Of everything I get asked about with printing, why do my prints come out dark is the one that comes up more than anything else, and if it's happening to you then you haven't done anything daft. You've more than likely just taken the same advice that I did before I figured this out.
Why it happens
Your screen is backlit. It's basically a light source, the same as a lamp is, and a print isn't. All a print can do is bounce back whatever light is already in the room, so you've got one thing making light and one thing borrowing it, and those two are never going to look exactly the same.
However, here's the bit that catches everybody out. If your screen is throwing out more light than a sheet of paper could ever hand back to you, something happens while you're editing that you won't notice at the time. The picture looks bright because everything on the screen looks bright, so you bring it down a little, and what you've actually done is make that adjustment based on the screen rather than the picture. Then you send it to the printer and the paper has got no way of lifting it back up again, so you get the version you darkened on a surface that can't do anything about it.
The number
What you'll normally hear is to set the brightness in your calibration software to somewhere between 80 and 120 candela, and more often than not you'll hear 120. Sure, 120 looks great. It's bright and everything on it has a bit of life to it, but that is exactly what's wrong with it. It looks great because it's throwing out far more light than a sheet of paper is ever going to give back to you, and all the time it's sat there looking great you're quietly pulling down pictures that were never too bright to begin with.
I'm not saying everybody else has got it wrong. I'm saying I took that recommendation and my prints were always too dark.
The histogram is the only thing telling the truth
When your screen is calibrated too bright it's kind of lying to you, and the histogram is the only thing you can really trust. Change your screen brightness and to your eye the picture completely changes, but the histogram doesn't move, not one bit, because nothing has changed in the actual file. It's the one thing in this whole process that doesn't care how bright your monitor is.
The settings I use
After a lot of trial and error, and I do mean a lot, along with a fair bit of wasted paper, ink and money, I settled at 60 candela for my screen in my workspace, and I've not had a print come out too dark since. The software you calibrate with may well look different to mine, but the numbers transfer straight across:
luminance 60 cd/m2
white point D65
gamut Adobe RGB
gamma 2.2
black point 0.5 cd/m2
Most calibration software will offer you presets, and the photography one will typically drop the luminance in at 120, which is the whole problem, so I ignore the presets and put the numbers into the custom target instead. The black point is worth a word too. By default it'll be sat at absolute zero, and I lift mine just a touch to 0.5, because even in the darkest parts of a picture I still want a bit of detail; at absolute zero it's just completely black.
I'm not saying 60 is the right number for everybody either, because the room you're working in comes into it.
The room your screen is sat in
Your eyes don't judge a screen on its own, they judge it against everything else around it, so a bright screen in a dark room appears brighter than it actually is, you darken more, and it's the same problem again only now the room is making it worse.
Two things matter. Enough light in the room, and light that's fairly neutral rather than a warm domestic bulb, because a warm bulb will have you making decisions about colour and that's a whole different problem. In my office the main light stays off and I use a Screen Bar Halo 2 from BenQ instead, which throws light down onto the desk without any of it hitting the screen and puts a glow on the wall behind the monitor. That glow on the wall is the bit that does the work, because it cuts out the extreme contrast of sitting in a dark room with just a screen on.
If you haven't got a calibrator
You can still get most of the way there for nothing. Take a print you've already made, hold it up next to your screen, and bring the screen's brightness down until the two of them look pretty much the same. That's it. It won't fix your colour and it isn't calibration, but it's a down and dirty way to stop your prints coming out too dark.
Whichever way you go at it, going from 100 down to 60 or thereabouts will look far too dark at first, and it is quite a jump, but just let your eyes adjust and they will after a few minutes, and very quickly it'll seem normal.
