Assign Function key to do screen grab

Can I assign F19 (I have the extended wired keyboard on a Mini-Mac w/Mavericks) to replace the key combination:

Control + Command + Shift + 4 for cross hair selection tool

so that I can press one key to print a screen?

Not sure where the Ctrl key came from(although it works) but for the cross hairs it's just Cmd Shift 4. To grab the whole screen, it's Cmd Shift 3.

Can't help with the F19 key though.

The control key is not recognized and is extraneous in that combo.

edit:
That is wrong - adding the control key to the screen shot shortcuts copies it to the clipboard.

Apparently Did not make myself clear

I apologize that I have not made myself clear. I understand that I can use the three-finger combination to print a defined portion of the screen. What I am trying to do is to use a Function Key (in this case F19) to duplicate those three simultaneous strokes so that I can press just one key to perform the function.

Similarly, there are 18 other function keys that could be assigned other functions. They would not be there if there weren't some way to program them. My question really is how do I program any of these keys (step-by-step) and what can and can't I program into the key. Surely Apple has some instructions somewhere -- I just can't find them. Thanks in advance for any assistance.

System Preferences - Keyboard - Shortcuts - Screen Shots

highlight the item you want to change then click on the existing shortcut - press the key(s) you want to change it to

Also you might want to check the 'Use F-keys as standard function keys" box.
Lot of those keys are already programmed for cut, copy, paste etc.

Thanks

bobtomay said:

System Preferences - Keyboard - Shortcuts - Screen Shots

highlight the item you want to change then click on the existing shortcut - press the key(s) you want to change it to

Click to expand...

Thanks -- this worked perfectly!