# Canon SL1



## owlwayswithlove (Jul 23, 2014)

My husband bought me a canon SL1 for our anniversary as a new beginner camera. I love it however I can not get it to go lower than f/ 5.6 while in Manual. I know it goes lower than that because it does in the Av setting. Do I need to change a different setting to bring it down lower than that?


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## Derrel (Jul 23, 2014)

On MANY smaller lenses, f/5.6 is the "lowest numerical value" that the lens has when the lens is zoomed out a bit. For example, on an 18-55 f/3.5~5.6 zoom lens, the lowest f/number is f/3.5 at 18mm, but the maximum aperture size changes as the lens is zoomed, and the f/number's value hits f/5.6 somewhere in the zoom range. So....again, it's quite possible that the reason you can not get the *f/number value* LOWER THAN 5.6 is that...the lens has been zoomed out a bit.

Another example, in Av mode, you can use the command dial on the camera to set the lens to f/3.5 with the zoom set at its shortest length, like say 18mm...but when you manually zoom the lens to a longer length, like say 28mm, it might drop to oh, say f/4.5 or f/4.8, and then to f/5, then to f/5.6 at the longest end...which is the way a variable maximum aperture zoom lens of any brand will behave.


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## owlwayswithlove (Jul 23, 2014)

ok thanks! that makes so much more sense!!!


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## TCampbell (Jul 24, 2014)

The focal ratio (aka "f-stop" or "aperture") is the number of times the diameter of clear aperture can be divided into the focal length.

Using an EF-S 18-55mm f/3.5-5.6 lens (the lens most commonly included when you buy the camera + lens as a "kit") the maximum diameter of clear aperture is about 10mm wide (9.8mm) when zoomed to the 55mm focal length.

It turns out most variable focal ratio lenses aren't very "linear" in terms of the reduction in aperture.  E.g. while the lens can do f/3.5 at the 18mm end, typically by the time you get to the mid-point of the range, a lens will have already arrived at the smaller sized aperture (smaller aperture openings have higher f-stop values because a smaller diameter can be divided into the focal length more times.)  That means you will likely find that somewhere around a 35mm focal length the lens will already have a  lowest-possible f-stop of f/5.6.

There are other lenses which physically collect more light... which means the glass and opening is wider.  The Canon EF-S 17-55mm f/2.8 IS USM lens is an example... it covers nearly the same focal length range as the 18-55mm lens... but it can provide the f/2.8 aperture opening at all focal lengths.

It's a bit more complicated for a lens designer to pull that off... the glass elements must be physically larger.  When they get larger, the dispersion increases because the edges of the larger lens want to behave like a prism and split the light into a "rainbow".  That means they have to add more corrective elements, which also must be physically larger.  Lower focal ratio lenses tend to be larger and heavier... and also more expensive.


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