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Review: Samyang 135 mm f/2 Astrograph

March 6, 2016 9 comments

IMG_2663

By calling the Samyang 135 mm f/2 lens an astrograph in the title, I’m giving away a bit of the conclusion, so let me just state the conclusions up front: I found this lens to be sharp from corner to corner with a reasonably flat field across a full-frame sensor. This is better performance than all but the best prime telephoto lenses, and also better than many telescopes I’ve owned claiming to be astrographs. Even more impressive is the fact that it accomplishes this at a focal ratio four to eight times faster than “fast” refractors. Finally, it’s hard to beat the price: currently just over $500.

While the performance was adequate at f/2, I found that stopping down the lens one full stop to f/2.8 improved sharpness. This is true of any lens, and even stopped to f/2.8, that’s still four times the light gathered per sensor area than an f/5.6 telescope. It’s easy to forget how fast this is, but my first night using the lens reminded me. My usual telescopes are two f/5 William Optics Star 71s and a Takahashi FSQ-106ED, also f/5. I usually shoot narrowband exposures of 20 minutes with these. So the combination of broadband and f/2.8 put me in the realm of 30 to 120 second exposures—anything longer at ISO 800 overexposed the stars.

The infinity focus point is about 2 mm left of the mark on the lens barrel, so you’ll have to carefully dial in focus. There isn’t much tolerance for error at such fast focal ratios, as the zone of focus is very narrow.

Vignetting is substantial in the corners, but it is more reasonable if you move slightly inward.  For very fast optics, this is typical, though it does lead to lower SNR toward the corners. I was able to keep the full frame images without cropping by using good flat frames, but this is essential.

Master Flat created from 33 pictures (Average)

Master Flat created from 33 pictures (Average)

The quick 99×1 minute image below of the Rosette Nebula area gives you a sense of how wide the view is with a Canon 6D. In the center is the Christmas Tree/Foxfur/Cone Nebula area, with huge dark nebula Barnard 37 prominent. This was taken without a CLS filter, so light pollution prevented me from adequately revealing the Foxfur nebula well. The Rosette Nebula shines brightly to the left, though. (Note that this image is reduced to 25% of the actual image resolution.)

Samyang 135 Rosette_widefield 25 percent size

As you can see from the full resolution close-ups below, the lens is impressively sharp across the Canon 6D’s entire field of view, with very minimal distortion even in the extreme corners.

Samyang 135 corner performance

The 9-blade diaphragm of the Samyang results in a pleasing radiant around bright stars, but the lens does exhibit some internal reflections.

Samyang 135 reflection

I look forward to using this lens as my (very) widefield astrograph. Depending on your sensor size, the ideal targets for this lens will vary, but I’m looking forward to shooting:

  • The Orion Molecular Cloud Complex
  • IC2177 and Thor’s Helmet area
  • The California Nebula to Pleiades area
  • Orion’s Head/ Meissa Nebula
  • Heart and Soul Nebulae area
  • The Rho Ophiuchi area
  • Cygnus
  • The IC405/IC410 area
  • Taurus Molecular Cloud
  • The Milky Way’s Pipe nebula region
  • Sagittarius

After complex mosaics and multi-night narrowband CCD projects, it’s a joy to throw a simple setup like this onto the mount to grab bright widefield images in a few hours.

Pros:

  • Fast focal ratio
  • Sharpness
  • Flatness of field
  • Price (currently ~$529 USD)

Cons:

  • Some internal reflections
  • Limited targets available for this focal length
  • Will require adapter to fit CCD cameras
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