I am scanning slides to my computer, by taking photos of them in macro mode, 2 slides at a time. Now I want to split each photo into the two separate slides. Is there a batch way to do this? They are all to be split down the middle. Subsequently I will crop each individually to the actual slide size. Thanks!
That's Jpeg compression. Same amount of information, just compression is applied to make the image a smaller file size, although too much compression like using a quality below 8 can result in visible artifacts and so can saving a jpeg over and over.
If you save the Jpegs with a Quality of 8 to 10 that's about the best compromise between size a quality.
If you want better quality and the ability to edit and save the image repeatedly you should use psd of tif for saving.