can someone help explain this to me?

alright guys so i've been doing some research on flash techniques and how to properly expose.. and i was reading here and i don't understand what this underexpose 2 stops means. i don't even know what the "2 stops" means. is that the f stop, or the exposure meter? here's the entry from where i was reading...

quoted from strobist.blogspot.com . .
"Bearing in mind your original ambient exposure was a 60th at f/4, you want to drop the ambient down, say, two stops. This will create your shadows - but with detail. Assuming your camera can synch at up to a 250th of a sec, you have several choices. You can stay at a 60th and go to f/8, for depth, but your flash will have to work a little harder to put out the extra light to support that aperture.

You can keep the aperture at f/4 and go to a 250th of a sec., which might be a good choice if you are powering with AA's and/or want faster recycle times for better chance at grabbing moments.

You could split the difference and go to 125th at f/5.6. Whatever. The idea is to build an ambient-light-only exposure that would result in an underexposure of 2 stops. That will be your lighting ratio. You can choose another ratio (and you should experiment) but 2 stops is a good starting point."

i don't understand how flash is in conjunction with the fstop, but i understand how it is with the shutter speed.
can someone help explain this to me? i really want to learn and have been reading nonstop about a lot of things, and maybe it's just that i'm cramming too much in at one time...

help?

Read this thread. Just remember light is light, whether it's ambient from the sun or created by the flash.

The thread Tim linked to will give you the basics of photography lingo and relative amounts of light.

Flash is immune to shutter speed because it's faster than the shutter. But changing aperture or ISO will change the flash exposure just like it changes the ambient exposure.

Strobist was describing a specific technique for taking a shot using flash and underexposing the ambient. He was pointing out that if you underexpose the ambient by using a smaller aperture, you'll need more flash power (four times as much) to compensate for it. But if you underexpose the ambient by using a faster shutter speed, no change in flash power will be required.

Curtis N wrote in post #5591712
Flash is immune to shutter speed because it's faster than the shutter. But changing aperture or ISO will change the flash exposure just like it changes the ambient exposure.

Ah of course! That's how we're able to catch another camera's flash in our photos (i.e. the duration of flash happened during the exposure which is relatively long)

ignore me. Just had a revelation

f/stop - Back in the day, camera lenses had click stops on them that caused the manually adjusted ring that changed the aperture to stop at certain points. Each one of these stops cut the light by 1/2 or doubled it, depending on which way the ring was turned. These click stops were always set at a particular ratio of the lens focal length to the size of the hole in the iris diaphragm, which is what changed to let more or less light into the lens. This ratio of the focal length to size of the hole was expressed as a fraction where f was the focal length and a number represented the size of the hole that let in the light. So, the numbers were f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, and so on. Each of those numbers lets in 1/2 as much light as the preceding number and they are known as f/stops. Shutter speeds share the same relationship, only it's easier to see that being open 1/250 of a second lets in half as much light as being open 1/125 of a second. The standardized shutter speeds that halved or doubled the light got to be 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, and so on. Don't ask me why it jumps a tiny bit between 1/60 and 1/125, but it appears to work and has been pretty much standard over the years. There were "stops" on the shutter speed dials as well. Changing the shutter speed by one of those increments changed the exposure just exactly the same amount as changing the f/stop by one increment. Those increments of light got to be known as stop, whether it was adjusted by aperture or shutter speed. So photographic light is referred to in "stops" and they are nothing more than the even steps that result in halving or doubling the light. You can open the lens up or close it down by however many stops, you can change the shutter speed by a however many stops, or you can measure the light coming from two sources and describe it as having so many stops difference.

Since I just wrote this down straight out of my old man head, I probably got something wrong, but I think you get the idea.