Copying a Photo Out of Photo Library Alters and Compresses It <sad face>

That is what File->export->unmodified does. You get the original file, warts and all.

Export photos, videos, slideshows, and memories from Photos on Mac - Apple Support

I guess you're using drag and drop from the Photos window? That yields the Preview, mid quality version of the image.

Use File -> Export. You have more options. Just bear in mind that quality has the biggest impact on file size, and size refers to the dimensions (length by breadth) of the image, measured in pixels.

YMMV really applies here.

However, for transferring to the PC you might want to transfer the originals. (File->Export->Unmodified) You will lose any edits, but you will not lose any information from the files due to multiple compression cycles.

Hello WolandSkip,

We understand that you're looking for options to transfer your photos from your Mac to a Windows PC without compression. Do you use iCloud Photos? If so, a good option would be to install iCloud for Windows. If you'd like to look into this option, check out the articles below for full details.

What is iCloud for Windows? - Apple Support

Download iCloud for Windows - Apple Support

So, that we can research a bit more for other options, let us know exactly how you are currently transferring photos from your Mac to your Windows PC.

Best regards.

Thanks Yer_Man, yes, but File -> Export makes the files larger than they were in the Library. Is that expected?

Yes.

Longer answer: when you take a shot with your camera the sensor is exposed and the data interpreted by the computer in the camera, and this is processed into the photograph. That photograph is then compressed into a jpeg file. You have no choice in the amount of compression the camera uses.

When you export from Photos the app takes a copy of the photograph and puts it into a new file, and saves it as a jpeg. But now you have a choice in the amount of compression used. So, you can apply less, and then you get a larger file than you imported.

There is no secret sauce here. This is plain as day, and perfectly transparent if you understand how digital cameras and, in particular, jpeg compression works.

What export settings are you using?

You can adjust the amount of compression used by selecting a higher quality image.

You have no choice in the amount of compression the camera uses.

Sorry I'll correct that:

You have no choice in the amount of compression the camera uses - unless you have one of Keith's cameras

:)

Seriously, does that include the Jpeg compression? I shoot Nikon and but I've never noticed a setting for that.

Not true. On my Canon's you can choose from several compression settings. You can also choose a non-lossy compressed RAW (which might not work with Apple's Raw support.)

Then your library is hosed. Are these RAW?

Note that with RAW, 4:3 is usually the standard size of the image and the 16:9 is just a "crop".

The D850 has it, at least.