ISO

The third and last key variable that determines exposure is ISO, which determines how sensitive the sensor is to light. A high number means high sensitivity, which can allow you to take pictures in dim conditions but typically causes pictures to be "grainy".

If you use "full auto" mode (the green square), you don't need to sweat ISO, as the camera will choose it for you. If you're in the "creative zone", you generally want to take pictures with the lowest ISO setting possible given the shooting conditions. As I've said before, I usually use "full auto" or Av mode. When I use Av mode, I set the ISO based on these guidelines.

- In bright sunlight, I use ISO 100 to ensure the sharpest image possible.
- If it's a little cloudy, I'll bump down to ISO 200. This minimizes the chance that pictures will be blurry
- Inside, especially if I'm taking pictures close up, I tend to use ISO 400. This is a good utility setting and, in my experience, the one most often selected by the camera in "full auto" mode.

My Canon Rebel XT also has ISO settings of 800 and 1600. I almost never use these as the pictures tend to be really grainy. If I want to take shots at night, I either use a flash or shoot long exposures. I'll write more on long exposures later.

What can go wrong if you choose the wrong ISO setting? Well, if it's too low and you're using one of the modes in which the camera chooses the shutter speed, your pictures may be blurry as the camera will choose a slow shutter speed to ensure the enought light hits the sensor to capture an image. If you use too high an ISO setting, pictures can become overexposed (too light) because the camera can't open and close the shutter fast enough. As mentioned previously, a high ISO setting can also cause grainy photos.

Shutter speed...

Shutter speed controls how long light is allowed to enter the camera (and hit the sensor) to form an image. The term refers to a moving part inside the camera called a shutter. Think of this as a little door or gate that keeps light entering via the aperture from hitting the sensor. When you press the shutter button (the button you press to take pictures), this little door opens (very, very quickly usually) and an image is captured on the sensor. For more information than you're likely to ever need on shutters, click here.

I usually think about shutter speed in two situations:
1. At night to take long exposures (an advanced topic I'll cover later)
2. To manipulate how much blur or crispness I capture in a picture that contains moving stuff, e.g., people, water

That having been said, 95% of the time I don't think about shutter speed at all. I use either "full auto" or "Av" modes so the camera chooses the shutter speed. In my experience, it is much better at this than I.

Anyway, most of the time you'll want the shutter to open for a fraction of a second. You can play with shutter speed a little to see the effects you get. I think the easiest way to do this is to use full manual mode. If you're new to photography, I would recommend using full manual mode to play around only. As its name implies, in this mode you control everything. I've taken easily over 100,000 shots and can still screw this up! More on full manual in a future post.

By the way, when I first started understanding the key variables that effect exposure, I used full manual ("M") mode a lot. I thought it made me some kind of hardcore photographer. All it really made me was a stress case with a lot of bad pictures. I now use full manual on very, very rare occasions as the Rebel XT is really good at choosing the right shutter speed (see above). I would assume most amateur photographers are in the same boat.

Aperture...

An aperture is a hole. In terms of your camera, the relevant hole is the one that lets light from the lens into the camera and onto the sensor where images are captured. I use aperture in two ways:

1. To let more light in when I'm shooting in dark conditions, e.g., at night
2. To control how deeply object are focused in my pictures.

The first one is simple enough: choose a low aperture number (also called "f number") so more light gets to the sensor. I explain how to set aperture further down in this post.

The second one takes a bit more explanation (but only a bit). With a low aperture number, you can focus on something in the foreground, like Grandma, and the background will be out of focus so Grandma really stands out. With a high aperture number, things close to you and far away from you will be in focus in your pictures. Use this one when you're shooting landscapes. Smarty pants like to refer to how deeply things are in focus as "depth of field". Google this phrase for more info.

See below the same picture taken with a low aperture number (blurry background) and a high aperture number (more stuff in background in focus). The lights on the tree in the background are particularly illustrative.

To manipulate the aperture number, you have to use one of the "creative zones" on your Canon DSLR. On my Rebel XT, I use Av mode. Don't ask me why (yet), just use it if you want to play with aperture! By the way, in this mode you control the aperture number with the "wheel" on top of the camera (in the picture below, in the lower right behind the shutter button). The aperture number shows up in the display on the back of your camera, probably to the left of the number showing how many pictures you can take. Something like this: 3.5 [311]

3.5 would be a low aperture number. Spin the wheel (in Av mode) and you'll see this number change from 3.5 to around 22 (on my Rebel XT anyway).
BTW, I recently had a friend who was having trouble with his DSLR. It seems only the right side of the pictures he was taking was in focus; the left, not so much. I discovered that the problem was really that the aperture was set to a very low number. This meant that, for whatever reason, the camera was focusing on stuff on the right and everything closer to and further away from its focus point was fuzzy.

Exposure made simple...

Exposure refers to how light hits the sensor in your camera (and thus impacts the appearance of your pictures). There are three variables that determine exposure. I'll give you a summary here and just a bit more detail in separate posts.

Aperture
This setting is most often used to keep stuff up close in focus while the background is blurry (or out of focus). You often concentrate on aperture if you're taking portraits (to make the background blurry) or shooting landscapes (to keep things near and far in focus).

Shutter speed
This setting has to do with how long light is allowed to hit the sensor. It is typically measured in fractions of a second. Typically, the higher the number, the less time light is allowed to hit the sensor. In a bright environment, a high shutter speed keeps pictures from getting overexposed or "washed out".

ISO
This setting determines how sensitive the sensor is to light. A high number makes the sensor very sensitive. This may sound like a good thing, but can result in overexposed pictures and/or "grainy" pictures (pictures that look muddled, or unclear). The idea is to set the right sensitivity for a given set of conditions.

Stepping into the creative zone...

Sooner or later you'll likely want to start leveraging some of the power of your DSLR. That's when you start using zones on the mode dial that Canon calls "creative". Here are a few reasons while stepping outside the "green box" (all auto zone) can be worthwhile:

- taking pictures when the subject is backlit (by the sun etc.)
- taking pictures in conditions when "full auto" seems to be over or underexposing pictures (making the too light or too dark)
- controling how "deep" the focus is in your pictures (are things up close the only ones in focus or are things near and far in focus).

Before you start playing in this zone, you need to understand the basics of exposure.

Editing software...

The internet and your local electronics/computer retailer are full of software that will allow you to edit and organize your pictures. As you talk to friends and google around, you'll hear Photoshop mentioned a lot. This is a great editing package but, as of this writing (Dec. 23, 2007), is way too complicated for most "normal" folks. I use Microsoft Digital Image, a great combination of easy-to-use and advanced features. Unfortunately, the ne'erdowells in Redmond have discontinued it. If you can still find a copy, I highly recommend it for getting started with editing pictures.

If you know of good, simple editing software, please post a comment and let folks know.

Composition

Composition is a fancy-schmancy word for how you decide what shows up in your pictures. The internet is rife with advice on the topic so I won't bore you with the details (I'll let the folks who created this content do that). I have learned a few simple lessons though that made my pictures better:

You can use software to fix pictures that have bad composition etc. I'll cover some of this later, but don't be too disappointed if a picture isn't exactly what you want. With enough patience, you can fix a lot of errors.

Rule of thirds: Google this yourself (or click the link) and read about it. To make it simple, don't always stick your subject(s) in the middle of the frame (this is a natural tendency that leads to boring pictures).

Implied motion: If the subject of the picture is in motion, leave a bigger gap ahead of it than behind it (and don't stick it in the middle). For whatever reason, this technique communicates motion to the person viewing the picture.

Get down to the level of kids to take their pictures. I'm a pretty tall guy and was taking too many pictures of scalps in the beginning.

Don't be afraid to get close for portrait shots. Some of my favorite pictures of my kids are taken from a foot away or so. Their whole head doesn't always fit in the picture!