Shooting the Sun is different depending on if you are in the path of totality or not.
If not in the path of totality then you'll need a safe solar filter (preferably an ND 5.0 filter which blocks out 16.66 stops of light.
With that filter you could shoot at ISO 200, f/8 and 1/1000 sec. exposure (again that's WITH the ND 5.0 filter in place) and get a pretty good exposure. (use manual exposure... auto metering is likely to be fooled by the fact that most of the image is black and will attempt to brighten it). You could also use ISO 100, f/8 and 1/500 sec with the ND 5.0 filter.
You can use other f-stops as well... but f/8 tends to be a sweet spot for most lenses and since there's enough light to use any f-stop you want, you may as well use f/8.
You'll also likely want to switch to manual focus because auto-focus depends on a focus point being right on the edge of the sun so it can detect contrast. The sun's disk will not like offer much contrast and of course neither will the blackness of space around the sun.
The entire event will take about 3 hours for most of us (the actual time varies by location and how much of the Sun is eclipsed by the moon).
You could set an invervalometer to capture images about once every 2 minutes and you'd end up with around 90 images.
Of course the Earth is spinning during this time and that means you'll need to keep tracking the Sun or nudging your tripod along.
If you ARE in the path of totality, things are very different. As totality approaches, you can safely remove the ND 5.0 solar filter about 20 seconds prior to totality (and no sooner than 50 seconds prior). This completely changes the exposures.
Around 9 seconds before totality you may see the "diamond ring" effect.
Around 1.5 seconds before totality you may see the "baily's beads" effect (this one is very fleeting and timing is critical. Usually the camera is under computer control to catch this.)
During totality you can capture the solar corona... but the corona has quite a bit of dynamic range. The part nearest the sun is bright and the part farthest from the sun is faint. It isn't possible to get the entire corona to expose nicely in one shot. It's recomomended that to catpure the entire thing, you'd need between 10-12 stops of "bracket" exposures.
That means you take a shot at, say, ISO 200, f/8, 1/1000th sec.
Then you'd change to 1/500th and re-take the shot
Then you'd change to 1/250th and re-take the shot
Then you'd change to 1/125, then 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1/1, 2, and 4 seconds (that's a 12-stop bracketed sequence).
All of those images would be merged into an HDR image result (using software such as PhotoMatix Pro ... although lots of programs can generate HDR images.)
Once totality ends you have another chance at more Baily's Beads and another Diamond Ring... then the filter goes back on.
Once the filter is back on you can continue to shoot the partial phases until the entire event ends.
Being on top of all the exposure changes is tricky and it can mean you end up working on the camera instead of enjoying the event -- which is tragic because you really should experience totality without worrying about the camera.
To do this, you can use computer software to control the camera.
If you own a Mac, then Solar Eclipse Maestro is the go-to app for eclipse photography.
If you own a Windows PC, then either Eclipse Orchestrator or SETnC are the go-to apps for eclipse photography.
For more tips, see: http://mreclipse.com/SEphoto/SEphoto.html
Tim Campbell
5D III, 5D IV, 60Da
