter density varies, as does sun angle and the atmosphere so exposure can vary. A later blog post will describe correct exposures to use for different phases of an eclipse. At totality and for a base landscape image, you will remove the filter so you’ll want something that comes off easily and quickly but is secure when on the lens.
OD 6 materials will provide the most comfortable naked eye viewing for binoculars, scopes and glasses.
There of course is a wide range of slip-on, clamp-on and screw-in filters with a wide range of prices. Filter materials can be glass, plastic or thin films. The most economical OD 5 filters are made with thin films. There are 2 types: aluminum coated film (Baader solar safety film) and dyed film (Symour solar film, Thousand Oaks solar film) and some others. More expensive options include standard 16 and 20 stop slip in glass or plastic filters and glass filters for telescopes. Amazon is not selling sheets of filter material is several sizes that can be used to make your own economical filters for your equipment. https://www.amazon.com/gp/search?ie=UTF8&tag=clark0c5-20&linkCode=ur2&linkId=8ff087b1acf8c454ce9f98cf40ac8230&camp=1789&creative=9325&index=aps&keywords=thousand%20oaks%20optical%20solar%20filter%20sheet
Ready made and materials for home made filters can be purchased. The aluminum film is easily scratched but it more neutral in color. The dyed films are more durable but have a stronger color cast. You’ll have to color correct all these dense filter images though. These films are so thin, they can not visibly degrade image sharpness even on a ultra-tele. The atmosphere will do plenty of that anyway.
There are several ways of making your own filter using thin films. A sheet can cover face shields, camera lens, binoculars and/or telescopes. A typical sheet of 9×12 can be shared by two or more of you. Here are instructions for one way of making these: http://astrosolar.com/en/information/how-to/ You can also find Youtube instructions for example using filter step-up rings.
Both types of film sheets and ready-made OD 5 filters can be purchased from Amazon, Adorama, B&H, Symour, Thousand Oaks, Orion Telescopes and Focal Point. Mike at Focal Point is getting Marumi OD 5 glass filters up to 77mm. Currently Formatt Hitech is the only maker of 13-stop filters in slip-in and screw in glass. Formatt claims these block UV-IR and visible light. But 13-stop filters are NOT dark enough for visual use!
It would be wise to purchase your filter or materials soon as they are likely to sell out as the 2017 eclipse approaches.
EDIT 11/20/16: I purchased a sheet of dyed polyester solar film from Seymour & tested it when it arrived. I found it was closer to OD 7 (20 stops) and much too dark for photography or viewing. Shutter speeds required w/ such a dark filter were 1/50″, too slow for long tele even on a tripod.
I got a refund & was told they are on the end of their last batch. They will be getting new film in January. Apparently, QC of the production process is not so good & this is a somewhat common problem for solar filter films from both Seymour & Thousand Oaks. So test your material when you get it & be sure you can get a refund.