The ergodox is a great community project, and it’s awesome to get people building their own keyboards, but to be honest I find the thumb section really annoying, and I gave up on it after several weeks of use. It has however inspired me to figure out how to make my own custom keyboard with a new layout.
It has several advantages over a standard keyboard:
(1) It can be split, with the two sides independently rotated and tented
(2) It has a column-based rather than row-based stagger, which, once you spend a couple weeks learning it, makes it easier to reach keys
(3) It has at least 2 usable thumb keys for each hand (the other 4 thumb keys are stupidly placed, but at least don’t do harm)
(4) It has fully programmable firmware, so you can set it up to work any way you like
It also has some disadvantages:
(1) It’s not the most portable – it takes 2 pieces, and you might want some kind of palm rest and/or support for tenting, so you can’t carry it around as easily as e.g. a 60% board
(2) It takes at least a couple weeks to get fully up to speed with the new layout
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With respect to Photoshop or similar apps, specifically: you can very easily set up an arbitrary mapping of key presses to output, so you can make a special "photohop" mode where all your common shortcuts are on very easy 1 or 2 key shortcuts of your choosing, and you can do it in firmware on the keyboard, so you don't have to change the global settings within photoshop on every computer you use, or make a standard keyboard stop behaving the way it always does.
As long as you’re willing to put the time in to figure out what you want the keyboard shortcut setup to look like, a fully programmable keyboard like the ergodox is a dream for pro apps like Photoshop (or CAD software, or music/video software, or complex computer games, or whatever).
Likewise, to answer your other question: you don’t at all need a separate numpad. Just program the numpad into the right side of the ergodox, on a layer, and you’ll be all set.
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There are a few upcoming keyboards that you might also want to consider. The
http://www.keyboard.io people are making a keyboard that I think is going to be really great (though I’m not sure how soon it will ship). Matias’s ErgoPro
http://matias.ca/ergopro/pc/ is a bit closer to a standard keyboard layout, and less programmable, but should be pretty nice (and does at least have a split spacebar with one half usable as backspace).