As we know there is an eclipse happening on April 8th. Having shot a few - though by no means an expert - I thought I would put down a few thoughts of what you need to do to prepare.
First: Your lens. Is your focal length long enough to get a decent amount of resolution for the sun?
The sun (and moon!) takes up 0.5 degrees of sky. How many pixels will that be on your sensor?
The relationship between the angular field of view and focal length is given by:
FOV = 2*arctan(frame size/(2*focal_length))
where frame size is the dimension of the sensor in mm. (We will focus on the long dimension for a 3:2 sensor, but since we have square pixels the short dimension would be equivalent)
For this post, we will look at 3 cameras as examples: A T6S like mine, an APS-C sized sensor with a 22.2 mm long dimension, A R6MkII, A Full Frame sensor with a long dimension of 36mm and a R5C a FF also with a long side of 36mm.
The FOV for the T6S and a 600mm lens is 2.1 degrees; for a 200mm lens it is 6.4 degrees.
The FOV for the FF cameras are both the same: 3.4 degrees for the 600mm lens and 10.3 degrees for the 200mm lens.
Since the sun takes up 0.5 degrees the fraction of the FOV it takes up is 0.5/FOV.
For the T6S: 0.238 for the 600mm lens and 0.078 for the 200 mm
For the FF cameras: .147 for the 600mm and .049 for the 200mm lens.
But what really counts is how many pixels the sun will take up.
The width of the sun in pixels is the fraction of the FOV calculated above * the number of pixels in the long dimension.
For the T6S at 600mm: 0.238*6000 = 1428 pixels; for the 200 mm .078 * 6000 = 468
For the R6M2 at 600 mm: .147*6000 = 882; For the 200mm lens: .049*6000 = 294
For the R5C at 600mm: .147*8192 = 1204 pixels; at 200mm: .049 * 8192 = 401.
The upshot is that the smaller sensor holds its own in resolution. There is no hard and fast rule for the minimum resolution, but I would think that you want at least 800 pixels to capture the detail. It is also clear that 200mm is right out, the image of the sun being way too small.
Now that we know what lens we are using, we need to protect it. There are a lot of choices for eclipse filters, but I settled on one from Orion Telescope made of optical glass. Mine is number 07730.
Here are a few views of mine and it attached to my 600mm lens:
The next step is the way you get to Carnegie Hall: Practice, Practice Practice. This is a one time event, so the more you do to prepare, the better. Take your camera, filter and tripod out on a sunny day and start shooting. Shoot one time in P mode to get into the ballpark, and then switch to manual so that you can play with the exposure and ISO. You will have to decrease the exposure compared to the camera suggestion, possibly several stops. You want the sun to turn from a white featureless ball to one that has color - the exact color depending on your filter. You might see some sunspots, too.
Once you are comfortable with your setup, you are ready for the eclipse to get images like this:
I find it relatively straightforward to get reasonably sharp focus with the solar filter in place. What I'm less confident about - is this focus setting going to be the same, or very, very close, after the filter gets removed during totality? It's impossible to practice for.
Your thoughts?
