a few thoughts on aperture...
wait... a few thoughts on LENSES first. Lenses first, because the type of lens you have decides how open you can go with aperture. There are 3 types of lenses out there, and I will try to keep my bias out of this.
1- kit lenses (these are the lenses that comes with a camera kit) i.e. purchase the Nikon D300 or D90 at your local store and it will have the option of being bundled with a lens that offers a fairly wide range of zoom and a variable f-stop (3.5-5.6). At wider angles this lens offers f3.5 but as you zoom in, the f stop is only capably of 5.6 as the largest aperture. Usually the focus motor on these kit lenses is par (not real fast and sometimes prone to wear out before the camera body does)
2- professional lenses - "fast-glass" As far as I can tell, they are called fast for 2 reasons... because they are f2.8, you have a larger aperture and allow more light in. That allows for faster shutter speeds at lower lighting conditions. Secondly, they have a faster focus motor in them. In addition, they are generally made more robust than the kit lenses. These lenses usually cost about 3 to 5 times more than the kit lenses.
3- third party lenses (like Sigma and Tamron) If you are seeking fast glass but can't afford the name brands (like Nikon or Canon) these can offer a comparable alternative. They can be had for about 1/2 to 1/3 of the price of the fast glass.
So with that out of the way...let's move into APERTURE.
Dept of focus is often a sign of a professional lens versus a kit lens, (and coincidingly, a professional looking picture and a point and shoot (PAS) look)
Question: How do I get a smaller dept of field?
3 ways - wide aperture (like f1.4 or 2.8), zoom lens, and put your subject far from their background. I don't have time to go into the details of why, but check this out for the mathematically-inclined or this site or this site for a lot of detail or this site for the basics
An example... I used all of the previous techniques in this shot... f stop of 4 (I believe), 300mm (450mm on my D50 body), and background quite far from subjects (like 50 feet or more)
What f-stop should I use for portraits?
Most of the time I use between 4 and 5.6. If you go too big on the aperture, your front row will be in focus while your back row is out of focus. Also, remember to have all of the people in your portrait about the same distance from the camera. If the group is large (8 or more people), you probably will do better to bring the end people closer (like a semi-circle) so that all the people are the same distance from the camera. This allows for the photographer to keep a 4 or 5 f-stop and maintain good lighting.
Another reasons for 4 to 5.6 is that they bring in more light than the f8-f11 and allow for faster shutter speed (less blur from movement in your subjects is pretty critical).
Also, if you are using artificial light (flashes AKA "strobes") they have only so much power to put out. As you decrease your aperture (go toward 22) your flash has to work harder and harder to give you the same amount of light. This is another discussion for another day, but the short answer is that your flash output is directly related only to your f-stop, not to your shutter speed.
When would I use Aperture priority (A) or Manual (M) mode instead of shutter priority (S)
A: when you are concerned with depth of focus, usually close-ups or portraits
Times when you would not use aperture priority is landscapes or on a vacation where you want your subject and your landscape both in focus.
Or you probably would not choose (A) when there is a lot of action because it can force the shutter speed to be slower if lighting is low
Pick of the day: found on flickr
f/22 46mm ISO 200
I love the composition of this shot. Love the framing of the rider with the horns. I like the rider looking to the side.
There are a couple of changes I would have liked to see with this shot. The sky is obviously blown out (stark white) and the brightness and focus of the mountains behind the rider are drawing attention away from the rider.
It appears that the rider was originally quite shadowed, so they hit this image with the fill light feature in Photoshop or Bridge (you can see the graininess of the rider, which shouldn't be the case with ISO 200) . Furthermore, I have noticed that fill light gives some detail back to the shadow areas (the subject) but doesn't give the subject the due attention that additional light would.
So, at your own risk of ticking off this bull with a big reflector or a flash: big silver/gold reflector lower left or an off-camera flash lower left would really help the subject stand out more. If I used a flash I would probably underexpose by a couple of stops and set my flash on plus a couple stops (this will be it's own blog). But the intention here would be to expose more correctly for the sky and decrease the attention given to the bright hills behind the subject. (Yet another blog will be on syncing your flash above shutter speed of 250). Simplified, just use a fill flash on this subject and call it good.