Why would a grayscale png file change saturation next to grayscale tiff?

[Lisa32370745popf]

I have two grayscale photos placed on a page - a tif file and png file. They are close to each other with the frame edges touching but not overlapping. They appear the same in saturation and levels and 8 bits/channel, dot gain 20%. I need the two images on consecutive pages with one or two pages only needing the png file. I have pasted in place the two photos and deleted the tif image when not needed. When the tif file is deleted, the saturation of the png file visually becomes darker. It appears darker in the InDesign file and low-res PDF but appears correctly in high-res print files. If I paste the tif file back on the page (anywhere on the page) the saturation immediately lightens again. I have looked into trapping, transparency, and compared setting in Photoshop. What would cause the saturation to change with/without the tif file if both are printing as black?

[Eugene Tyson]

Hard to know without seeing it.

The only time I've seen things like this happen is when there's transparency in the image. 

When you paste in place - where are you pasting from? 

Why use PNG? 

Trapping wouldn't have anything to do with it. 

Need a lot more info to be able to help you out. 

[Lisa32370745popf]

Thanks for the reply. It helped me narrow my focus.

I saved the PNG to a tif but it still had a saturation shift. The fix was in the transparency blend space and ouput/color conversion and destination. The saturation still changes in InDesign which I'm not worried about - the pdfs look better. 

[rob day]

Hi @Lisa32370745popf , is the document intent print or screen viewing? If it is for print turning on Overprint Preview displays a grayscale as it would print on the press Black plate--the document's CMYK profile handles the preview (which  isn't Dot Gain 20%). Any transparent object on the page also displays the CMYK preview.

If the Transparency Blend Space is set to RGB, the grayscale would be converted to 4-color CMYK on export or output. Separation Preview would show the expected output values.

[rob day · Accepted solution]

Also, this thread on how InDesign handles place grayscale objects might help:

https://community.adobe.com/t5/indesign-discussions/greyscale-jpegs-or-psd-images-are-going-much-dar...