Sigma-XQ 500mm f/8 Mirror Ultra-Telephoto is prime lens for 35mm SLR cameras via T-mount adapter. It works well on mirrorless camears via adapter.
It lacks a holdable tripod foot, and can be somewhat clumsy to use. It looks like they stuck a standard drawer-pull on the top to make it more manageable.
Unusual rear-focus mechism can be difficult to use, as you have to get your fingers between the rear of the lens and the camera body.
Unusually high-contrast for a mirror lens. Very good centre sharpness, getting softer toward the edges.
Adapts well to mirrorless cameras, and does a credible job — better than most of the dozen or so mirrors i've had over the years, second only to the outstanding Olympus OM Zuiko 500mm ƒ/8 Reflex, but two stops brighter.
According to reports on the web, the front corrector element seems especially prone to fungus infection, so make sure to store it in a dry place — and carefully examine any used one you consider. One reviewer disassembled and tried to remove the fungus, to no avail.
Unfortunately, this lens costs as much and weighs as much as an off-brand refracting lens of similar focal length and aperture, and is probaby more of interest to the collector than the serious user.
Posted by: Jan Steinman Date of publication: 19.02.2019