“If the lenses don't infinity focus with the camera bodies they're designed to be used for don't even waste time trying to adapt to an astro camera, there's something wrong with the lens or what you're doing to focus it.”
That is the exact problem I had with the Samyang 21mm lens. It will just barely focus at infinity on my Sony cameras, but when I attached it to the ASI533 all I got was very out-of-focus stars. I do not yet have a 58-52mm step-down ring that is needed to attach my nebula filter to the front of the lens so working with the ASI533 had been without any type of filter in the imaging train.
As far as lenses go, I started with the Meike 35mm and 50mm manual focus lenses that were originally used for city/street shooting. These were for my Canon EOS-M6MKII camera The lens mounts are marked EF-M. I also have the Sigma 24mm and 45mm lenses for my Sony a7RM3. These, of course, are full frame lenses with the Sony E-Mount.
The E and FE on Sony lenses have nothing to do with the lens mount. That can be confusing because the FE series lenses are designed for full frame while the E lenses can only be used on their APS-C. Hence the F in the FE means it is a full frame lens. Sony only has one camera mount (their A-mount is not part of this discussion) and that is used on their E lenses and their FE lenses. When an E lens is used on, say, the a7RM3 camera, there is a very interesting image produced. Sony handles this by providing an APS-C/Super 35mm setting on their full frame cameras. This results in a normal image on a full frame camera albeit using only the crop sized area of the full frame sensor.
Getting back to my Sigma lens. When I ordered the ZWO Sony Adapter for ASI Cameras it was my intention to use these Sigma lenses on the ASI533. At that time, I did not realize that the focus and aperture on these lenses were controlled by camera electronics. With this discovery I ordered the Meike 25mm f1.8 for E Mount. This lens is designed for APS-C cameras and works just fine on my a6600 and also on my a7RM3 when I have that set to APS-C/Super 35mm. The mount on this lens is just marked E-Mount. Again, the FE just means the lens is a full frame lens and has nothing to do with the mount itself.
The Meike 25mm worked just fine on the ASI533 with and without a nebula filter. Because of this I decided to try the 35mm and 50mm Meike lenses on the ASI533. They too worked just fine. All I needed to do was remove the EF-M lens mount from these cameras and attach the E-Mount. This is not a big deal since there are only three small screws holding the mounts in place and there are no electronics to play with.
The 25 and 35mm lenses require a 49-52mm step-up ring for the nebula filter. The 50mm has a 52mm filter thread so the filter attaches directly to that lens.
The Meike lenses focus well past the Infinity mark so there is no problem getting sharply focused round stars. That was not the case with the Samyang 21mm lens.
The theoretical back focus distance of the ASI533 with the attached ZWO Sony Adapter for ASI Cameras is 17.5mm. That is too great a back focus for the Samyang 21mm lens that I had. What I observed when trying to get stars in focus convinced me that the lens was either defective or the back focus distance for this lens needs to be in the 16mm range. I am hoping that after hearing from the dealer that there have been many complaints about the infinity focusing of this lens that mine was just defective and the replacement will be better. I HOPE!
