My "other" passion is astronomy... I've been planning this event for the last four years.
If you are in the path of "totality" then an eclipse has four "contacts".
C1: The first contact occurs when the moon first touches the edge of the sun and begins to block it. You must have a solar filter on the camera to shoot these partial eclipse phases (as the moon continues to move in front of the sun). You'll need an "ND 5.0" solar filter. I use Thousand Oaks brand solar filters. I'll put a link below.
About 10 seconds before C2 is when things start to get very interesting... you may remove your solar filter NO SOONER than 50 seconds prior to C2, but you definitely want that filter off by 20 seconds prior to C2.
BEFORE you remove that solar filter (prior to C2) you can do a final frame & focus on the Sun. The solar filter MUST be on your lens when you look through the camera to do this. If you do not own a "tracking" mount (Sky Watcher "Star Adventurer" or iOptron "SkyTracker Pro" or similar products) then the sun is going to move during totality and you need to frame in anticipation of this... so here's some math.
The Earth spins at a rate of 15 arc-seconds per second of real time. The longest duration of the Ecipse is roughly 2 minutes 40 seconds if you're lucky enough to be in that spot. That means that the Sun will move 2400 arc-seconds (40 arc-minutes or about 2/3rds of a degree) during totality. The Sun and moon are only 30 arc-minutes wide (from edge to edge). That means it will move about 1.3x it's entire width.
But this assumes you framed it at the moment that totality beings... and really you need this thing framed in about 1 minute before then. In that 1 minute the Sun will move about 1/2 of it's width. This means REALLY you want to compose the frame so the Sun starts on the left side of the frame... maybe in the left third (but that depends on your focal length) and it will migrate to the right as totality is happening.
At about 9 seconds or so prior to C2 you'll see the "Diamond Ring" effect ... and that will give way to the "Bailey's Beads" effect. Also, you may notice the shadows on white surface will have a strange rippling pattern called "shadow bands".
C2: The second contact occurs when the moon has finally completely covered the Sun and "totality" begins. You will see the solar corona and the brighter stars in the daytime sky (such as Regulus which will be 1º left of the Sun).
At this point the solar filter is off and you can visually see the corona (and so can your camera) but getting the entire corona takes a HUGE bracketing sequence... about 10-12 stops worth of dynamic range. The exposure that can capture the outer region will over-expose the corona near the edge of the sun. The exposure that doesn't blow out the corona near the sun wont capture the outer areas.
See:
How to Photograph a Solar Eclipse
"Mr. Ecipse" is Fred Espenak. He's the retired NASA physicist who came up with the eclipse prediction that all of us are using AND he's also probably the foremost expert in photography of the solar corona.
During this period you REALLY want the camera on auto-pilot so YOU enjoy the eclipse instead of having your head in the camera.
If you're lucky enough to own an Apple laptop, you want a program called "Solar Eclipse Maestro" which will automate everything with staggering precision. (Free for non-commercial use but the author appreciates donations. Commercial use requires a proper license and since we photographers are pissed when people steal our work... software developers are likewise pissed when people violate the license terms. So if your work is to be enjoyed by you and your friends.... it's free. If you think you would like to sell your work, you'll need to get a paid license.)
If you have Windows, the program to have used to be "Eclipse Orchestrator" but from what I can tell, it's not being maintained anymore. So whether or not it will be able to support your camera is in question (depending on the age of your gear). I've used Eclipse Orchestrator (on the day of the Eclipse I'll be using Solar Eclipse Maestro on the Mac) and it worked well enough for my gear. I'm told the other program that people are using on Windows is called SETnC. But the caveat is that I think SETnC might only be able to control Canon EOS cameras (I'm not certain, I haven't evaluated that particular program.)
C3: The third contact occurs when the Sun starts to peek out on the opposite side of the Moon. Totality is over... but not the fun.
At this point you get (a) another set of "Baily's Beads", another "Diamond Ring" effect, and may see more shadow bands.
BTW, prior to C2 and after C3 you may see the shadow of the moon approaching / receding ... especially if you have a higher vantage point and can see off into the distance (above the crowds).
Within 50 seconds of the end of "totality" (after C3) you MUST get that solar filter back on your camaera ... and you can continue to photograph the partial phases until...
C4: The fourth contact occurs when the moon moves completely off the Sun and the Sun is completely revealed... the eclipse is over.
I use Thousand Oaks solar filters and I own a lot of them... for different telescopes and camera lenses. Thousand Oaks solar filters render the sun in an "orange" color. Baader solar film (a different filter) tends to render the sun in a "white" color with a bit of a blue cast.
Here's the link to Thousand Oaks:
Solar Filters (“White Light”) – Thousand Oaks Optical
They do make filters that "thread" onto the filter threads of your lens, but eclipse photographers tend to prefer the "cap" type filters (they don't have threads... they're like a cap that fits over the front of your lens. If you get the "cap" type then they are sold based on the inner diameter... measure the outer diameter of your lens body and buy whatever diameter is just barely bigger (they come with a felt tape that you put on the inside to give the filter a snug fit.)
You NEED to buy this soon for two reasons: (a) as the eclipse draws near the price tag goes up and (b) they do actually run out of these things. Prior to the 2012 Transit of Venus (and that's "just" a Transit of Venus ... most of the non-astronomy public doesn't know about it) the manufacturer ran out of filters about a month before and they were so backlogged with orders that those who didn't yet have filters were waiting to see if they'd even have a filter in time.
YOU should have solar eclipse viewers for your own eyes (to avoid having to buy the white cane with the red tip a day or two later when your retinas stop working ... retinas do not have nerves that can feel "pain" so you wont know your damaging them and it takes between 24-48 hours for the damaged retinas to stop working. So you can think you're ok... and a day or maybe two days later you wake up and suddenly realize you don't see so well.
In addition... you MIGHT want to pick up an eyepatch for one eye. Principally this is so you can look like a pirate. But one side effect is that it takes a human eye about 40 minutes to become completely dark-adapted, but you'll be mostly dark-adapted after about 20 minutes. Totality only lasts a couple of minutes so your eye would never get dark-adapted in that amount of time. So the idea is that if you put a patch on just ONE eye about 30 minutes prior to totality, then that eye will be mostly dark-adapted (you must NOT cheat and take that eyepatch off for any reason... not even for a second... until totality occurs). You can now remove the eyepatch at totality and THAT eye will be dark adapted and see (a) a much larger solar corona and (b) loads of stars. (All those people not wearing an eyepatch 30 minutes prior wont see that... and also wont look as cool.)
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