Estimating the Image Size
Will your camera and lens produce a close up of the eclipse or a little dot?
One way to answer that question is to determine what the size of the eclipse will be in your image relative to the imaging plane. This is the image size.
The image size (S) is calculated from the lens focal length (FL) and the angular size of the object to image (A).
S = 2 * FL * tangent(A / 2)
The measurement units for the image size (S) will be the same as the focal length (FL). In most cases this value is expressed in millimeters. Compare the value computed with the size of the imaging plane to get a feel of how large the image will appear when displayed or printed.
An image of a solar eclipse typically includes the moon and solar corona around it. The lunar/solar disk is approximately 1/2 degree in angular size. Eclipse photographers often reference the corona in terms of solar radii or diameters. That is, you might try to image the corona out to one solar radius (an additional 1/4 degree in angular size on each side) or a full diameter (add 1/2 degree to each side).
Another way to answer the question is in terms of field of view. The field of view, expressed in angular units, describes how much of the sky your camera/lens set up can see. To determine this you apply the above formula solving for the angular size (A) using the size of the imaging plane (XY).
A = 2 * arctangent(XY / (2 * FL))
Once again, it is strongly recommended that you practice using the camera and lens set up by taking pictures of the moon. The practice shots will reveal the actual image size of the moon. Remember that during the eclipse we are not as interested in the lunar details. We want to image what surrounds it in the sky (the corona and prominences). When practicing we are interested in the lunar details to get the best focus.
Prominences are about the size of large craters on the moon and the corona extends several lunar diameters away from the center. Sometimes selecting the right lens to use results in using multiple cameras - a practice to be avoided by most eclipse chasers since the enjoyment of the eclipse is greatly diminished.
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