When you add the drop shadow you are adding transparency, so ID renders the preview using the separations values, as it does in Overprint Preview. There is currently no support for grayscale profiles in ID, so the gray values in the image are treated as if they are in the black channel of the CMYK working space (and that's basically what you want since grayscle is black ink, except it would be nice if there was a value adjustment for mismatch, as there is for RGB or CMYK, but there isn't). There should be no actual change to the numbers or how they output, only in how they are displayed, which you can verify by exporting to PDF with and without the drop shadow and comparing the desities using output preview in Acrobat Pro, or by turning on Spearations Preview and comparing the density to Photoshop (but ID uses single pixel sampling, so you may need to adjust the sample size in Photoshop to match).
I've never found this to be a problem, personally, but I was trained to do grayscale conversion more or less "seat of the pants" and make adjustments in Photoshop that would often seem too light, knowing that dot gain was going to darken the image in print. If you want an exact match between what you see in Photoshop and InDesign you should consider creating a custom color settings file in Photoshop that uses the black ink channel from the CMYK profile as the the gray profile. To do that, Edit > Color Settings... and click the More Options button (if it is showing). From the grayscale profile dropdown choose Load Gray... and in the next dialog navigate to and select the CMYK profile that matches your ID working space.
Thank you Peter,
I'll look into that, but wouldn't you think ID would sort this issue out? I'll see how I go, but may contact you agin re this issue if I still have questions! Thanks.
ID is doing what it is designed to do. It's putting all black ink on one plate. The only issue here is one of preview on screen, the numbers are not changing. If the print is too light with a drop shadow applied, it is going to be too light without it as well.
As to sorting it out, I'm not sure what you mean. Even with High Quality Display enabled you are looking at a preview in ID, not the actual image pixels as you do in Photoshop. You can turn on Overprint Preview to get a more accurate rendering (overprint preview and separations preview are tied together) by forcing ID to calculate the separations values instead of using the image previews, but this will force any RGB content to be rendered as CMYK on screen which might not be desirable if the output is intended for the screen rather than print. It will also slow down the application slightly, but whether you notice that is going to be dependent on your system specs.
but wouldn't you think ID would sort this issue out?
In addition to Peter's comments, ID isn't used exclusively for print, so there needs to be a way of previewing for both screen and print media. When there's no transparency on the page and you turn on Separation or Overprint Preview you get a CMYK print preview (grayscale previews on the black plate via your doc's CMYK profile). With Separation/Overprint Preview off you get a preview for screen media—Gamma 2.2 (sRGB).
When you add transparency to the page the Transparency Blend space is used—if it's CMYK you get the print preview, if it's RGB you get the screen preview. You can match profiles as Peter suggests by loading your ID CMYK profile as the grayscale profile in PS. Unless you are going to a web press US Sheetfed is probably a better choice than the default US SWOP. It allows for more dot gain—you'll get a darker preview.
Thank you to Rob and Peter.
My query is about a book (ie print) I am producing, and it will be b/w throughout (all greyscale images with drop shadows + black text), so CMYK and RGB are not a consideration in this case. I'm sorry, I'm not as tech savvy as you both are, but can I assume that I will get what I see in Overprint preview?
but can I assume that I will get what I see in Overprint preview?
Only if your document's CMYK profile is a reasonably accurate profile of the destination press conditions and you have decent monitor calibration and profiling in place. If you are printing on a typical sheetfed press then US Sheetfed will probably be a better profile choice than the default US SWOP.