What is photography’s greatest scourge? Cellphone cameras? MySpace self-portraiture? Neither even comes close to the insidious, creeping threat that is your camera’s built-in flash. Here’s when and how you should—and more importantly, shouldn’t—use a flash.

Avid photographers, you already know the score, and this isn’t a guide for you. Nor is it for the dude with the brand-new 5D Mk II with an external flash gun, or the weekend strobist. This is a reference to be passed around as a public service; a quick guide for the aquarium-flashing, face-flushing, baby-blinding friends and family you all know and tolerate love.

When You Shouldn’t

At Large Events
Every time I go to a nighttime sporting event or concert, I see hundreds of starry flickers coming from the stands. When I see them, I die a little inside. For your average point-and-shoot, the effective range of your built-in flash is about 15 feet. You might stretch this to 20 feet if you jack up your camera’s ISO settings to 800 (or God forbid 1600), but under no circumstances will your camera’s flash reach down to the field or stage.

Every little flash you see in the photo above represents a failed photo, unless the intention was to get a well-lit out-of-focus shot of the dude sitting two rows forward. Shooting artificially lit events may be hard, but letting your camera’s automatic flash have its way won’t help. Shut it down.

Through Glass
Walk into any aquarium for a classic flash infraction: Shooting through glass. People press their cameras up to the fish and everybody goes blind. This almost never works—ever notice that giant white explosion where the fish was supposed to be? We don’t have an aquarium in our office, so I put Kyle, our new intern, in a glass conference room for a similar effect. He now has a glowing orb for an eye. Thanks, flash.

Shooting Gadgets, or Anything With a Screen
This one may be a bit of a tech blogger pet peeve, but please, turn off the flash before taking pictures of your gear, especially if it has a screen. Even the brightest, matte-est screens act as flash mirrors, as do all manner of plastic and metal finishes. It’s nearly impossible to take a good photo of a gadget with your flash on, and there’s rarely a reason to: Gadget generally won’t move unless you tell them to, so find a way to stabilize your camera and treat your subject to a nice, loooong exposure. On point-and-shoots, this usually requires nothing more than manually turning off your flash and staying in auto mode—the camera will figure out the rest.

On Anything That Isn’t Moving
Know what I said about shooting gadgets? Honestly, it applies to all inanimate objects, and even animate objects, assuming you get get them to sit still enough. Set your camera on the table, prop yourself against a tree, make an improvised monopod out of a lamp—if your subject is still, the only person to blame for not turning off your flash is yourself.

On Humans
It’s not a hard rule, but it’s a good guideline: built-in flash units emit whitish xenon light, and generally make your subject look like a malnourished villager from medieval Europe, often with horrifying red pupils. If you can help it, avoid the flash. (If you can’t, we’ve got some tips below for making your shots look less ghostly.) Photo by Flickr user busbeytheelder

In a Baby’s Face
Because as adorable as this overdramatic baby is, flashing blindingly bright light into your newborn’s pupils seems like bad parenting. And babies don’t usually move too fast.

When You Should

In Daylight
Counterintuitively, one of the only times your camera’s built-in flash is genuinely useful is when it’s bright and sunny out, and you’ve got a shadow problem. Ideally you should try to illuminate a subject with natural light, but in the event that your photo is lit from behind or above, like this here cat, knocking out a few shadows is a reasonable excuse for using flash. Why? Because the mix of ambient and flash-bulb light is much less harsh than straight flash. Photo by Hoggheff aka Hank Ashby aka Mr. Freshtags

When It’s Totally Dark
Because you have no other choice.

How to Avoid It

Stabilize Your Camera
Keeping your camera still isn’t always easy. If carrying a tripod or Joby-style stabilizer isn’t an option, you can always do it yourself. From our piece on hacking together camera accessories on the cheap:

Shooting long exposures without something to prop your camera on is a pain in the ass, not to mention a blurry mess. So is carrying a tripod. This video shows how to build a pretty effective foot-looping camera stabilizer out of some string, a bolt and a washer. The results are surprisingly good.

And another! Here’s what I call the David Pogue Special, and it’s great: Many lampshade mounts share a diameter and thread size with the tripod mount screw on the bottom of your camcorder, point-and-shoot or DSLR, providing quick and dirty stabilization in a bind.

If You Absolutely Have To

Reduce the Flash’s Intensity
Many cameras will have a setting for flash intensity. Find it. This will essentially just turn down the brightness of your flash, which will avoid overexposing your subjects’ faces, albeit at the expense of range.

Improvise a Diffuser
External flash units turn out better photos because they have bigger, better bulbs, mostly, but also because they’re often fitted with a diffuser. These accessories soften your flash’s harsh glow, but they’re both expensive and generally impossible to fit onto your mom’s point-and-shoot.

Luckily, you can fashion them yourself, sometimes in a matter of seconds. Again, from the DIY camera accessory roundup:

A coffee filter held in front of a flash, a translucent film canister with a notch cut into it, a simple piece of A4 paper or even a piece of matte Scotch tape over the flash lens will measurably improve your drunk party photography.

Tricks like this tend to take a little trial and error, but you’ll love the results. Top image via SharperFocus

Still something you wanna know? Can’t get your brother to stop flashing himself in the mirror? Send questions, tips, addenda or complaints to tips@gizmodo.com, with “Giz Explains” in the subject line.