i have the rebel t1i with the original lens that came with it. i believe it's a 18-55mm lens. i see that the lowest f/number i get is 5.6. are f/numbers dictated by the type of lens you got?
First off it looks like you 'shift key' isn't working.
5.6 isn't the lowest f/number you have.
Here's what the numbers on your lens mean:
At 18 mm the maximum aperture of your lens is f/3.5 (which is actually a big number because it's a fraction). The opening in your lens would be 1/3.5 as wide as 18 mm or 5.14mm wide.
At 55 mm the maximum aperture of your lens would be 1/5.6th of 55 mm or 9.82 mm wide.
Both numbers only describe the maximum aperture your lens can achieve at the extremes of the zoom range 18mm and 55 mm. you can adjust the aperture to much smaller numbers on the order of f/16, f/22, f/32, etc. (I'm not familiar with the lower limit of that lens)
Your lens has what is known as a variable aperture zoom. If you set the aperture to f/3.8 when the lens is at the 18mm focal length and then zoom the lens to 55mm it will automatically change the aperture as you zoom until it is at f/5.6 when you reach 55 mm.
Constant aperture lenses cost more, because they are more difficult to manufacture. A constant aperture lens has just one number after the zoom range, like 80-200 f/2.8.
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