UKC

Well it's a puzzle to me.

I had a few thousand jpegs, including a lot that I scanned from negs at too high a resolution or size. So the scanned images were about 10 times bigger than my normal camera jpegs, which are around 3 to 5 MBs.

So I used a batch processor which was supposed to convert all photos to 4000 px by 3000 px, (a little larger than my biggest camera jpegs) but no pictures would be enlarged if smaller. Dpi was to be maintained.

Therefore, I figured all the pics that were too big would be reduced, and all the pics smaller would be untouched.

However, I find that all my camera jpegs are now reduced in size from about 3 to 5 MB down to less than 1MB. Enlarging the before and after pics shows no loss in quality. So everything is equal except the file size.

What am I missing here please?

In reply to Fredt:

Has it saved them at the same resolution but a lower quality setting? JPEG can do varying levels of compression. They may have been barely compressed previously. You'd probably notice at least some loss of quality though. Have you got the original files still around for comparison?

In reply to Luke90:

Yes, I compared the originals with the new ones, can't see any difference in quality.

The originals will have been exported from Lightroom at maximum quality.

In reply to Fredt:

JPEG compression seems the most likely answer to me. 3-5MB down to around 1MB seems like a plausible compression ratio and wouldn't necessarily show hugely visible artifacts. I'm no expert, though.

The smaller files will have more compression applied.

This means they will also have more 'compression artifacts'. For JPEGS theses include blockyness, jagged lines and banding (rather than smooth colour transition). Google it so you know what you are looking for.

If you still can't see them, then the amount is low enough for your use, and you've saved yourself a lot of storage space.

Most photographers would still advise to keep the originals however.

In reply to Fredt:

Most jpegs have some detail about the quality and the encoding process used to create them in the exif data

There's at least 40 lines of info in the exif data on a photo taken by by camera phone, and 177 lines of info on one taken on my Nikon D3100.

Obviously a lot of that is about the camera, focal length, lens, ISO rating etc, but there's some stuff about compression and quality



So, you could extract all the exif data from one of your originals and then from the "same" file that's been converted and see if you can spot the differences there...

Not sure what tool you'd use in your OS, but in linux "exiftool" spits out loads of info on each file.


Also, I think some jpgs include thumbnails. Your conversion program might have stripped those out?

In reply to Fredt:


Just a point of information but this statement is meaningless. dpi refers to the 'dots per inch' when a file is printed. If you print it big then it will have a lower dpi than if you print it small. This has no bearing on the original file size, it is just an output setting for printing.

I agree with the others advice about Jpeg compression but would also add my support to the advice not to re-save as a jpeg. You can never add quality to a Jpeg so each time you save it, the file get s a little bit worse, even if you save it at a 'higher quality' (less compression). Keep originals since you can't get better than those.

Alan