Photographing the Solar Eclipse with a Mirrorless Camera

[esrevernidellepS]

Hello,

I am planning a trip to South Carolina to shoot the total solar eclipse in August. I would like to shoot it using my Sony A7R, but I have a few concerns about frying my sensor that I was wondering if the forum could address.

While there is a fair amount of information regarding shooting a solar eclipse, very little of it discusses using a mirrorless camera. With a proper DSLR, the mirror is down while composing the shot/shooting the sun during the partial phase of the eclipse, so the risk to the sensor is minimal (one should simply ND to bring the exposure within the shutter speed range of the camera). However, since the A7R is mirrorless, the entire composition must be done with the sensor exposed. Compounding that, I would ideally like to use a very long lens (1000–1400mm). I would stop down to f8 or 11, but I'm sure the magnification power would create a lot of heat. The sun will be in the sky at around 2:45PM, so it will be quite strong.

Would a 10-stop ND filter be enough for this? Is there any filter out there that would protect my sensor from a few minutes of sun exposure?

Thanks,
Dave

[Bart_van_der_Wolf]

Would a 10-stop ND filter be enough for this? Is there any filter out there that would protect my sensor from a few minutes of sun exposure?

No, you need a specialized Solar filter (regular ND filters may transmit enough IR radiation to fry your sensor). This website gives a lot of information on the subject: http://www.mreclipse.com/MrEclipse.html and has a section specifically about filters: http://www.mreclipse.com/Special/filters.html

With a mirrorless camera, you'll need to find a filter that allows very long exposure times of the solar disk. You need to figure out how long a specific part of the sensor will be exposed with (part of) the solar disk as it travels across the sensor. The focal length you use will determine how fast the sun travels across the sensor. You need to calculate the angular movement and the on-sensor magnification to determine that. Then the actual exposure time must be determined, which may result in having to increase the ISO.

An alternative could be to shoot the reflection off of the surface of a black bucket filled with some water, but testing is still required. Maybe the website I mentioned has more tips, I haven't read all the information there.

Cheers,
Bart

[esrevernidellepS]

Thanks Bart,

There is a lot of research I have to do, but it's reassuring that at least it's possible to find a filter that should work. Thanks for that website. I hadn't seen it.

-Dave

[Bart_van_der_Wolf]

Thanks Bart,

There is a lot of research I have to do, but it's reassuring that at least it's possible to find a filter that should work. Thanks for that website. I hadn't seen it.

I know Dave, but you are right that there may be little specific info about mirrorless cameras. If the camera has a physical shutter to protect the sensor between shots, things become a bit easier because the sensor will only be exposed by the image projection of the sun at that spot on the sensor (assuming short enough exposure time to freeze motion for a given focal length/magnification). But without physical shutter things will get tricky pretty fast.

Some things can be calculated in advance, others need to be tested, so the website may be helpful in getting the number of experiments down. At least we know that the earth rotates once in approx. 24 hours, so the sun appears to travel at a speed of 360 degrees per 24 hours, or 0.00416666... degrees per second. If you know/calculate the angle of view of the lens + sensor, you can calculate the time it takes the image of the sun disk to traverse the full image FOV on the sensor, or during the exposure time (less than 1 pixel for a freeze motion).

You can also calculate the diameter of the sun's image projection on the sensor for a given focal length. The Wolfram Alpha website gives info about the date of the next solar eclipse and the sun's distance at that date and diameter. The magnification factor of the lens at infinity focus of a given focal length then allows calculation of the projected diameter on the sensor.

The exposure time for the sun (even if partially obscured by the moon's shadow) with a proper Solar filter should place the hump/spike in the histogram some 2.5-3 stops above a spot exposure measurement to avoid clipping.

I don't have the time to do all the calculations, and I'm not able to enjoy the phenomenon from where I'll be located, but the above should give some guidelines if the earlier mentioned Website doesn't give the answers you need. Maybe some of the other LuLa members with more astrophotography expertise can fill in some blanks.

Cheers,
Bart

P.S. The website I referred to has a rule of thumb for exposure time:

For your image to be in sharp focus, here is a formula for the longest exposure allowable with no clock drive:

Exposure (seconds) = 340 / focal length (millimeters) . For longer exposures, you need a clock drive to compensate for the Earth's rotation.

I don't know if that applies to film or digital sensors (which may be a bit more sensitive to motion as the sensel pitch gets smaller), so use that as the slowest shutterspeed.