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Occasional Contributor
Posts: 5
Registered: ‎02-01-2018
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Lens astrophotography

Hello,

 

I have a Canon 6D and I would like to get a lens for astrophotography.

I don't have a massive budget (less than a 1000$).

 

What would you recommend ? After reading for hours on internet, it is clear that Canon lenses are out of the question (too slow) so I am left with Rokinon, Tamron, Samyang and Sigma.

 

I saw numerous recommendations for the Rokinon 14mm f/2.8 IF ED UMC and some others, but every time I read the negative comments (I use B&H and Amazon), I get huge doubts whether or not I should take it.

 

Thank you,

 

Kevin

Honored Contributor
Posts: 6,748
Registered: ‎08-13-2015

Re: Lens astrophotography


Kev74 wrote:

Hello,

 

I have a Canon 6D and I would like to get a lens for astrophotography.

I don't have a massive budget (less than a 1000$).

 

What would you recommend ? After reading for hours on internet, it is clear that Canon lenses are out of the question (too slow) so I am left with Rokinon, Tamron, Samyang and Sigma.

 

I saw numerous recommendations for the Rokinon 14mm f/2.8 IF ED UMC and some others, but every time I read the negative comments (I use B&H and Amazon), I get huge doubts whether or not I should take it.

 

Thank you,

 

Kevin


Rokinon 14mm lenses are popular for wide idle astrophotography, but there are many complaints about production quality.  I have purchased three of the Rokinon 14mm T3.1 Ciname lenses, and they have all been razor sharp, no issues.

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"I don't rent software. I use Photoshop CS6, ACR 9.8 and Lightroom 6.8 ."
VIP
Posts: 9,804
Registered: ‎12-07-2012

Re: Lens astrophotography

"...but every time I read the negative comments ..."

 

Exactly why I don't own one.  I don't want the hassle of maybe getting a good one.  I have no idea how B&H puts up with such nonsense from a company like Rokinon. I know of no one that got a good one right from the get go.  But you could always be the first !

EOS 1DX and 1D Mk IV, along with, a lot of other stuff.
Esteemed Contributor
Posts: 3,637
Registered: ‎06-11-2013

Re: Lens astrophotography

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
Occasional Contributor
Posts: 5
Registered: ‎02-01-2018

Re: Lens astrophotography

Hey,

 

A BIG thank you for the detailed answer ! I wanted to ask how to spot a decentered lens when you have one and you gave me the exact steps I need to follow. I will consider other options like the 24mm f/1.4 or anothher model ! Now that I know how to make sure my lens is not flawed.

The only problem with the 24mm is that I own a 24mm-105mm and I feel bad paying 500$ jjust to get the f/1.4. I will see though.

 

Thank you so much again and have a great day.

 

Kevin

Esteemed Contributor
Posts: 3,637
Registered: ‎06-11-2013

Re: Lens astrophotography

...and a tracking head is about $300-400 depending on the one you buy (cheaper than the lens).  With a tracking head, you can align the axis to be parallel to Earth's rotation axis and that means that as the Earth spins from west to east, the head rotates from east to west ... at exactly the same rate.  And since their axes of rotation are exactly parallel... they cancel each other out.  This means you can take very long exposures if you have a tracking head and STILL have tack-sharp stars (but the land would blur.)

 

Still... you could take a long exposure of the sky, then another of the land... and composite them in Photohshop.

 

This is a single 8-minute test exposure on a tracking head (used to validate that my head was tracking without smearing -- which confirms that I've got a good alignment)

 

IMG_2719.JPG

 

You can see the stars are tack-sharp ... no elongation in any direction (which means the head had a very good alignment).  Also since I took this at f/10 (as I recall) you can see the brighter stars have diffraction spikes (created by the aperture blades in the lens).

 

But this was used to create this exposure (and HDR shot at f/2 involving sets of exposures taken at 2 minutes, 1 minute, 15 seconds, and 3 seconds... all combined to create this.

 

Orion HDR.jpg

 

This sort of thing is only possible with a tracking head.  BTW, these were taken using a 135mm lens and an APS-C size sensor  (without tracking I would have been limited to about 2.3 seconds... not enough to get anything usable.)

 

Tim Campbell
5D III, 5D IV, 60Da
Occasional Contributor
Posts: 5
Registered: ‎02-01-2018

Re: Lens astrophotography

Beautiful and stunning pictures !

A tracking head was my next item on the bucket list. I thought it was a lot more expensive thought.
Can I ask you for a few good references ? Also, how does it work, is it something I mount on my tripod or is it a special kind of tripod itself. Do you need a computer with you to make the tracking ?

Thank you so much for all the information ! I will keep this thread in my favorite and read it when I need good tips and explanations.

Cheers,

Kevin
Esteemed Contributor
Posts: 3,637
Registered: ‎06-11-2013

Re: Lens astrophotography

The two major vendors are Sky Watcher and iOptron ... and both companies make two units with the difference being how much weight they handle (build quality is higher to handle heavier loads).

 

Sky Watcher makes:

  • Star Adventurer Mini - handles a 6.6 lb payload
  • Star Adventurer - handles an 11 lb payload

iOptron makes:

  • SkyTracker Pro - handles a 6.6 lb payload
  • SkyGuider Pro - hanles an 11 lb payload

 

They include an alignment aid which helps you find the celestial pole (for northern hemisphere observers the celestial north pole is a point very close to Polaris (the North star)... Polaris is about 2/3rds of a degree away from the pole but the alignment aid helps you find the spot in the sky by position polaris in the right spot).

 

You could just use the tilt of the tripod head to do this... but it's difficult to make precise adjustments.  Both companies make an  optional equatorial "wedge" ... it has knobs with a worm-gear that gives you very fine control to get the angle just right.  The angle needs to match your latitude on Earth.  I live at roughly 42.5º North lattitude... which means I use the scale on the side of wedge and adjust it to that angle.

 

You do want a solid camera tripod ... as the head tracks... the camera is moving around ... so the weight is shifting.  This can result in some "flexure" in your tripod that causes the stars to smear (especially when using longer focal length lenses).  So another option they both offer is a counterweight.  Whatever the weight is on your tripod... it will remain constant as the device tracks (it wont shift because as the camera & lens weight is always offset by the counterweight).

 

You do not need a computer.  

 

You set the tracking direction & speed (you can set it for northern vs. southern hemisphere rotation.  If you were to watch the stars around the north celestial pole, you'd notice they all go around the pole in a counter-clockwise direction.  But if you traveled to the southern hemisphere and watched the stars go around that pole, you'd notice they all go around in a clockwise direction (flat-earthers have to come up with some pretty insane ideas to explain THAT phenomena).  

 

The speed is normally set to "sidereal".  "sidero" was the Greek word for iron.  The Greeks thought the stars were made of iron because anytime they found a meteorite... they noticed it had a lot of iron.  So they logically concluded that metorites, which came from "falling stars" meant that stars were actually iron.  (Today we know better -- but who could fault them... it seemed pretty logical.)  So "sidereal" in astronomy means "of the stars" ... "sidereal tracking rate" means "the rate of the stars"... the rate the stars appear to move across the sky.  It's about 15 arc-seconds of angular rotation for every 1 second of time.

 

The moon and sun move at a different speed.  So if you were using it for lunar photography or for solar photography (only with a proper solar filter on your lens to avoid damage to the lens & camera) they have tracking rates for those objects as well.

 

You can also usually set rates such as 1/2 sidereal speed (it allows you to take night landscape shots and get away with doubling the normal max exposure time).  

 

Anyway they are fairly easy to use.

 

Put it on a decent photo tripod.  Align it to the north pole (they have an alignment aid that comes with it).  You can put the camera on it and point the camera anywhere you wish.  

 

And then start taking long exposure photos.  

 

Tim Campbell
5D III, 5D IV, 60Da
Respected Contributor
Posts: 1,853
Registered: ‎12-02-2012

Re: Lens astrophotography

Rokinon and Samyang are the same company.  Same company as Bower too. 

Scott

Canon 5d mk 4, Canon 6D, EF 70-200mm L f/2.8 IS mk2; EF 16-35 f/2.8 L mk. III; Sigma 35mm f/1.4 "Art" EF 100mm f/2.8L Macro; EF 85mm f/1.8; EF 1.4x extender mk. 3; EF 24-105 f/4 L; EF-S 17-55mm f/2.8 IS; 3x Phottix Mitros+ speedlites

Why do so many people say "FER-tographer"? Do they take "fertographs"?
Highlighted
Occasional Contributor
Posts: 5
Registered: ‎02-01-2018

Re: Lens astrophotography

Yeah, I don't know why I put both of them.

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