Originally posted by TerryL What exactly does variable aperture mean. I've always preferred fixed aperture lenses in the f/2.8 range, but am looking at lenses for my Sons Canon Rebel and a lot of them are variable aperture. He would like to have a nice UWA lens.
The f/stop = the focal length of the lens divided by the diameter of the aperture. So with a 35mm prime at f/4 the aperture will have a diameter of 8.75mms (35mm/4). But on a typical 18-55mm zoom, because the focal length is not a constant, it is not cost effective for the lens to be designed to maintain the same effective aperture. At shorter focal lengths it may be f/3.5 and at the long end it could be f/5.6 or f/6.3.
"Constant aperture zooms" for any camera will generally be more expensive. For the Canon, Meike makes an UWA constant aperture 6-11mm f/3.5 for under $300:
Meike MK-6-11mm f/3.5 Fisheye Lens for Canon EF
Tamron, Sigma, Tokina, and Canon also make constant aperture zooms in the 16-50mm range for $300-$500 that you can filter to find on the B&H's website for Canon mounts.
Based on your description, I'd recommend the Sigma 10-20mm f/3.5 that is currently on sale for $400:
Sigma 10-20mm f/3.5 EX DC HSM Lens for Canon EF 202101 B&H Photo