"Are the F stops on a digital SLR the same as on the old SLR cameras? Or are we supposed to be using ISO? I only hear that someone "shot it with xxxx ISO" and not the F stop"
Quick and easy answer is...yes. The f stop means how wide open the lens is, so that hasn't changed. Yes, you are supposed to be using ISO too, as well as shutter speed...it all works together.
Now for the not quite so quick, though hopefully still fairly easy answer.
I think of f-stops as the light and depth setting, shutter speeds as the light and motion setting, and ISO is the light and noise setting. Notice something they all have in common? They ALL help control the amount of light that gets into the camera. They are all tied together.
ISO is actually easy. The smaller the number the less light your camera gets and the less noise your picture has. The larger the number the more light your camera gets and the more noise your picture has. Ok, so that is the VERY basic explanation of the ISO setting. If you are the math nerd type and want to know all the technical bits...you can check out this
and I'm sure there are many more sites with loads of technical stuff on them. For me though, that looks too much like work, and I honestly don't care what ISO stands for or the logarithms that went into determining how they got the numbers. I am just happy with knowing how they work on my camera.
I try to start every picture with ISO 100, and I up it if the other settings I am using just won't allow enough light for me to take the picture.
Shutter speed is also easy. I know it wasn't asked about, but the three settings really are tied together so much, I have to talk about it for just a bit. If you want to stop motion, use a faster shutter speed, if you want to show motion, use a slower one. The slower it is, the more light it lets into your camera, but the more chance for motion blur you have. It is always a good idea for a photographer to get used to how slow they can hand-hold their camera without getting blur. Yep, your breathing and even your heartbeat can cause motion blur (as can wind, traffic on a bridge, earthquakes, landslides, horseback riding, motocross...umm...where was I?). I can hand-hold safely to 1/125, ie., 1/125th of a second, and if I hold my breath I can usually manage at 1/100. Slower than that and I need either a flash or a tri-pod. I'm wobbly though, but shooting kids since they move, unless you are using a flash, it is generally recommended not shooting below that range anyway. Obviously some animals require even faster speeds. There are some people who are lucky freaks of nature and, without a flash, can get sharp pictures at 1/80 or even 1/60...but we don't talk about them, it isn't fair.
If you don't have to worry about blur, or WANT blur (milky water on a stream, showing a car's lights looking like a red streak on a road), you can use a long shutter speed which allows more light in and can give you more leeway with the other two settings.
Now the setting that I think is generally the most confusing for people...the f-stop.
First, I did painting and other art forms before I got into photography. So...think of this before I jump into the pseudo-technical aspect of f-stops.
You are taken to a canvas, a mad painter hands you two brushes, one big and fat and one teeny. He looks at you with hysteria, the one eye you can see underneath his lopsided beret rolling wildly. He tells you that you have to finish his painting in 1 minute. He's got the foreground in, you just have to finish the background. So...to paint in a hurry what do you do? You grab the big brush and slop that background in. No detail, but you've got something there and with a crazy breathing down your neck...who needs detail?! If you had all the time in the world and no insane painter you could use the little brush and show all the detail as far as the eye can see.
Yes, I do have a point.
The reason the f-stop can confuse people so much is the number seems counter-intuitive. The smaller it is, the more light it lets in. Wha?! How did they come up with that? Well, they thought they'd be obnoxious and confuse everyone. The f-stop is actually a fraction. You may see 1.8, 5.6, or 22 but what it actually is though, is 1/1.8, 1/5.6 and 1/22. Freaky, the bigger number is actually the smaller number...I feel like I just did some magic.
It has to do with how wide the lens is open. Like with the ISO there is tons of technical stuff out there and it is nice to look into, but my brain fizzles out quickly so I don't bother with it. Remember though, the f-stop not only controls light, but DOF (depth of field) which is how much of the picture is in focus. Ha, I wasn't just off on some random tangent...here is where the brushes come in! The wide open lens is like painting the background with a huge fat brush and the "stopped down" lens is like using a little brush. With the same lighting conditions and the same ISO...you can get a picture faster (ie., faster shutter speed) with a wide open lens than you can with a closed down lens, and with the faster picture you'll have less of the background in focus than you would with the slower picture. So, little f-stop number can mean a faster shutter speed but has a shallower DOF where as a bigger f-stop number (forget you ever heard that they were fractions and pretend you don't know it means how wide open the lens is) means greater DOF but you'll need a longer shutter speed.
DOF is actually affected though by distance from a subject too. Try taking a close-up picture of someone at 1.8 and you'll see that while they have one eye in focus, the tip of their nose sure won't be, and probably even their other eye won't be as sharp either...yet put that same person, with the same settings 20 feet from you and their whole face will be in focus. That is just more to confuse us, so I wouldn't worry about it till you are comfortable with the basic idea of the three settings...or like even more math!
It truly is all about compromise. Pick what you NEED for your picture, be it fast/slow shutter speed to get the motion right, shallow/deep DOF to isolate the subject or give you a feeling of space, or is your biggest problem that you can't stand noise? I think with most people the ISO is going to be the least important, can't adjust no matter what sort of setting and gets upped if the others will have to compromise too much.
If you have to shoot at ISO 800 so that you can get a picture of your child on a horse for the first time in a stable where you aren't allowed to use flash...go for it and who cares about the noise?! If you want to stop motion completely to get the hummingbird's wings frozen, open your lens up and bump up the ISO so that you get all the light possible from those two settings allowing for the camera to only need a micro-second to take the shot. If you want to get that landscape picture clear throughout, stop down the lens, pick the biggest number you can, go with as low an ISO as you can and adjust the shutter speed to get the exposure right.
Oh, and to be honest...I think a lot of the reason you might hear "I shot at ISO 1600" or whatever, is because they are proud of how little noise they have in their picture!