Samyang 135mm f/2

A telescope that fits a camera mount

Astrophotographers talk about this lens the way they talk about a good refractor, and the comparison is fairer than it sounds. At 135mm and f/2 the aperture works out at around 67mm — a small apochromat, in other words, with a field flat enough to use wide open and no focuser to sag. The difference is that it costs a fraction of what a comparable telescope would, and it screws straight onto a camera body.

What it buys you is the middle ground. A 14mm lens gives you the whole Milky Way and a telescope gives you one nebula; 135mm gives you the region — the North America and Pelican together, the whole of the Rho Ophiuchi complex, Orion's belt and sword in one frame with the Flame, the Horsehead and the Running Man all present. Those targets are too large for most telescopes and too small to register on a wide-angle lens.

Wide open, which is the point

Fast lenses are usually a compromise: you buy f/2 and then stop down to f/4 to get acceptable corners, at which point you have bought an f/4 lens. This one is genuinely usable at f/2. There is a little coma in the extreme corners on full frame and a trace of purple fringing on the brightest stars, both of which clean up in processing or disappear entirely with a modest stop down to f/2.8.

At f/2 the light-gathering is dramatic. Two-minute subframes on a modest tracker reach nebulosity that a slower setup would need ten minutes and autoguiding to match, and short subframes are exactly what an unguided tracker wants. That combination — fast optics and short exposures — is what makes the lens forgiving of imperfect polar alignment.

Living with a manual lens

There is no autofocus, no electronic aperture and no communication with the body at all, which means the camera records no focal length or f-number in the file. For astrophotography none of that matters. You focus once on a bright star using live view at maximum magnification, tape the focus ring so it cannot creep, and shoot all night.

The focus ring itself is superb — long throw, heavy damping, and precise enough to find critical focus without fighting it. The lens is also heavy at around 830 grams, which is worth planning for: on a small star tracker it needs to be properly balanced, and a lens collar or a counterweight is not optional at this weight.

Practical notes

• Dew is the enemy. The front element is large and recessed only slightly. A dew heater band and the supplied hood together will get you through a humid night; neither alone reliably will.
• 77mm filters fit. A light-pollution or dual-narrowband filter in this thread turns it into a genuinely capable suburban emission-nebula instrument.
• Focus shifts as it cools. Check it again an hour in. The shift is small but at f/2 the depth of focus is smaller.
• Flats still matter. Vignetting at f/2 on full frame is real. Take flats before the lens comes off the body.

By daylight

It is also, incidentally, a lovely portrait lens — the rendering wide open is creamy and the sharpness is there when you want it. Manual focus at f/2 on a moving subject is a skill rather than a convenience, so this is not the lens for photographing children, but for anything that stays still it is excellent.

Buying one

Sold as Samyang in most of the world and Rokinon in North America; the lens is identical. It exists in every common mount, and because it is fully manual the adapted versions behave exactly like the native ones — there is no electronic communication to lose in the adapter. That makes it one of the few lenses genuinely worth buying in a different mount if the price is right. Used copies are common and, with no autofocus motor and no electronics to fail, there is very little that can be wrong with one that looks clean.

Pair it with the A7 III and a tracker and you have a widefield deep-sky setup that fits in a small bag.