Some Technical Questions

[Oblivion]

I've got a few semi-technical questions here, and if it makes any difference at all I've got a Canon Rebel XS.

1. From what I've understood, you ONLY use higher ISO for taking pictures in lower light, but it seems like sometimes people use higher ISO (200-400) in pictures with ample light. What else does ISO effect? I've heard that it changes the effect when using a flash... and also some depth of field stuff, but have never been clear.

2. Along those same lines with depth of field (DOF), is there any way to change it other than specific lens and F-stop? I recently got a 50mm 1.8 and it takes amazing pictures, but for portraits (with more than one person) the DOF is pretty extreme, often making subjects directly behind the focal point look blurry. I imagine with a prime lens there isn't much you can do, but if I remember right there was a formula for DOF when using a zoom lens.

3. And finally: White balance. I have been previously keeping my white balance on auto, but after messing with the presets the white balance seems to be a pretty powerful tool. Is it a good idea to set custom white balance during important shoots (or just in general)? The presets looked pretty extreme, so I couldn't see myself using those much.

Thanks for the help guys =)
-Mark

[Anbesol]

1. ISO just changes the speed, i.e. the rate at which the sensor perceives light and color. So, a shot at 1/60th - ISO 100 would look equally exposed to 1/120th - ISO 200. Each ISO rating is a stop different. It works in doubles, so its 100-200, 200-400, 400-800 and so on. Personally even though ISO 100 offers the best quality, I stick more often around 200, the difference in todays age with image grain is miniscule and that extra stop advantage sure is helpful.

2. controlling distance from focus point, whether with your legs or with the lens, the further you are from the subject/focal point, the wider dof is. With that 50mm f/1.8, you should expect that very narrow dof in most workable range, in close ups you should even expect the tip of the nose to be the focal point, then focus falloff starting at the side of the face/side burns. The soft effect is really cool when used properly. There is also plenty you can do with a prime lens... stop it down.

3. Always custom, all other wb options are pointless (except the flash wb, that can actually be useful, auto white balance on a lazy day). Custom white balance is very easy, set to 'customize', then point at something white in the light and shoot, presto bango - it should say something like 'white balance at 6300k' plus a filter set (depending on gear). If you are shooting JPEGs, custom white balance becomes critically important, make a habit of setting the white balance, its very easy. Auto white balance usually does a good job in outdoor light, but it will constantly have white balance issues indoors in tungsten/incandescent/fluorescent house lighting.

[Frog]

Just because your 50mm has a 1.8 aperture, doesn't mean you have to use it.
Increase dof by stepping down the aperture but it still depends on how close you are.
I always shoot auto white balance because it is so easy to change in photoshop if you shoot raw format.

[brmill26]

This may help you:
http://www.kenrockwell.com/tech/shutteraperture.htm

Exposure is basically a 3 variable process - ISO, shutter speed, and aperture. As Anbesol said, ISO controls sensitivity to light. Each level you go up is one stop more sensitive. In other words, ISO200 exposes twice as fast as ISO100. ISO400 exposes four times as fast as ISO100. Assuming you're in daylight, you would only want to go to a higher ISO if you need faster shutter speeds (such as sports) or if you wanted a high F/stop, such as for landscapes.

So, since ISO controls sensitivity to light, it contributes to how your flash exposes. Your flash can only be so strong, but if you make the camera more sensitive to light by increasing ISO, it has a similar effect to using a stronger flash. Thus, you can either get more distance out of your flash coverage, or in dimly lit areas, higher ISOs will bring up the level of ambient light to you don't get that "flash in a dark room" look.

DoF is related to focus distance, but it is primarily controlled by aperture (both have an effect, but aperture has a much greater effect in normal shooting distances). Higher aperture values give you greater DoF. With your group portraits, for example, you should be able to get everyone in focus at F/4, or if not then, by F/5.6.

White balance is a hugely important tool. You can mess with it shot to shot, or if you shoot RAW, I find it's much easier to simply change it in post processing.

[Oblivion]

Okay, thank you very much guys =D. All of your answers were extremely helpful. I am now off to experiment with my new found knowledge!

-Mark