samyang - Catadioptric lens: equivalent focal length and focus at infinity

I recently bought a Rokinon (=Samyang) 300 mm catadioptric lens for use with my Fujifilm x-e1 mirrorless camera. It seems to work OK, but there are two things I don't understand about it:

1. 
   

   This review says that "on Fujifilm X and Sony E-mount cameras [the equivalent focal length] is 450mm." What does this mean?

   
   


2. 
   

   When I rotate the focus ring all the way counterclockwise, distant objects (even several miles away) appear out of focus. To get them in focus, I have to back off a little. The white pointer seems to be under the ∞ symbol when I do this. At full rotation, the indicator is under the symbols m and ft, which are beyond the ∞ symbol. This is very awkward. I normally expect to be able to rotate the lens all the way in order to get a focus at infinity. Am I misunderstanding something? Is this a design or construction flaw in the lens? An incompatibility with my camera? What would be the purpose of having a lens that can focus on a converging bundle of rays, which is something we never encounter when photographing real-life objects without some other optical element in front of the camera? Is there an adjustment that I need to make to my lens?

   
   

If the answer to #2 is that I need to adjust my lens, how do I do that?

possibly related: Do I need to calibrate my mirror lens?

Answer

As you know, the 35mm size film camera has been the one at the top of the photographic food chain for almost 100 years. Because of its popularity, most photographers are highly familiar with the way these cameras preform as to angle of view and magnification. This format measures 24mm height by 36mm length. For lens selection, we calculate the corner to corner measurement of this rectangle. That value, the diagonal measurement is 43.3mm. Keep this value in your mind.

Now your camera is based on a smaller format introduced 20 or so years ago. This format was named APS-C which stands for Advanced Photo System – Classic format. This format measures 16mm height by 24mm length and the diagonal measurement is 28.8mm.

The difference is 43.3 ÷ 28.8 = 1.5. In other words the APS-C is 1/1.5 = 0.66 X 100 = 66% the size of the 35mm film format. We grayhairs can use the 1.5 value to get a handle on what lens to mount on the APS-C to get an equivalent angle of view and magnification to a lens mounted on a full frame (FX) camera.

It goes like this: A 300mm lens mounted on your Fujifilm will perform equal to 300 X 1.5 = 450mm. In other words, if a 450mm is mounted on a full frame 35mm, it delivers a specific view. If you mount a 300mm on your camera, the two cameras will deliver an equivalent view. One more thing, the 1.5 value is called a magnification factor or a crop factor. Now about the focus question: The symbol for infinity (as far as the eye can see) is ∞. Almost all properly fitted camera lenses are factory set so that when they are racked in as close to the camera body as possible, they will be hard focused on infinity. Evidently this lens’s focusing mechanism goes too far and thus you are required to back off a tad to get a hard focus for infinity. Not the happiest of situations but doable. So don’t let that issue trouble you, it’s no big deal.

When you achieve a hard focus for infinity, the image forming rays from a distant subject are correctly converging on the surface of your camera’s imaging sensor, so again, no big deal. So my best advice is, use and enjoy this lens, never mind the minor stuff.