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If you want to capture those streaming corona then up to 15 radii of the sun would give a calculation for the ideal focal length for that type of shot.
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Yes sort of 1 solar radius (Rs) is 1/4 degree
15 radii from the limb is 16 from the center and if we assume a symmetrical corona is 32 side to side or 8 degrees. Solar max corona are typically more compact though more symmetrical than a solar min corona which typically has big streamers coming from the equatorial region and little "polar brushes" north and south.
So that's a 100mm lens on APS and a 150mm lens on full frame if I use the short dimension of the sensor.
I don't want to mislead you. Capturing that extent of corona is by no means easy. Druckmullers 2006 image is a composite made from about 230 images taken with 5 lenses 200mm, 400mm, 600mm, 1240mm to 1640mm (GSO RC8) on canon 5D bods.
You can get an idea of the extent and shape of the corona by looking at the LASCO C3 image. But you can't translate where the corona's luminosity will get lost in the sky glow during an eclipse. I think I've seen Peter's shot from 2006. I was referring to one by Miloslav Druckmuller also taken during tse 2006. The Sun was very high in the sky that day. The solar min eclipse was assymetrical so he put the long streamers diagonally.
http://www.zam.fme.vutbr.cz/~druck/eclipse/Ecl2006l/Tse2006l_39r3_102s_26d33_231n_v1/00-info.htm
Looking at this image again, I think he is measuring the diameter in radii which would double those focal length calcs ie 200mm for APS-C and 300mm for full frame.
One other thing I could have mentioned in my last post. If you are using a camera lens, then auto focus can sometimes work very well providing the camera AF zones lock onto the lunar limb. But I've also seen out of focus images and/or cameras run by computer where the AF couldn't lock and therefore wouldn't fire the shutter. Owner came back at end of eclipse to find no images.
I checked out your astrophotography site today - fantastic work!
Joe