Now recently I was looking for a fast focal ratio lens for astrophotography, as it would be great to be able to image at F/2.0-2.8. As the ZWO ASI178MM-cooled has a crop factor of 4.8x, it would mean a 135mm focal length lens would be ~650mm, and a 200mm focal length lens would function at ~960mm. Hence I had bought the Samyang 135mm F/2.0 lens in Canon EOS mount for this purpose.
Reasons to choose the Samyang 135mm F/2.0 lens:
- Great affordable price £329 from Park Cameras; that is much cheaper than getting a Hyperstar/Fastar imaging system.
- It is sharper than the Canon 135mm F/2 L lens.
- There are some impressive astro images on Astrobin I’ve seen shot with this lens wide open.
- It could function well as a portrait lens with my Sony A7R.
Now this lens arrived in early January ’16 but I’ve not had the opportunity to use it for astro-imaging, as I did not have the correct adapter for it – I am still awaiting the arrival of the Geoptik Canon CCD adapter with attached finderscope mount for my guide scope.
I have to say that this lens is not that heavy compared to a telescope, so the iOptron ZEQ25 should handle this setup with ease. The front and rear elements look huge, and this lens takes 77mm front filters.
It does look to have very modern coatings on the optics.
And it does look good and fits the Sony A7R perfectly – I used a Kipon EOS-NEX adapter to mount it.
Now to put this lens to the toughest test, I shot it wide open at F/2.0 using my Sony A7R to see the center and corner sharpness, and it looks to have performed very well with sharp corners; the left bottom does not look as sharp which may be due to slight decentering of the lens or more due to the subject distance being significantly different.
As you can see, the images are very sharp even wide open, and even to the corners of the Sony A7R’s 36 megapixel sensor. So my thought was that it should not be an issue for the much smaller image sensor of the ZWO ASI178MM-cooled.