SLR camera lens: Since the aperture can be set to a fixed value (such as F3.8), in this case, turn the zoom ring aperture can change?
This is called the fixed maximum aperture, that is, in any focal length below can use F3.8 this maximum aperture. But also can reduce the aperture, such as F8, in general, can be reduced to F22
Because it is not a constant aperture that is the telephoto end and wide-angle end of the largest aperture are fixed
Such as Nikon 24-70MM F2.8 Whether it is 24mm wide angle or 70mm telephoto end maximum aperture can be set to F2.8
Also, such as the lens is written on the 18-135mm F3.5-5.6 that this lens is not constant 18MM wide-angle end of the big aperture is 3.5 135MM telephoto end is 5.6
Note that your lens should be marked with words such as 18-135mm f3.5 / 5.6
This means that when you use 18mm end when the maximum aperture is 3.5 and 135mm side when the maximum aperture is 5.6
So when you use the 18mm end, the aperture can be fixed at f3.5 but when you turn the zoom ring, the focal length becomes longer when the maximum aperture of less than 3.5 will vary with the maximum aperture allowed by the focal length. If you set the aperture above 5.6, then no matter how you turn the zoom ring aperture value will not change.
