Be careful not to underestimate the influence of the ambient light while you're editing your photos. I've seen many people comment that when they edit photos at night, then look at them during the daytime (or vice versa) they look markedly different.
Of course, when you edit some photos (especially if you're tired), then look at them again a few hours or days later, you're always going to notice some things you'd like to do differently or better. That's human nature, I guess, and in a way it's a good thing - while you're perpetually critical of your own work, you'll keep striving for improvement. And as long as you do all your Photoshop editing with layers, it should be very easy to make whatever adjustments you need.
But I'm not here to pester you about layers again. I want to discuss light. The light that's around you while you're doing your post-processing.
It's gotta be bright enough
Some people say you should edit in the dark, or at least that the computer screen should be the brightest thing in the otherwise dim room. That's completely ridiculous. What kind of idiot stares for hours directly at the brightest light source in the room? How is that good for you? And what about if you need to jot something down, or read something you jotted down earlier? And how on earth are you supposed to compare your prints to your screen to see if your calibration is accurate? (Actually, the answer to that last question is: you're meant to buy a fancy viewing booth. I've got better things to spend my money on, frankly.)
Any time somebody asks me about light, I always give the same response: "I think you should only edit in light that you'd be happy for your kids to do their homework in." You wouldn't like them to be risking eye strain by reading or writing in too-low light, and I don't want you to risk it either. I hasten to add I have no scientific basis for this - it just seems sensible to me.
It's gotta be white enough
In Ask Damien I field many questions every week about photos in print not matching the photos on screen - most commonly: "My prints are too yellow!". Most people are quick to blame the lab, the calibrator, or the screen, but in the vast majority of these situations, it's the light. The prints aren't too yellow, they just appear too yellow because they're being viewed in too-warm light.
When the photographer changes their light bulbs, or views the prints in a different room with white light, or even takes them outside to some nice white daylight ... Voila! The prints look fine.
It's gotta be consistent
This is crucial. Your light has to be the same every time you edit. And that's not always an easy thing to achieve.
As it happens, I got incredibly lucky. The bloke who owned our house before us was a musician, and he built himself a windowless (and soundproof!) room downstairs. When selling, the poor old real estate agent must have been wondering "how the heck am I going to market a windowless room??". But it's perfect for me! I've got some nice white flourescent bulbs in the ceiling, and no matter when I'm editing - day or night - the light is exactly the same. It plays a huge role in the consistency of my work, and that's important.
Actually, the middle of the day isn't such a problem - not here in Queensland, anyway. That light is pretty "white". But it's the golden tones as the afternoon goes by, and turns into evening, that are problematic. Lovely for taking photos in, dreadful for editing in.
Not many people will be as lucky as me, I guess. Some of you might be able to find a windowless nook in your home, away from that nasty changing sunlight. But most of you will have windows to contend with. So you have to do the best you can.
Heavy curtains are the best answer, of course. Cardboard is also good, but doesn't flatter your house! Anyway, you've got to block out that outdoor light, and install some good indoor light.
The million-dollar question - what is good indoor light?
Lighting has become confusing in the last few years. Well, to me, anyway. The advent of LED lighting and "energy saving" bulbs threw everything we knew out the window, didn't it? You used to screw in a 100W white bulb and everything was dandy. Now ... gosh.
So I urge you to do what I did - go to a lighting shop. See light for yourself. With the kind permission of my local Beacon Lighting store, I took a photo of the wonderful little booths they have to display their downlights:
By looking at those booths on the wall, I was able to easily compare the light sources, and immediately rule out the ones that were too warm or too cool. It made my decision dead easy. (As it happened, I was choosing lights for our kitchen, not my office, but the nerdish satisfaction of viewing carrots at their exact correct colour can't be overstated!).
I know we live in an age of online shopping, but I urge you - put some pants on and leave the house for this purchase. Go and see the bulbs for yourself.
Some technical stuff
My lighting store guy was very generous with his advice. He told me that among modern bulbs (LED and fluro) we should probably look for temperature of 4000-4200K. Lower would be too yellow, and higher too blue. He said to be wary of descriptions such as "Daylight" and "Cool" as their meanings could vary between manufacturers.
He told me that the brightness was measured in Lumens, and usually we would look for bulbs in the 600-900 lumens range. But he hastened to explain that the brightness should be dictated by the size of your room and the height of your ceiling. Whereas a single 600lum bulb might be ample in a small alcove, it would be pitifully dim in a larger dining room, for example. (He loosely suggested a total of 1800lum for a standard 4x4m bedroom, from either two 900s or three 600s.)
Which leads me back to the previous point - go to a store. Talk to the professional there for the best advice. You won't regret it, I promise.
One more thing to mention:
Often people ask me what I think about laptops for editing. I don't like them. Not because of the laptops themselves (yes, some have really bad screens, but others have perfectly good screens), but because of the very thing that makes them attractive - their portability. If you've got a laptop, you're tempted to edit on the couch, or in the hammock, or wherever. Please be careful about this. Give a thought to the light you're in, and what impact it might be having on your edits.