I have a greyscale image that I placed into two separate InDesign CS6 documents. However, this image appeared darker in Doc B than in Doc A (Doc A showed the image just as it appears in Photoshop). Obviously some setting in Doc B is darkening the image but I am struggling to find anything relevant.
Can anyone suggest things for me to check?
Do they look the same in both files with Overprint Preview active? If not, it's probably a case of using different CMYK working spaces (grayscale profiles are not recognized in ID and the gray values are passed unchanged to the K channel in the document working space). If they do it's probably transparency on the page in one file and not the other.
If it's transparency, you should expect both files to output the same, and the way you see it in overprint preview.
Overprint Preview was indeed the answer. It was switched off in Doc B and switching it on made the greyscale image look as it did in Doc A and Photoshop.
I had already ensured that both docs used the same CMYK working space but had forgotten all about Overprint Preview, mainly because I only use ID once a year. Many thanks for reminding me.