If by "converting to jpeg" you mean saving the file as a jpeg, then that should be the absolute last thing you do. When you save as a jpeg, the computer compresses the file to make it smaller. In doing so, it discards some information. If you save the file once as a jpeg, and then reopen it, you won't see any difference. But if you save it as a jpeg, open it later and make some changes, resave it as a jpeg... etc, pretty soon you will have discarded enough information that it becomes noticable. The general rule I live by is to save it as a psd (photoshop) file until you're done with all your changes, THEN save the final file as a jpeg.
Now, as to the sharpening, you'll want to resize the image before you sharpen it. The next thing you want to do is determine the screen resolution and view percentage to use. My formula is to take the width of the screen in inches, and multiply by 75 (I'll expalin the 'why' of 75 in a minute). In the case of your 15 inch monitor, that gives us 1125. Next, we want to search the available resolutions for the one closest to 1125 pixels wide. My monitor has a 1152x720 resolution available, and that's very close, so I'd recommend that one if you have it. Now open the file in Photoshop, and set the view to 25%.
Now, let's examine why we've set it up this way. On a 15 inch monitor, a width resolution of 1152 pixels gives us a screen resolution of 76.8 pixels/inch (as close as we can possibly come to our 75 pixels/inch target. At 25% view, we're seeing one pixel out of every four (that's how Photoshop gets the image to appear that size on the screen. So the
effective resolution of the image on the screen is 76.8 times 4, or 307.2 pixels/inch.
Remember now, the native resolution of most printers is around 300 pixels/inch. This resolution, at this view percentage, gets the onscreen image as close as possible to that resolution. Therefore, the image you see onscreen is VERY close to what the print will look like. Now when you sharpen, you can see how the settings will affect the print.
We have one more obstacle to overcome. Photoshop, in sharpening an image, applies the most sharpening to the areas with the highest contrast, and the least sharpening to the areas with the lowest contrast. That is the exact opposite of the outcome we want. Here's how to fix it:
You already have the image open. Duplicate the background layer (Layer> New> Layer Via Copy or (cmd/ctrl+J)). highlight the top layer, and go to Layer> Layer Style> Blending Options. At the bottom of the dialog box, you'll see the "blend if:" sliders. Use these settings:
Now when we do our sharpening, the darkest edges (below 40) and lightest edges (above 215) will receive no sharpening. Between 40 and 65, and between 190 and 215, the sharpening will be graduated. Between 65 and 190, the sharpening will have full effect. This concentrates the sharpening in the midtones, which is exactly what we want.
All that's left is to do the sharpening itself. Make sure your working on the top layer, and go to Filter> Sharpen> Unsharp Mask. This command has three inputs: Amount, Radius, and Threshold. Threshold is about useless- there are much beter ways to control where the sharpening is applied, as illustrated above. I generally set the threshold to 1 pixel and leave it there.
Of the two controls that are left, Amount controls how much contrast is applied, and Radius controls how far back from the edge to apply the contrast. If you make the radius too wide, it can be seen by the eye on the print, and that's not good. If you don't make the radius wide enough, you don't get the maximum sharpness from the print.
Here's one of those important tips that I'm going to ask you to accept on faith: extensive testing (mostly trial-and-error) has shown me that sharpening halos that are 1/100th of an inch wide provide the best overall sharpening, and are still small enough not to be seen. When I sharpen, that's my goal: 1/100th of an inch. That's why it's SO important to take the printer's native resolution into account- so that the printer neither enlarges nor reduces the sharpening halos. That's why we resized the images to 300 ppi. At 300 ppi, a 3 pixel wide radius will give us what we want. Set the Radius value to 3 pixels.
The rest is easy. Adjust the Amount slider while watching the onscreen image. Make it sharp without looking blocky or pixelated. Remember, the image on the screen very closely relates to what the print will look like. When you're done, merge the layers and save.
In your case Dan, the printer is going to reduce some of your images to get the print size you want. This means that it's also going to reduce the sharpening halos, and the image will lose some of its sharpness. But they will still look considerably better than they would have if you hadn't done any sharpening.
I know that this has been long and fairly technical, but I hope it has helped explain why I did things the way that I did. Any questions? LOL.
-Medley.
There are only 10 types of people in this world- those who understand binary, and those who don't.
