I'm trying to combine a number of black and white pencil and ink drawings onto one page but the separate images keep ending up with slightly different tints from each other and from the white space. Is there some easy way to fix that? Thanks!
I'm afraid there is no completely easy way to fix this, but the principle is simple enough: You need to edit each of your images in Photoshop to make sure that the lines are as black as wanted and that the paper is totally white with no dots. Even the slightest tone or sporadic dots in the paper tone can result in a visible light gray rectangle around the drawings. This rectangle is likely to be even more visible on print than on the screen.
If all the drawings are scanned on the same scanner and drawn on the same paper with the same tools by the same artist you might be able to repeat the exact same treatment on all images, but if the images are different in some way they probably need a slightly different treatment.
Pure black and white ink drawings are pretty easy to fix since they only contain black lines and areas on white paper. There are no midtones or smooth gradients to protect.
Open the image in Photoshop and create a Levels adjustment layer. Hold down Alt and start moving the white point slider to the left. Now every pixel which contains just the slightest amount of black will be shown as plain black. Move the slider left until you reach the point where the all the paper is white. Don't move it further than necessary since it will result in jagged edges on the contours.
Now hold down Alt and start moving the black point slider to the right. The opposite happens. Every pixel which is lighter than completely black will be shown as plain white. Move the slider until all the lines are black, but not further.
You can also use the black and white point samplers on the left, but I prefer the more manual approach.
Be aware that there might still be some unwanted smudge present in the image so you need to inspect the whole image and perhaps manually correct problems with a brush.
Pencil drawings are way trickier. They often contain lots of subtle shadings which you want to preserve, but sometimes these shades are lighter than the paper texture or some of the shadows on the paper, so simply using levels might be too harsh.
Generally my advice about digitizing pencil drawings is "don't do it". There is a reason why pencil drawings traditionally hasn't been reproduced as much as ink drawings (it's time consuming and sometimes even impossible). But if you must, it would be easiest if the the whole paper is filled with pencil shading so the paper doesn't need to be completely white. Or at least use very white and smooth paper without texture, push hard with the pencil to get good contrast, keep the white areas as clean as humanly possible and use a good scanner.
All this said, of course it's possible to reproduce pencil drawings, but it requires a lot more finesse. You can start out with the same procedure as above, but as you might discover you'll often start ruining the finer details before the paper is totally white. And since pencil isn't as black as ink you probably want to hold back on the black point so only the darkest pixels gets totally black.
If you can't get the paper white using levels you might need to manually paint the problem areas or use masks to apply different amounts of levels to different parts of the image. Other adjustments like Curves, Exposure, Selective Color etc. might also come in handy.
If the drawing is well defined with white paper all around you might be able to use Magic Wand to select the paper, expand the selection a little bit to include all dots and then contract it a little bit to stay clear of the drawing, apply some feather to the edge and fill the area with white, but be careful not to fade the contour of the drawing.
I don't really know which example to show you here, because pencil drawings can vary a lot and the method to use all depends on the artwork.
In InDesign you can use View > Output > Separations Preview to quickly check if the backgrounds of your drawings are completely white. Set View to Ink Limit and set the percentage to 1%. Now InDesign shows in red where there is more than 1% ink coverage. Beware that this method isn't 100% bulletproof. There might still be non-white areas with less than 1% ink coverage and you need to zoom a little in on the images to be able to see every tiny dot.
Drawing © Freja Hougaard Bagge