Converted black and white RGB photos to grayscale loose contrast in InDesign

InDesign does not have a grayscale space, so gray values get displayed as if they are on the InDesign CMYK black plate when Overprint Preview is turned on.

The gray output values you get on a conversion from RGB to Grayscale in Photoshop depend on the source RGB profile and the chosen destination Gray profile—you would get different values if the destination gray profile is the default 20% Dot Gain vs. Gamma 2.2 profile.

The default 20% Dot Gain profile causes a lot of confusion, because it would not be the correct profile if your InDesign CMYK output destination is ISO Coated V2 300% (shown in your screen capture). The destination Gray profile used in Photoshop should be the matching Black Ink-ISO Coated V2 300% black ink profile.

These threads have more on how to color manage grayscales and how to create Black Ink profiles from a CMYK profile:

https://community.adobe.com/t5/indesign/dot-gain-or-gray-gamma/td-p/8365606?page=1

https://community.adobe.com/t5/indesign/exporting-rgb-cmyk-and-grayscale-to-print-pdf/m-p/10990825?page=1#M179498

There's a reason for this:

Output values for InDesign color settings. Using your Euro ISO coated stock color settings, you can not only count on 20% dot gain (where "25%" would reproduce at 25% + 2.5% + 2.5%, with InDesign indicating that your "25%" would print at least 30% on press without image correction, but you can also account for the general "dulling" you see as there's compression of the dynamic range within the lightest and darkest shades you can print (highlight dot/shadow dot). The settings you see as you save your PDF from InDesign are directly copied from the color settings you have set for your InDesign document.

No compensation for the color settings within Photoshop. Franz is steering you right that you should compensate for the end result directly from Photoshop to preclude surprises. Start by using the same settings in Photoshop that you're accounting for within InDesign. But I can also offer you another trick that'll minimize the "dulling" effect that is getting you down.

Try this:

1. Make a copy of the original image in Photoshop. You always want to be working with copies of the original file for any image corrections, as that preserves the original image if your experiments should go awry.
2. Rather than change the image from RGB color mode directly to Grayscale, change it to the Lab Color space. You won't at first see much difference, but trust me, you will see much difference — and I believe a difference you'll prefer — by the time we're done with it.
3. Open the Channels panel. Delete the "a" and "b" channels by first selecting them, then clicking on the little trash can in the lower-right corner of the panel. You should get the impression of higher contrast (or slightly higher contrast) like your original RGB shot.
4. Now go back to your Image>Mode menu command. You'll see that your image is now shown as Indexed Color... and this is the time to change it to Grayscale. You may note that your image darkens slightly, but when you place it in InDesign, you won't see the dulling effect like you were getting in your InDesign document. If you match up the color settings in Photoshop with the settings you're using with InDesign, you shouldn't see any surprises at all.

This will hopefully get you results more like you'd expect.

Nice art, by the way. Please let us know how this works for you, and good luck with your book.

Randy

That is what profiles do, so why not convert to 'CMYK', in this that is only K, with the correct profile (greyscale with wanted profile) in Photoshop and place that in InDesign?