There are a number of lenses in this category, made by various manufacturers. Their main advantage is that they are quite compact. Optical performance varies, but they all share come common traits. They all have a secondary mirror in the center of the front element that makes OOF highlights look like donuts, which can be distracting. They are all constant aperture, meaning you are always shooting at f/8, and cannot stop down further. More importantly, you're also composing and focusing (manually of course) at f/8, which can be difficult. They typically have somewhat lower contrast than standard lenses. Some are much worse than others in this regard. Even the best of these designs are not as sharp as a good refractive optic. The inexpensive (typically Russian-built) ones that you will see on ebay are of just OK optical quality. You may or may not be satisfied with the results.
The combination of logn focal length and snmall aperture means that a tripod is almost a requirement. Handheld, you'd need at very least 1/500s @ f/8, which means you can use 100 ISO only in full sunlight.
Bottom line, if you know what compromises you're making, they can have some interesting applications, but they are niche lenses to be sure.
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