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Photography Question 
Barefoot Photography by Tina Doane
BetterPhoto Member Since: 1/3/2005

My canon 28-135 lens

I have a 30D and a 5D, I have a 28-135mm lens and I've noticed I cannot zoom out as far when I am using the lens with the 30D. Do focal lengths change depending on the camera? I am obviously missing something

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2/20/2007 7:43:44 AM

Alan N. Marcus   Hi Tuba,

The Canon 5D sports a “full frame” sensor chip. This chip is exactly the same size as the picture area used on a 35mm film camera. This chip measures, for all practical purposes, 24mm x 36mm. Now the Canon 30D uses a chip that is 15mm x 22.5mm, that a little smaller.

Now the job of the camera lens is to project an image of the outside world on the surface of the light sensitive chip inside the camera. Because the 30D has a smaller chip, not all the projected image cannot fit on it. The 30D can only see the center portion of the image. Thus a portion of the image from all sides will be lost.

Think of it this way, you are watching a movie in a classroom at school. The movie is being projected on a large screen. Due to poor scheduling you are made to give up the screen. You are given a smaller screen. You continue watching using the same projector and lens and the projection to screen distance remains as before. Sorry to report, the new set-up only allows you to view the center portion of the picture. The wonderful and wide angle picture you were watching is cropped off and thus lost. In other words the revised image has been reduced on all thus the angle-of-view is reduced.

Hope this helps,

Alan Marcus

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2/20/2007 8:26:11 AM

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