It’s interesting to note how many photographers, even advanced shooters, are mistakenly under the impression that the depth of field (DOF) of a variable-aperture zoom lens changes in relation to the effective aperture of the lens as you zoom toward the telephoto end of the optic’s zoom range. But as the character Sportin’ Life sang so long ago in Porgy and Bess, “It Ain’t Necessarily So.”
The reason f/3.5 slows down to f/4.5, f/5.6 or f/6.3 as you pile on the millimeters is because variable-aperture zooms are inherently inefficient at transmitting light. You’re fine at the wide-angle end of the zoom range, but the further you zoom in, the light levels continuously drop off as you zoom into your subject.
What doesn’t change is the DOF at any set aperture. If you set your lens to f/3.5, f/8, or f/11, the DOF remains the same as it would be on any lens set to the same aperture and at the same focal length regardless of how much light is lost along the way.