The following assumes you will be using a solar safe filter for photography during the partial phases of the eclipse. Filters are removed during totality. Look here for my discussion of filters. http://emeraldphotographic.org/2016/10/17/filters-for-photographing-viewing-an-eclipse/
Those of you wanting to make telephoto captures of the sun during the eclipse face the significant challenge of how to successfully nail the focus on the sun using your gear. This is very challenging because atmospheric turbulence is constantly causing blur on much of the sun’s surface & edges. Plus even when there are visible sunspots, the sun has no high contrast detail to aid focus. Since we are approaching solar minimum, frequently there will be no sunspots. You will be trying to focus in bright daylight, which will make viewing your finder and monitor difficult. Add camera shake as you try to manual focus a long tele and now you know why this is hard.
Here’s two short videos of the sun. One has no sunspots and uses a 1200mm lens with no tracking mount. Note the ripples in the sun’s edge due to atmospheric turbulence. The other is on a day with sunspots using 840mm FL, a tracking mount but it was a windy day. Both these are in focus.
Here’s what works. First you must have a shaded and magnified view of your finder or monitor. If you are using a DSLR without a tilt screen monitor, I highly recommend you use an angle finder. There are several inexpensive units available of varying quality that can be used on most brands of DSLRs. If you have a tilting monitor, I suggest a hoodman knockoff unit. I got this one off Amazon for $30: https://smile.amazon.com/gp/product/B00XL2BYK4/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1
I selected this one because it attaches with elastic cords and I could easily modify it to fit my Olympus. It opens allowing a view of the whole monitor. There are lots of others and all it takes is a drill and some elastic cord to fit it to your camera. Flip-out to the side monitors are a bit problematic but they still work with this but will flop down when released. One of these can be used for viewing a cell phone screen attached to a scope in bright daylight. Mike at Focal Point may have something usable too. Best of all, with a tilt screen, you won’t have to bend your neck back at a steep angle during the eclipse.
Here’s the unit after modification & attached to the camera. Note I looped the cords around the “prism” hump to keep everything tilted up. Not all camera top plates will allow this and a different arrangement will be needed.
Now that you have the gear you’ll need, here’s the technique that will help. After centering the sun in the field of view and setting to manual focus mode, turn on magnified live view and image stabilization. IS will help reduce camera shake when you adjust the focus ring. If there are sunspots, use those and adjust for maximum contrast. If your camera allows, increase the live view magnification & fine tune the focus. If first contact has occurred, use the moon’s dark edge for focus. If there are no sunspots & first contact hasn’t happened yet, use the sun’s edge and again maximize contrast. Depending on your camera, you may find focus peaking or zebras helpful but I haven’t.
After you think you’ve adjusted correctly, make a test capture and examine it on your monitor, cell phone or tethered computer. I’ve had no success using a tethered computer for focus even when shaded because the image is always soft. But when viewing the test image, you’ll likely find you are not sure its in focus. Unsharpened solar images are always soft. You can’t be sure you’ve nailed focus until you get the images home on your computer. That’s why practice is required to help you gain confidence in these skills.
Note that the eclipse lasts over two hours from first to fourth contact. The temperature of your lens will increase as the day progresses. This often causes focus drift so periodically recheck your focus. Especially just before totality.
Here’s an unadjusted solar image with sunspots that is in focus.
Here’s a 7 image HDR with sunspots after (too)strong processing showing not only the spots but also solar granulations – boiling plasma. There’s lots of detail on the suns surface but it takes strong adjustments to bring them out.