I didn't have a heck of a lot of time, but if I did I probably would have used shutter priority to give a greater difference in aperture. Instead I had it on Program.
Kyle, I think an explaination of exposure and metering options might help you sort some things out.
Exposure is a simple equation: Aperture X Shutter Speed = Exposure.
The first devil in the detail is how we compute the needed exposure for an image. Each ISO sensitivity (or film speed) has an "ideal" amount of exposure (call it "E") to light to make an ideal image. Once we know the "E" value needed, we can adjust the aperture and shutter speed variables in the equation (within the limits of the camera and lens) until we know that enough light has hit the sensor/film to match the needed "E" value for the sensitivity/film speed.
The most sure fire way to compute the needed exposure needed is with a manual light meter that measures the amount of light falling on a scene (sun, room light, studio flashes, etc.). You'll see why it's the best method later on...
Seeing as most people don't want to lug around a light meter, people prefer to use the handy light meter built into their camera. And they do a pretty darn good job... most of the time. In-camera light meters measure "reflected" light that bounces off of the subject and they also aim for something known as "18% gray" (Actually, Thom Hogan argues that it's
actually 12% gray). Camera makers determined that the "average" photo in our world, if it were a finger painting that had its colors swirled together, would mix into a light shade of gray. The meter calculates the needed exposure to make the picture average out to that shade of gray.
The reflected nature of the in-camera meter, along with its 18% gray target, is the main short-coming of in-camera meters. If you have a scene that deviates from "average" (the classic examples are a black cat on a coal pile or a snowman in a blizzard) or has several different lighting situations in the frame, the camera meter will often fail you. This is why the two exposure settings in your two images are so different.
These differences will exist regardless of which meter sitting (other than manual) you are using. All auto-exposure settings work to meet the same exposure amount the meter determines the scene needs. What differs is in how the Aperture and Shutter Speed variables are adjusted to give you the same Exposure.
Aperture Priority: You set the aperture, the camera changes the shutter speed so the "proper" exposure it met. You'd primarily want to use this setting when "depth of field" is important to you.
Shutter Priority: The opposite, you pick the Shutter Speed, the camera sets the Aperture as it sees fit. You'd primarily want to use this setting when it's important to assure the shutter speed remains at a desired level (as in the case of sports).
"Program": There's lots of different types of "Program" or "Scene" modes. Basically, a program mode will allow both Apeture and Shutter Speed to vary (though making sure that Aperture X Shutter still equals the needed Exposure) based on what it thinks will work best. For example, if the camera sees that a shutter speed (based on the lens of the camera) would be too slow and likely to result in blur in the image, the camera will select a wider aperture in order to keep the shutter speeds up to an higher level. In a similar manner "sports" modes will select a higher shutter speed at the expense of depth of field.
So, all things being equal, Aperture Priority, Shutter Priority, and Program should give you the same "look" in a photo. The only thing that will differ is the depth of field and the shutter speed used. One shouldn't give you a lighter or darker look... unless it's something like a "snow" scene mode that adjusts the exposure compensation.
The differences in the light/dark nature of your photos is due to what you metered on. They sky is a whole lot brighter than the light reflecting off of the trees, therefore when you metered on the sky it attempted to make the sky "18% gray" and had to darken the rest of the image to attain that.
Also, keep in mind that scenic filters such as graduated filters can have an effect on your in-camera meter. The meter may attempt to "over-ride" the filter and lighten in image (again, with the "18% gray" target in mind). It's probably best to determine the proper exposure setting for an image, use that setting in manual, and then slip the filter on. The alternative would be to use the filter maker's "filter factor" for a given filter and dial that into the camera's exposure compensation setting. For example, see
Hoya Filter Factors
