I'm fairly new to graphics and Inkscape, so please be patient with my ignorance. I created a brochure with text on one layer 
over a .pdf of a photo at 300 dpi on another layer. I set the opacity of the photo at 15% and exported the file to .pdf and sent 
it to my print shop. On screen both on my Mac in Inkscape or Preview and on their Windows machines with Photoshop the 
photo in the background was clearly visible. When they print it with a color inkjet printer it looks good too. Any darker made 
the text too hard to read. When they printed it on their high volume printer you could not see the photo at all, however. They 
looked at it closer and told me the darkest area of the photo was 8%.

Can anyone explain this and more importantly can anyone tell me how to adjust the opacity to make it come out 15-20% in 
the final file. This is a fairly large run, so trial and error doesn't work. I don't know whether to guess that the opacity is 7% low 
and make it 22%, or is it only half of what it should be and make it 30%.

The problem is that Inscape and .svg are not formats they typically work with, so not providing the file from AI or PS pretty 
much puts me in the position of having to prove it's not my fault and/or figure out how to fix it.