How do I automatically sync photos between MBA, iphone & iPad (& iCloud)?

Thank you so much. Just 1 or 2 questions. Where does the sync originate from, ie, I only take photos/videos using my iphone, so will my other devices 'take' the photos/videos from my phone? In the past I seem to have used iTunes and exported from my iPhone to my MBA (my iPad is quite new). At least I think that's what I did; it seemed messy anyway. With hindsight, maybe I have never synced! Sorry to sound stupid, but syncing like you advise, could it delete/overwrite anything? I suppose that's why I ask which way the sync works.

I have got to the part on MBA in Photos app where is states: please see screen shot. And then got scared off! I believe I have enuf memory but it looks like I have not. This confuses me as if my devices just sync then I am not using up any extra space as the items in my Photos app at present are the ones I have in my MBA, iPhone and iCloud. For the last few days i've been going thru them all to check and apart from one or two items they are quite similar. I just want them to be synced exactly and to update automatically, hence all these questions. Don't give up on me, see me more as a challenge. You talking to someone who knows what they are doing would be so boring for you (not?).

I have compared all my 4 'devices', namely, Photos/MBA, iPhone, iPad & iCloud after taking a photograph with my iPhone. The same photo automatically appeared on all 4. Using another new photo taken using my iPhone (which as above appeared automatically on all 4 devices), I then deleted the same photo on my iPhone and the ONLY device it did not delete on was my MBA/Photos. I used the same method but this time deleted a photo on my iPad and the ONLY device it did not delete on was again my MBA/Photos.

Also after comparing all 4 devices, whilst all 4 have the same number of videos, my Photos/MBA has 3 less photos. Also, Photos has always had a few photos that appear out of order. I suspect that I have deleted photos in Photos/MBA and not the iPhone, and vice versa. I have about 1600 photos and 200 videos on all 4, including iCloud, so I have probably used methods to download photos over the years that may or may not have included syncing. I recall using iTunes but never liked using it. Now I go in a circle as I don't want to use this method anymore. Getting back to the screen shot I submitted, I may have answered your last question, as in why my photos are already in iCloud. iCloud & Photos are ticked in Sys Prefs. But in Preferences in Photos on my MBA, the iCloud Photos box is NOT ticked. And then this leads me to my screen shot which confused me as whilst I would say nearly all my photos/videos are in the cloud already (although sync does not seem to be in use), if I then tick the iCloud Photos box, why I am 'told' that I may not have enough space? I'm afraid of doubling up somehow or deleting by ticking the automatic upload & store box. Hope you can advise. Thanking you.

Ignore my last missive! I have now ticked all the boxes and yes you are right, far too many photos and vids are being synced. Oh well, there is another days work to delete duplicates. Thank you for all your advice including the apple doc I said I didn't want; it is easy to read. One more thing, why do I need the My Photo Stream if my devices all sync?

Thank you. In 4 hours the syncing has uploaded 3 photos! Need to upgrade my broadband. But at this rate it would be best to stop the syncing. Thank you once again.