how to export full quality in photos

In iPhoto you used to be able to export a modified picture in 'current' quality, which was full quality as shown in iPhoto.

Not sure what you mean by "full" quality, but exporting at the current setting in iPhoto simply yielded the iPhoto preview - a medium quality version of the photo.

When you're talking about edited Jpegs there is no concept called 'full quality'. Image can be exported as High, Low etc. but if you want to mimic the 'current' setting use the medium setting. If you want high quality Jpegs then use the High quality setting. That will be better quality than the medium/current setting.

You misunderstand what 'quality' means in this context. Quality is about the amount of compression applied to the image when the Jpeg is created. Is had very little to do with the quality of the photo. A well exposed image will print pretty much the same from a 1, 2 or 5mb image, especially from a domestic printer. You might see artefacts on a very high quality printer, but it's really not that mooch different.

Jpeg quality really comes into play if you use a lossless editor after exporting. This is in the nature of the Jpeg format. It's not an image format at all, it's a compression format attuned to images. Essentially its a special kind of zip file. When you open a Jpeg it's decompressed, so that 5mb Jpeg could contain a file that uses 50mb of RAM.

However, if you edit the image, then the file is recompressed and that means that some data is thrown away. In theory, edit a Jpeg enough times and you'll have an empty file. To overcome this, when you edit in apps like iPhoto/Photos the original file is not touched. The edit decisions are recorded in the db and only committed to the file when exported.

Exporting create a new file, containing the same photograph. But the quality of the file - which is the amount of compression applied - can be selected. If you found that the Preview in iPhoto always matched the reported size of the original then either a: that was a fluke or b: the image wasn't edited.

Medium gives about the same quality as the Preview, that's all.

The best quality to export at depends on the next use the file gets: if it's going to editing further, then high. Other than that it doesn't make a huge amount of difference.

Hello Terence and thanks for your reply to jonny12's query. I have also noticed that despite selecting the High option when exporting edited JPEG images from Photo, the file size at destination is usually around 3MB - approx 60% smaller than when I exported similar edited images from iPhoto with its High option. The image size is not affected, of course. As Photo's compression-rate appears to be much higher than iPhoto, it seems to me that image quality must be more than slightly reduced and would be quite noticeable when printing at 10" x 12" or more. For printing this size, do you think it would it be better to export an edited JPEG as a PNG or even a TIFF? And is there a possibility that Photo is also extra-compressing images at the import stage, not just during export?

I am aware that using RAW makes this discussion a bit academic, but I have only recently started shooting in JPEG+RAW and still have 14,000 JPEG images, some of which I would like to include in a show.

, it seems to me that image quality must be more than slightly reduced and would be quite noticeable when printing at 10" x 12" or more.

Trial and error is the only way to find out if this is the case. And that will be even more affected by the quality if the printer than the quality of the image. again, there is no simple connection between the quality of the photo printed and the MB size of the file. A large file of a poorly exposed shot will make a bad print no matter what.

I am aware that using RAW makes this discussion a bit academic,

Not in the slightest. You can't print a Raw. You need to process it and that becomes the Jpeg (or whatever) that you print. If you don't process they raw then you're printing the automatically generated preview.

but I have only recently started shooting in JPEG+RAW

Bit of a waste of space, as any Raw you import then get's a Preview so you effectively get Raw + Jepg + Jepg

Really, it's trial and error, that's the only way to test the print quality.