they react the same as wide-angle lenses. And that is true.
However, one of the ways I think about lenses is that the view-angle is narrower for a telephoto lens than for a wide angle lens. For a 250 mm lens (as was used for the photo of the bridge), the angle is fairly small. People interpret the size based of an object partially based on how much of the image the object takes up.
Normally, a 55 mm lens (for 35mm film cameras) is around what the eye sees. When we look at an image taken with a 250 mm lens, the difference in how much an object takes up changes. Because we know how big the bridge is, we interpret the pelicans as proportionally larger when in fact they are just closer.
The one thing that cannot be changed is atmospheric changes due to distance. A 250 mm lens will still have as much atmosphere (fog in this case) between it and the bridge as a 55 mm lens. That helps provide context to the sizes, making us know that the pelicans are much closer to us than the bridge. It's also one of the reasons that photos of the astronauts in space look so strange, we lose some context.
All of this is good to know, but the real goal is to take good photos. Just as a warning, although the Canon 7d allows you to manually set the aperture and shutter speed while it handles the ISO (in auto mode) for perfect exposure, it won't let you under or over-expose it by using the dial. It will let you bracket though.
Tuesday, July 6, 2010
These three images are taken from a sequence I took with the new Canon 7D. There are 7 totals images, all taken within 1 second showing the redtail hawk launching off of the lamp post.
One of the problems I had with my old camera (a Canon Digital Rebel XT) was getting exposure right. You could tell the camera to be in all-manual mode, shutter preferred mode, or aperture preferred mode. The ISO was only set by hand.
On occasion I wanted an additional auto-adjustment. The Canon 7D also has an auto ISO adjustment which is very close. I set the shutter speed and aperture, and the camera figures out the ISO needed to get the shot. Given the wide range of ISO (100- 6400) that allows for a very broad range of exposure. This particular series was ISO 640, which would be very noisy on the Rebel XT but quite reasonable here.
Especially if you use a technique I read about (sorry, I can't remember where) where you stack multiple layers of images and average across the layers. I haven't had a chance to play with this yet, but it looks really intriguing.