Total solar eclipse on 21st of August 2017
On 21st of August people from the US and not only will participate at a celestial show. America will witness a total solar eclipse. This hasn’t been seen for over 38 years. If you have missed the previous article on that take a look at the article here.
Solar filters
During the partial phase of the solar eclipse you must use a solar filter. You can imagine that if 99% of the sun is covered by the moon, the remaining 1% crescent is dangerous to view with the naked eye and can cause serious eye damage or blindness.
Safe solar filters for cameras and telescopes are available as either “Full-Aperture” and “Off-Axis” filters. Both of these filters fit over the objective (front end of the telescope) or camera lens.Full-aperture solar filters are the preferred filters of choice. This is because the filter completely covers the front of the telescope so the entire mirror or lens is used. No refocusing of the telescope or camera lens will be needed when you remove the filter at the beginning of totality or when it is replaced back on the telescope/camera lens at the end of the total phase.
Gear needed to photograph the eclipse
You can photograph the solar eclipse with any types of camera: full frame, crop factor, mirorrless or even point and shoot camera with safety in mind. By no means should you look in the Sun or through the viewfinder with your naked eye. Also you will need to bear in mind that not all camera type offer you manual control over the exposure.
The longer the focal length of your lens, the large the image of the Sun will be in your frame. For DSLR cameras you can combined a telephoto lens with a teleconverter to increase the focal length of your lens. For some FX sensor cameras you can increase the focal length by changing to DX crop mode. Additionally you will need a sturdy tripod to get the shot.
But actually what focal length do I need to fill in the frame?
The focal length you will need depends greatly on what you will want to have in the frame. Therefor if you want to fill in the entire frame you will need a focal length of:
- less than 2000mm for an FX camera
- less than 1300mm for a DX camera
If you plan to have also the corona in the frame during the phase of total eclipse these numbers change as follows:
- less than 1400mm for an FX camera
- less than 900mm for a DX camera
What ISO, aperture and shutter speed should I choose?
Below you can see the phases of a total solar eclipse. You will see something similar. During the partial phases of the solar eclipse you will need to use a solar filter. This is very important because the Sun emits quite a lot of energy. For comparison a solar filter will attenuate the Sun’s infrared energy by a factor of 100,000. Therefor any ISO can be used as you will have quite abundant light. The actual filter factor and choice of ISO will play critical roles in determining the correct exposure.
Because the Sun’s brightness stays the same throughout the partial phases, no exposure compensation will be needed. You may also decide to bracket your exposures to ensure that you photograph the solar eclipse with a perfect exposure.
Photographing the Totality Phase of a Solar Eclipse
The challenging part of photographing the solar eclipse is to get the correct exposure. During the total phase you need to remove all solar filters as the Sun’s corona is million times brighter then the Sun’s surface. Consequently you will photograph the corona without any filter. The best strategy is to choose one aperture and bracket the exposure over a range of shutter speeds. These should range between 1/1000 of a second to 1 second. Additionally you should rehearse this action as you can be easily distracted and forget to take pictures.
Finally if you need additional help you can get over and check Mr. Eclipse’s Solar Eclipse Exposure Guide here.
