i wanted to ask why gray photos differ between pc and offset printing. 86% gray seem whitish gray on pc but became black on offset printing , is there any solution to see how appear grayscale images on printing ?
In order to appreciate how a printed job will compare to your production ink jet print, laser print, dye sub print, etc., you will have to ask your printer for a "contract proof." It will be an additional cost that should be included in the job quotation. It is false economy to cut this to save production expenses.
A contract proof is a legal document, a guarantee that a printed job will look as shown by the proof print or better. They are usually produced by an inkjet printer that has been calibrated to match the work of the printing press.
If a better, higher quality, higher fidelity proof of expected quality is needed, then a "Press" proof made from the actual printing plates on the press that will be used for the job is required.
It's probably because of the gamma correction of your PC and the dot gain on offset printing.
PC show images darker in general and have a gamma of 2.2 while Mac has a gamma of 1.8. So that's one reason why it may look darker on your display. Your printer probably uses Mac so everything is calibrated with a 2.2 gamma and their own machines, and they also use their own profile for the dot gain.
Then there is the dot gain. This is a way for printers to lighten up a bit the ink according to the way the paper will react to it. For example, when they print a black that is at 100%, they don't apply the ink at 100% because it would be too much ink for the paper and it would create a black spot and make the images look more blurry. The way they fix this is by transforming all the 100% ink value to something like 85% instead (for example.) It's the same for all the CMYK inks.
That may be why you see your grayscale much paler. Your display shows you a black that is probably not a real 100% black but a 90% one, and the printer's dot gain adjustment lowers this value again by 5-10-15%! So your blacks could be in fact printed at 70-80% of their density.
The best way to verify this is by looking at the darkest part of what you think is pure 100% black on your grayscale image and see if it's truly at 100%. If you see your display has mislead you and it's not really 100%, you should calibrate it.
If your ink percentage value is alright, then it's probably only the dot gain of your printer. This varies from printer to printer, so you would need to ask him what he's using as value. You can help yourself by setting these in your color profile and it should help you to see what to expect once the images will be printed.
One thing to be careful: In general, printing on an uncoated paper will look a bit darker because the paper is more porous and the ink will expand more. Preparing images for coated or uncoated paper requires a different approach for this reason.