So, in case I do free cropping to the image I really want and that image does not fulfill the aspect ratios of the printing lab, then I can't print those in the sizes I want or the lab prints? Or is there a way to "expand" or "shrink" that image to fit the standard print sizes?
Okay, first I said what you are supposed to do. Let me explain what I really do.
I really prefer the native aspect ratio of DSLRs. I try to fill the frame and not crop. Just something I've done for years.
In the 'old' days it was something of a point of pride among photographers to print all the way to the edge of the negative and show the black lines that came through from the clear edges of the film. The idea was to show that what you printed was the exact image you took, without need for cropping.
Anyway, I still usually prefer the dimensions of a uncropped image. Fortunately, these days most of the better photo labs (I use MPix) have a wide range of sizes in a variety of aspect ratios. For example, you can order a 12 x 18 print or a 15 x 24 print that comes very close to the original proportions of the DSLR frame.
I seldom even think about aspect ratio when I work on a picture. I crop it to what I think looks best and can usually find a print size that comes pretty close to that same ratio.
Now, here are a couple of other things to know or consider.
When cropping an image in photoshop, if you take the crop tool and drag it out to the edges of your image so that basically it outlines the edge with no cropping then you can hold down the shift key and grab any corner of the frame and pull it in without changing the ratio. (This "constraining" works with almost all the Photoshop tools by the way)
So, say you know you want a 12 x 18 print (same dimensions as the original frame) but want to crop the image a little, just drag one corner in while holding down the shift key. Don't worry that the crop isn't exactly where you want it. You can do that with each corner until you have it about where you want it. Then you can click inside the frame and drag the image around until it is exactly where you want it. Get it where you want it, hit enter and the image will be cropped.
Another option. Let's say none of the print size choices fit the ratio you really want. For example, you've shot a panoramic image that is like 8 inches tall and 24 inches wide. Create a new layer and fill it with white (use the paint bucket--be sure the white square is the foreground color in the two squares near the bottom of the tool bar. Use the little arrow to toggle back and forth between white and black and background and foreground) .
Move that layer to the bottom, so it is underneath your picture. Now, go to "image" and select Canvas size. Since 8 x 24 isn't a standard print size, pick a size that is. Say 15 x 24.
Now, both layers will be over a background (most likely transparent) that is the full 15 x 24 size. Select your white layer and resize it. (edit menu - transform-scale) to fill the full 15 x 24 size. Now, select the layer that has your picture on it and do any minor adjustments on it that might be needed to get it to fit within the 15 x 24 canvas. (use the same edit menu- transform-scale to resize your picture, holding down the shift key so you don't change the proportions of the image.
Basically, you'll have your picture on a white background with the background matching a standard picture size and your actual image the size you want it. Save a jpeg (which will flatten the image) and then you can order a print in the standard size and it will end up as a print with a white background. You'll have to matte it of course to fit your image, but this is a cheap way to get a custom print size from printers.
As I said in the earlier post, I do this a lot when I want a quick print from Walgreens. I create an 8x10 white fill layer and then paste and resize my image to fit inside the 8 x 10 frame (it's usually about 7 x 10) I'm trying to attach an example. It's not a great picture, but it might help you visualize what I mean.
You can then order prints from anywhere in the standard size (8 x 10) but the final print will be your uncropped or cropped image with white around it. Again, you'll have to matte it to fit a standard frame.