Catadioptric lens: equivalent focal length and focus at infinity

Question

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 1

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.

Answer 2

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

It means the angle of view obtained with a smaller imaging area (sensor or film) will be similar to an equivalent focal length lens used on a camera with a "standard" 135mm film format size of 36x24mm. The Fuji X series and most Sony E-mount cameras (probably all of them at the time the statement was written) are APS-C cameras with a sensor size of approximately 24x16mm. Since the sensor is smaller, images obtained with such a sensor have to be magnified 1.5X more to display them at the same viewing size as an image from a 26x24mm sensor (often called a 'full frame' sensor). This gives the APS-C camera with a 300mm lens the same amount of magnification as a FF camera with a 450mm lens. If we view both images full screen on the same monitor, or print both images in the same size paper, the image from the APS-C sensor is magnified more by a factor of 1.5X.

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?

Modern AF designs avoid a hard stop at infinity so the focus motor doesn't beat the focus mechanism to death every time the system tries to focus at infinity. Zoom lenses that are not parfocal also vary the exact position of infinity as the focal length is zoomed in or out. With the prevalence of such lenses in the current marketplace (the vast majority of current lenses are either AF, zoom, or both - there just aren't that many manual focus primes in current lens offerings when compared to the overall number of lenses sold today), the expectation that any lens would have a hard stop at infinity is no longer the case as it once was a generation or two ago when most lenses were manually focused primes.

There are other reasons as well, even with fixed focal length manual focus lenses.

• Variation in temperature can affect the exact position of infinity focus. This is especially true of lenses that use multiple types of material for optical elements and/or multiple types of materials for the supporting structure of the lens. Different materials demonstrate differing amounts of expansion/contraction for a specific change in temperature. Most older manually focused prime lenses still around today were made of high quality metal and glass manufactured to very high tolerances. All of the glass in the lens was usually the same material. Most of the supporting structure was made of the same or very similar materials. This is no longer the case. Leaving a little extra room for the focus mechanism to move slightly past infinity is insurance against not having enough room to reach infinity focus at a specific ambient temperature.
• With so many photographers choosing to use adapters to connect lenses in one mount on a camera with a different mount, leaving a little extra room for the focus mechanism to move slightly past infinity is insurance against not having enough room to reach infinity focus if a cheaply made adapter is slightly too thick.
• Manual focus prime lenses from "third parties" such as Samyang are often made in many different mounts without much, if any, modification to the designs for specific mounts other than the actual mounting flange on the back of the lens. Leaving a little extra room for the focus mechanism to move slightly past infinity is insurance against not having enough room to reach infinity focus for a specific mount and doesn't require as much precision with regard to the flange focal distance of the lens.
• As the resolution of digital cameras continues to increase and as expectations from viewing images at very large display sizes (e.g. 'pixel peeping' a 24MP image at 100% on a 23" HD monitor is like looking at a small section of a 60x40 inch print) continue to reduce the acceptable margin of error, providing a hard stop at infinity becomes increasingly difficult to do with enough precision to satisfy current expectations. If the difference between what is considered a "good copy" and a "bad copy" of many popular consumer lenses when placed on a particular camera body is less than the current manufacturing tolerances for registration distance, how could we possibly expect a hard focus stop to be more precise that the camera-to-camera variation in flange focal distances?
• Lenses created to be used to record infrared or ultraviolet wavelengths (such as many catadioptric designs that are used for astrophotography) as well as visible light can also vary the point of infinity focus as the range of wavelengths for which the lens needs to focus is broadened.