Is a strong ND filter a safe alternative to a "disposable" solar filter for eclipse photography? [duplicate]

Question

I'm starting to gear up for next April's total eclipse since this time around they were considerate enough to route the path of totality near where I live.

I've also never actually seen anything near a total eclipse in person, and I've certainly never photographed it, so I don't really know what I'm doing. Browsing photography stores, I can see that eclipse filters fall into two classes, at least as conventional cameras go:

• One style is basically just a very strong ND filter, like this one. It's got a metal ring and threads onto the end of the lens like every other photographic filter I've ever used (not counting back-of-the-lens filters you might pop into an ultrawide -- but you get the picture). These will vary between 16.5-19 stops going off my quick skim of gear shops I have access to.

• The other style seems almost disposable: here's an example. It's a cardboard housing with what looks like a thin foil film you'd put in front of your lens similar to the material eclipse glasses are made out of. I get the impression that this one's more effective at blocking light. It's also an order of magnitude cheaper for the thread diameter I have, but with the foil being presumably malleable, I'm not sure what the effect would be on the image.

Is the first filter style measurably less safe for a sensor if you're pointing it at the eclipse? Does it make a difference whether I have a mechanical or electronic shutter (since with an e-shutter, the sensor's exposure to the sun would be much loner)?

Answer 1

You need more than an ND filter and a polarizer. You need a solar filter specifically designed for imaging the sun. The danger to your eyes and camera are very real if you are pointing the unprotected or underprotected camera at the sun.

• Most ND filters and polarizers only block visible light.
• The sun emits very high levels of UV and infrared radiation as well as high levels of visible light.
• A filter that only blocks visible light will not mitigate the energy contained in the UV and infrared portion of the sun's light.
• You can literally cook your retinas in a matter of seconds by exposing it to the sun's infrared energy focused through a telephoto lens such as your Jupiter 21m. Since our retinas have no pain receptors, you won't even realize it until hours later when the effects of all that heat cause scar tissue to form on your retina. The damage to your vision will be permanent and could be as severe as total blindness.
• Your camera is also vulnerable to IR and UV in such concentrated amounts.
• Even in the visible portion of the spectrum, it takes about 15 stops of neutral density to lower the light from the sun to a safe viewing level.

Lensrentals.com has posted a blog entry in which what happened to some of their rental equipment that were used without proper solar filtering during the recent total eclipse in the United States is shown in photos of the damaged equipment.

Damage to a shutter curtain:

Damage to a sensor:

Damage to the aperture diaphragm of a 600mm f/4 when the user used a rear positioned drop-in solar filter:

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You can make your own solar filters fairly inexpensively using film based solar filter material. I cut several circles from an 8x8 inch sheet of silver-black solar filter polymer made by Thousand Oaks Optical that cost about $20 and made two lens filters roughly 80mm in diameter and a pair of slightly larger than 60mm filters to fit my 16x60 binoculars for the solar eclipse in August of 2017.

For the binoculars and one of my lenses, rather than mount them in cardboard as many folks do, I cut holes in plastic lids used for cans that package food products and mounted them in the resulting rings.

A pair of lids from two plastic tubes of Lay's Stax potato crisps were just the right size for my binoculars. I cut a second set of rings from a slightly smaller pair of lids from a pair of Pringles cans and nested them inside the slightly larger rings made from the Lay's Stax lids with the filter material sandwiched between them.

_{Heavy crop of an image of a friend who was holding the binoculars with filters attached. From a sheet of 1/4" foam with self-stick backing I cut a couple of 3/8" wide strips that were attached around the circumference of the end of the binoculars' lens barrels. These held the DIY filters snugly on the end of the binoculars.}

A lid from a can of mixed nuts, with another slightly smaller plastic lid securing the filter media, fit the rim of the hood of my EF 135mm f/2 perfectly (I combined it with a 2X TC and used it on my backup camera, an APS-C Canon 7D, for most of the eclipse).

_{Heavy crop of an image showing the filter made to fit the hood of an EF 135mm f/2 L. You can see the sides of the inner ring made from a slightly smaller plastic lid protruding from the inside of the outer ring made by cutting a hole in the middle of a plastic lid from a can of mixed nuts.}

For my EF 70-200mm f/2.8 L IS II, I used a larger lid from another can of nuts and attached it to the end of a cardboard mailing tube that I had trimmed to about a 2 inch deep ring. I cut a hole in the plastic end cap of the mailing tube that fit snugly over the end of my lens (sans hood) and placed it in the back end of the ring.

_{Heavy crop of an out-of-focus area of a photo showing the 'mailing tube' mount for the filter attached to an EF 70-200mm f/2.8 L IS II. The large square is a piece of foam board with a hole just large enough to insert the 70-200mm lens from the rear. It fit snug enough that it held itself in place, and provided a shade from the sun to make it easier to see the LCD screen for magnified LV manual focusing and to operate the camera's controls.}

Here's an image taken through the 'mailing tube' filter:

_{EOS Canon 7D Mark II + Kenko C-AF 2X Teleplus Pro 300 DGX + EF 70-200mm f/2.8 L IS II, ISO 400, f/6.3, 1/400 second. Cropped from 5472x3648 to 1920x1280 pixels before being resized to 960x640 for web viewing.}

Answer 2

You can use a cheap filter during the setup, but during the totality the filter must go OFF ! Be sure to find a location with good weather and little stray light because the sun's corona is weak and you do want to catch it. You may need a different exposure to capture the solar flares. As you'll have less than a minute of shooting you will have to do some fast exposure bracketing, so .. practice, practice, practice. And bring the best tripod you've got and a remote control because with a focal length of 600-1000 mm vibrations are your enemy. With 1000 mm the sun and moon will travel almost the whole size of your image during that minute, so you must anticipate that when you make the final adjustment before the filter goes off.

Answer 3

The two types of filter materials do the same thing, and either can be used.

It is probably safe to say that the glass filter will be of higher optical quality and could render an image of greater detail with less artifacts. However, during an eclipse you are not really recording anything of terribly fine detail. So reduced optical quality may not be a significant negative.

Digital sensors have UV and IR blocking filters directly over them; which is why you do not need UV filters with digital, and why you have to modify the sensor (have the filters removed) to do full spectrum photography.

The 16+ stop ND's are made specifically for solar photography; photographing the sun. It has all of the protection a camera needs. The solar ND filters are bandpass filters which only allow light from near the middle of the visible spectrum to pass (~ 540nm), they block the majority of UV and IR. But many allow more light to pass than solar viewing filters (glasses); so to protect your eyes you may need to avoid using an optical viewfinder, and use liveview/EVF instead.

Using the ND solar photography filter is better for photography; more light, lower ISO, better IQ... especially for a solar eclipse as the actual light levels drop dramatically during the event.

Only solar filters that meet ISO 12312-2 are safe for direct viewing. There are solar filters of both types that are not safe (meant for photography through a telescope).

It's worth noting that most ND filters (non "solar") affect all wavelengths equally. Reducing near IR as well as visible light... some pass more IR, but IR has the least energy and is the least likely to cause damage.

Also note that ordinary glass blocks most UV (≤330nm), and near UV (UV-A) is also easy to block; i.e. polycarbonate lenses/panes block almost all UV; and UV filters are also highly transmissive of all other sprectra.