Ptyler22
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I wanted to know the same thing before I bought my 40d so I searched it. This article is about the 40d but the XSI will be exactly the same. I Copied this on Consumer reports. Hopefully I am not breaking and rules by copying and pasting this.
Sensor Size and Lens Magnification Factor
Although there are many new or improved features on this SLR, one element that remains consistent with its predecessor, the 30D, is that it still has the same size (APS-like) sensor.
That's important because that sensor is smaller than a 35mm film slide (which is the size of a full-frame sensor). In the camera body, that smaller sensor format effectively "crops" the image produced by the lens. To you, the effect is to magnify the image or increase the telephoto capability of your lens.
So, on the 40D, which has an APS-like sensor and a lens magnification factor of 1.6x, a 100mm lens becomes equivalent to a 160mm lens.
This is good news for nature enthusiasts who love to photograph animals or sporting events at a distance. The down side is that this magnification factor dramatically curtails the wide-angle end of the spectrum. A nice 24mm wide-angle lens, when used on the 40D, effectively becomes a not-so-wide 38.4mm lens.
So, in order to help clarify just what this magnification factor means, I shot the same scene in a conference room here in our headquarters in Yonkers, New York, one that has a big window in it, using the new 40D, which has a magnification factor of 1.6x and the Canon EOS 5D, which has a full-frame-sized sensor, with a magnification factor of 1x.
(http://blogs.consumerreports.org/.shared/image.html?/photos/uncategorized/2007/12/19/wideangle40dfinal.jpg) In the Photo A (Click on the link at left), I used Canon's EF 24-105mm f/4L IS USM zoom lens (which costs about $1000) on both cameras. In Photo B, I used a 70-300mm f/4-5.6 IS USM lens (which lists for about $600) on both cameras.
In Photo A , I set the zoom lens at its widest, 24mm setting and snapped both photos. You can see how the 5D lets you view more of the interior setting than the 40D. Indeed, the sensor within the 40D is actually cropping the image produced by the lens and turning it into the equivalent of a 38.4mm lens. Notice how, in the 40D shot, the chair and the telephone are cropped by each edge of the picture frame. In the 5D shot in Photo A, there is lots of space to the left of the telephone and to the right of the chair.
In Photo B (Click on link right)http://blogs.consumerreports.org/.shared/image.html?/photos/uncategorized/2007/12/19/telephoto40dfinal_2.jpg, notice again how the 40D appears to zoom farther in on the scene, cropping most of the cars out of the bottom of the scene as well as some of the tree trunk. In the 40D image in Photo B, the 1.6x magnification factor turns the 300mm end of the zoom into the equivalent of 480mm. On the 5D, it remains a 300mm focal length.
However, it's important to remember that although lenses may appear to change when used on a camera that has an APS-like sized sensor, the lenses retain their original characteristics. For example, a 24mm lens may appear to look like a 38.4mm lens, but it will still have the distortion and other characteristics common in wide-angle lenses. Say you’re standing too near someone's face for a portrait and are using a 24mm lens. The nose will still appear much larger than if you used the longer focal length. And that will be true when using a wide-angle lens on both the 5D and the 40D.
It's worth noting an SLR's magnification factor when you're deciding on a camera body, especially if you shoot wide-angle compositions. If you own a number of compatible wide-angle lenses and can afford its higher price, consider purchasing a full-frame sensor SLR. Or, if you go for a camera like the 40D, you may need to purchase lenses that are wider than the ones you own. Aside from the magnification factor, the 40D has a very strong set of features that make it a very capable, high performing camera for many types of photography.
Sensor Size and Lens Magnification Factor
Although there are many new or improved features on this SLR, one element that remains consistent with its predecessor, the 30D, is that it still has the same size (APS-like) sensor.
That's important because that sensor is smaller than a 35mm film slide (which is the size of a full-frame sensor). In the camera body, that smaller sensor format effectively "crops" the image produced by the lens. To you, the effect is to magnify the image or increase the telephoto capability of your lens.
So, on the 40D, which has an APS-like sensor and a lens magnification factor of 1.6x, a 100mm lens becomes equivalent to a 160mm lens.
This is good news for nature enthusiasts who love to photograph animals or sporting events at a distance. The down side is that this magnification factor dramatically curtails the wide-angle end of the spectrum. A nice 24mm wide-angle lens, when used on the 40D, effectively becomes a not-so-wide 38.4mm lens.
So, in order to help clarify just what this magnification factor means, I shot the same scene in a conference room here in our headquarters in Yonkers, New York, one that has a big window in it, using the new 40D, which has a magnification factor of 1.6x and the Canon EOS 5D, which has a full-frame-sized sensor, with a magnification factor of 1x.
(http://blogs.consumerreports.org/.shared/image.html?/photos/uncategorized/2007/12/19/wideangle40dfinal.jpg) In the Photo A (Click on the link at left), I used Canon's EF 24-105mm f/4L IS USM zoom lens (which costs about $1000) on both cameras. In Photo B, I used a 70-300mm f/4-5.6 IS USM lens (which lists for about $600) on both cameras.
In Photo A , I set the zoom lens at its widest, 24mm setting and snapped both photos. You can see how the 5D lets you view more of the interior setting than the 40D. Indeed, the sensor within the 40D is actually cropping the image produced by the lens and turning it into the equivalent of a 38.4mm lens. Notice how, in the 40D shot, the chair and the telephone are cropped by each edge of the picture frame. In the 5D shot in Photo A, there is lots of space to the left of the telephone and to the right of the chair.
In Photo B (Click on link right)http://blogs.consumerreports.org/.shared/image.html?/photos/uncategorized/2007/12/19/telephoto40dfinal_2.jpg, notice again how the 40D appears to zoom farther in on the scene, cropping most of the cars out of the bottom of the scene as well as some of the tree trunk. In the 40D image in Photo B, the 1.6x magnification factor turns the 300mm end of the zoom into the equivalent of 480mm. On the 5D, it remains a 300mm focal length.
However, it's important to remember that although lenses may appear to change when used on a camera that has an APS-like sized sensor, the lenses retain their original characteristics. For example, a 24mm lens may appear to look like a 38.4mm lens, but it will still have the distortion and other characteristics common in wide-angle lenses. Say you’re standing too near someone's face for a portrait and are using a 24mm lens. The nose will still appear much larger than if you used the longer focal length. And that will be true when using a wide-angle lens on both the 5D and the 40D.
It's worth noting an SLR's magnification factor when you're deciding on a camera body, especially if you shoot wide-angle compositions. If you own a number of compatible wide-angle lenses and can afford its higher price, consider purchasing a full-frame sensor SLR. Or, if you go for a camera like the 40D, you may need to purchase lenses that are wider than the ones you own. Aside from the magnification factor, the 40D has a very strong set of features that make it a very capable, high performing camera for many types of photography.
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