HEIF/HEIC or High-efficiency image format is a more modern file format for images. It provides more quality at the same or less file size. Think H.264/MPEG-4 AVC and H.265 (HEVC) for video. Without these compression standards high quality 1080p and especially 4k would not be possible especially for those with slow internet connections. HEVC will makes it possible to get reasonable sizes for 10-bit 4K HDR all the way up to 8K and also use the much wider Rec. 2020. The best TVs today can only show about 75-80% of the 2020 color space. Rec 2020 is also much wider than the P3 displays on iPhone 7 and newer and MacBook Pro 2016 and newer.
Well this new HEIF format was launched with iOS 11 and High Sierra. Now if your phone was set to “High efficiency” under the camera settings, the iPhone 7/7 Plus or newer will use HEIF. If you set it to “Most compatible” it will use JPEG.
Now here’s the story. Under photos settings in iOS, when transfer photos is set to automatic if will send HEIF if you’re using macOS High Sierra or newer and JPEG if you’re using anything else. If the other phone is on iOS 11 or newer then it will also get HEIF.
If you want to save in JPEG and always JPEG do this:
Open “camera settings” and choose Camera settings/Formats/MOST COMPATIBLE and then go to Photos settings and choose “KEEP ORIGINALS.” That’s it.
Now, it’s possible that photos saved originally in HEIF will not be able to be transferred as a JPEG on Mojave.
If this is the case then you have to convert these images back to JPEG using whatever software you want for that purpose.
Please keep in mind that sooner or later we will start seeing more and more support for HEIF. It is an official standard and not just some crazy format that Apple is backing. The reason why many places on the web don’t support it is because not everyone has it. JPEG is still the most common standard. Think USB-C vs. USB Type A. USB-C will completely replace type A but it takes a while.
Once most dSLRs and other camera support HEIF, JPEG will be history. People will either shoot RAW or HEIF. The thing is changes have to be forced otherwise people will stick to using what they know.
Hope this post is useful!