The issues with Rokinon is that sometimes they have "de-centered" optics. This means that *if* you happen to get a bad copy, you wont have a flat field of focus. The stars in part of your image might be sharp... but elsewhere they'll be soft and out of focus.
Those who get good copies, generally are happy with the lens and it's good optical quality. So it really depends on whether you get a good copy or not.
My suggestion should you choose to try it... is to immedaitely test the optics as soon as you get the lens... so you can return or exchange it if it is not a good copy.
Basically you want a nice flat surface with lots of detail (so you can inspect the quality). A brick wall would do well.
Position the camera on a tripod so that the camera is pointed straight at the wall (wall must not be at an angle... camera lens should be level (not pointed upward or downward ... and not left or right... you get the idea).
Take a test shot (or two).
Inspect the results.
It is normal that when you focus on the center... the corners may not be quite as sharp as the center. There are a lot of reasons for this, but one reason is that the field of focus usually isn't perfectly flat. There's a slight curvature (even though lensmakers try to make it as flat as possible). So a tiny degradation in sharpness may be noticed when you inspect the corners... but but all corners should be equal. You shouldn't have one or two corners that look sharp... and one or two that are obviously softer. If you see that, then you probably have de-centered optics and the lens should be retrned or exchanged.
Keep in mind that hand-holding the camera opens up the experiment to all kinds of human error ... so I would strongly recommend you use a tripod for the test.
Normally when you focus for astrophotography you find a nice bright star to focus on. But a technique for dealing with field curvature (the non-flat field) is that rather than putting the star in the center when you focus... put the star in the right-third or left-third of the frame. This means that the focus at that distance (1/3rd out) will be optimal, the center will be very very fractionally sub-optimal, but it also improves the sharpness to the corners. Basically instead of having perfect focus at the center... you're moving the area of perfect focus out a bit. On the whole... most of the image will appear sharper than if you had focused with the star in the very center.
And while we're on the topic... consider the 24mm f/1.4 as an alternative to the 14mm f/2.8.
An f/1.4 lens collects 4x more light than an f/2.8 lens. If you plan to shoot the milky way on a stationary tripod, you divide 500 by the focal length of your lens (since you are using a 6D ... a full-frame camera).
500 ÷ 14 ≈ 36 seconds.
500 ÷ 28 ≈ 21 seconds
So while on the surface it looks like the 14mm would be better because you can expose for much longer... you have to factor in the f-stop and how much light you can collect in that amount of time.
Since the f/1.4 lens is collecting 4x more light than the f/2.8 lens... it is "as if" the f/1.4 lens could expose for 4x longer. In other words... in 21 seconds it will have collected what would take an f/2.8 lens 84 seconds to collect.
This means the 24mm f/1.4 lens will be able to get roughly double the exposure vs. the 14mm f/2.8.
When you're trying to reduce "noise" in an image... it's all about the "signal to noise ratio" (SNR). Getting double the signal (collecting more than double the number of photons) will help you produce cleaner images.
Tim Campbell
5D III, 5D IV, 60Da
