Q: When importing AVCHD from my camera does Photos convert it?

I can't test right now, but you should be able to test this yourself.

Use the export option from the file menu to export an unmodified version of a clip to your desktop, you should be able to tell immediately if the file is still an .mts or if it's been converted to a .mov (or something else) by photos in the same way that iMovie etc do.

If it's unchanged you might want to undertake a further test by downloading the clip from iCloud.com to see if there's a conversion when it's uploaded via iCloud photo library.

Sorry, I'm about to go out for the night or else I'd do it myself for you.

I suspect you aren't loosing quality. AVCHD is fairly highly compressed to begin with, my guess is you are getting much the same in a different container and that file size is similar, but I wouldn't be surprised if it wasn't larger.

Dragging files out of Photos does change them (unlike the export option from the file menu), if I recall it changes the resolution as well (or it did last time I tested).

I wouldn't worry about the conversion from AVCHD, the format isn't all that much use for playback or editing, it's more useful for capture, if were ever going to edit it, you'd need to convert it anyway.

Yes, I see the same as you, photos keeps the files as h264 in a .mov container, kind of makes sense in hindsight, AVCHD is a poor playback format.

Not at my computer at the moment, had assumed the video and audio tracks were simply repackaged rather than recompressed. Will check tomorrow, what files sizes do you see before and after import.

I'm getting different results this morning importing in a different way, will get back to you, but in the meantime are you importing with the camera connected or from a file already on your drive.

OK, now here's a strange thing.

I don't use my camera's any more, video isn't what it used to be now that anyone can take reasonably good video at any time, so I like many just use my phone now. However I connected my old camera up and imported a short clip which imported as a .mov

When I took a second look, I'd copied the .mts to my desktop before disconnecting the camera, so I used this to import this time. It imported into Photos as a .mts (unmodified), it did however take a while to upload to the cloud and when I downloaded it again from iCloud.com it was a .mp4 file of about ¼ the bitrate/file size.

The other noteworthy point is that it showed up on my phone as a greyed out .jpg with the notification at the foot of the screen that it was adding a video. It's been like that since yesterday, so it doesn't seem to me that it's going to go anywhere now. I was wondering if this could have any bearing on the experience you quoted in your other thread.

I don't have any proof, but I don't think there is any additional compression taking place when they are imported from the camera, don't forget AVCHD is highly compressed to begin with.