El said:
Is there a way to keep (by default) existing jpeg compression factor
when saving a jpeg picture?
Thanks in advance
Not really. About the best you can do is choose something that gives the
same file size as before. Most JPGs don't have any record of amount of
compression that was used to save them. And even if they do, different
programs use different descriptions of the amount of compression.
Photoshop uses 0 (low quality) to 12 (maximum quality), Paint Shop Pro
uses from 1 (highest quality, lowest compression) to 99 (very high
compression but very poor quality), and Irfanview uses 1 (lowest
quality) to 100 (best quality). There are also number of methods of
chroma sub-sampling, different encoding methods and no standard
quantization tables, so different programs make different decisions on
exactly what to keep and what to throw away.