Converting multiple photos to HEIC using Preview

As an old bug Sequoia 15.5 Preview.app still exports wrong codec in batch mode.

With many images selected, Preview "Export Selected Images" exports many images with a wrong codec. For example:

1. Open many images (.heic, for example) in Preview.app.

2. Edit > Select All.

3. File > Export Selected Images... > Show Options > Select some other Format (JPEG, for example).

Result: All files have .jpeg suffix and the very 1st exported file(s) might really be .jpeg but the rest are really .heic (HEIF) if you inspect them with Preview > Tools > Show Inspector > General Info.

Exporting from Preview, JPG vs JPEG creat… - Apple Community

As you have noticed, the same bug happens if you try to export .tif as .heic in batch mode.

Kurt Lang wrote:

I can't replicate your issues in 15.5

I can replicate that issue also in a cleanly installed 15.5 and so could dialabrain in that other thread:

Exporting from Preview, JPG vs JPEG creat… - Apple Community

So if I duplicate a single a.tif multiple times in the Finder, and then open and batch export them via Preview as .heic, many of the exported .heic files are still using TIFF codec and are of course much larger than the correctly exported .heic with HEIF codec:

On the other hand, via Finder > Quick Actions > Convert Image > Format: HEIF all are correctly exported as .heic. Or use some 3rd party app like GraphicConverter or Photoshop for the batch export.

Because I haven't done comparisons for quite a while, I checked. Started with an uncompressed 105.8 MB TIFF image.

TIFF with LZW compression: 58.9 MB

PNG (largest): 55.8 MB

PNG (smallest): 52.5 MB

HEIC, maximum quality: 29.1 MB

Either way, PNG was truly lossless. Even zoomed way in with Photoshop, layering the PNG on top of the uncompressed TIFF, not a single pixel changed its color value. The real difference was the ridiculously long time it takes to write a PNG to smallest size with almost no gain in space.

LZW of course is also truly lossless.

HEIC, not so good. That extra 20+ MB of savings had to go somewhere. Even with the slider all the way to the right for maximum lossless quality, it still loses data. Will you notice it at viewing size? Probably not. But like JPEG, each time you make changes and save the file, it will lose yet more of the original color as you recompress the already compressed not-really-lossless data.

Also, you should be concerned about saving the files as 10-bit color. Not very many apps can handle it properly. I'm using the latest version of Photoshop. It had no problem opening a standard 8-bit HEIC file, but gave this error message when trying to open a 10-bit file.

Well, it is an HEIC file, with the same extension as the 8-bit image, but PS didn't know what to do with the 10-bit color space.

This is just so you know what an HEIC image is really doing, or not doing for you.

how do you select multiple files (images) in Preview?

In Finder select multiple images and let Finder open them in Preview.app. Or, in Preview > File > Open > [select multiple images] > Open. Or in Preview open one image, and then View > Thumbnails and then drag more images to the sidebar.

Then Preview > Edit > Select All to select all thumbnails [do NOT click the image area before doing this because that selects the image itself].

Then File > Export Selected Images... > and choose some other codec to export to.

For example, if you have multiple .tif (or some .raw format) images selected and choose to export as .jpg, then only the very first might be exported as jpeg codec, but the rest are exported as tiff codec but have .jpeg extension:

I sent Apple feedback about this in March.

I am amazed that this has not been addressed.

What has not been addressed?

With file selection for a batch export, you can click on any image in the left column. Doesn't matter which one. Then press Command+A to select all. Or from the file menu as you noted. Same thing.

Or do you mean Matti Haveri's note?

For example, if you have multiple .tif (or some .raw format) images selected and choose to export as .jpg, then only the very first might be exported as jpeg codec, but the rest are exported as tiff codec but have .jpeg extension:

I'm testing in Sequoia 15.5, and what you can't do now is mix file types at all. If you have a variety of file types open and try to do "Export Selected Images", you'll get this message:

So in a sense, yes, it has been fixed. Instead of incorrectly converting mixed file types, Preview just won't do it at all until you choose only JPEG images, only TIFFs or other.

Display your folder in List view (Command+2). Expand the folder if necessary to see the Kind heading. Click that to put all files of the same type next to each other. Do all of the JPEGs. Then all of the TIFFs, etc.

real-kloomnik wrote: I am using Preview to convert a very large number of scanned photos from TIFF to HEIC.

It sounds more like you are trying to use Preview. What OS are you in? My Preview doesn't seem to work like this. "Select All" selects all of the current picture and provides handles so I can change the selection from there.

Had you thought about using Photos instead of Preview? You could make a new Library, import the pictures, and then select them and export them.

You have to zoom way in, and be able to view one image on top of other, with the ability to turn the top layer on and off in order to see the pixel values change. It's subtle, but a loss is still a loss.

Still can't replicate it. I tested with a dozen 16 bit TIFF images. Did a batch convert with Preview to HEIC 10 bit. All of the converted images are just that.

What converted differently?

And I would still strongly suggest not using HEIC. You only get the choice for 8 or 10 bit in a Preview export if the bit depth of the images is more than 8. And in old TIFFs, they're most likely 16 bit.

You'd be far better off, quality wise, to use PNG so they maintain their 16 bit color, and are truly lossless.

That I can't replicate your issues in 15.5 suggests there's nothing wrong with either the OS or Preview.

You may have junk apps on your Mac that are interfering with the OS's operation. Like AV software. Cleaning/optimizing apps. VPN.

Download and run EtreCheck.

https://www.etrecheck.com/en/index.html

You don't need to purchase the extra features. When it's done examining your Mac, copy the report to clipboard. Start a new response and use the Additional text button to paste it into. The results are typically too big to paste into the main editing window. The button is in between the chainlink and portrait mountain scene icons.

EtreCheck automatically redacts any personal information in the report.

Oh, that!

HEIC is a container format. Yes, the image itself is written as compressed TIFF within the container. But, that doesn't mean there's anything wrong.

TIFF itself is a container format. Tagged Information File Format. In other words, HEIC is saving a container format (TIFF) within its own container format.

You can do the same thing with a TIFF. When you save one, you can also choose to compress it with LZW or JPEG. Well, when you choose JPEG, that's very literally what you're doing. You're saving a JPEG image within a TIFF container. At that point, it doesn't matter that the file extension says .tif, you've really saved the image as a .jpg within the container.

And that also explains the quality loss when you convert your TIFFs to HEIC. What it's really doing is save the image as a JPEG, roughly quality level 10 out of 12, within a TIFF container. And then it saves that within an HEIC container.

All the more reason to avoid HEIC altogether.