Approve the Cookies
This website uses cookies to improve your user experience. By using this site, you agree to our use of cookies and our Privacy Policy.
OK
Index  •   • New posts  •   • RTAT  •   • 'Best of'  •   • Gallery  •   • Gear  •   • Reviews
Guest
New posts  •   • RTAT  •   • 'Best of'  •   • Gallery  •   • Gear  •   • Reviews
Register to forums    Log in

 
FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial 
Thread started 27 Oct 2010 (Wednesday) 15:45
Search threadPrev/next
sponsored links
(this ad will go away when you log in as a registered member)

You don't need a telescope

 
VisualUniverse
Senior Member
455 posts
Gallery: 32 photos
Likes: 29
Joined Oct 2008
Location: Dallas suburbia, TX
     
Feb 09, 2012 17:21 |  #766

tkerr wrote in post #13857603 (external link)
How is it an issue when you're shooting a very distant object at infinity?
What is the issue in order to get tight stars is perfect sharp focus, not DOF..
DOF is the range of acceptable focus sharpness from a point starting in front of the object to a point behind it.
When you shoot the dark night sky, as far as the camera is concerned you're shooting a flat object with no foreground or background. Everything at that distance is either in focus or none of it is.
DOF is not an issue of concern for astro imaging. Focus as well as clear steady dark skies are the issues of concern for nice sharp stars.
When using a short fast lens a concern is the field of view(FOV) and how flat that FOV is. Typically very fast short lenses don't have a flat field and you run into distortion problems, especially as you near the edge of the field.

What I'm stating comes from common knowledge amongst astrophotographers, is taught by astrophotography instructors, and is a proven concept. I simply stated the facts and since my experience has affirmed what everyone before me in this activity has stated, I will continue the practice. If you want to debate this from your theoretical viewpoint, I suggest taking it up on the Cloudy Nights forum in the DSLR section where the experts hang out. At this point, I am done.


Website: VisualUniverse.org (external link)
Gear List - Canon and Nikon addict
flickr (external link)

  
  LOG IN TO REPLY
sponsored links
(this ad will go away when you log in as a registered member)
VisualUniverse
Senior Member
455 posts
Gallery: 32 photos
Likes: 29
Joined Oct 2008
Location: Dallas suburbia, TX
     
Feb 09, 2012 17:33 |  #767

tkerr wrote in post #13857603 (external link)
How is it an issue when you're shooting a very distant object at infinity?
What is the issue in order to get tight stars is perfect sharp focus, not DOF..
DOF is the range of acceptable focus sharpness from a point starting in front of the object to a point behind it.
When you shoot the dark night sky, as far as the camera is concerned you're shooting a flat object with no foreground or background. Everything at that distance is either in focus or none of it is.
DOF is not an issue of concern for astro imaging. Focus as well as clear steady dark skies are the issues of concern for nice sharp stars.
When using a short fast lens a concern is the field of view(FOV) and how flat that FOV is. Typically very fast short lenses don't have a flat field and you run into distortion problems, especially as you near the edge of the field.

I recommend reading this article by Jerry Lodriguss on using lenses for astrophotography...he talks about the effects:
http://www.astropix.co​m/HTML/I_ASTROP/LENSES​.HTM (external link)


Website: VisualUniverse.org (external link)
Gear List - Canon and Nikon addict
flickr (external link)

  
  LOG IN TO REPLY
FuzzSummit
Senior Member
Avatar
882 posts
Gallery: 60 photos
Best ofs: 1
Likes: 549
Joined Oct 2009
Location: Auckland, New Zealand
     
Feb 09, 2012 17:43 |  #768

The issue is not stopping down to reduce vignetting, CA, and other distortions (as noted by both Tim and Jerry) -- that goes without saying. The issue is your assertion that depth of field is important in getting sharp stars. As far as I know, whether a star is 500 light years or 5,000 light years, it's all the same to the camera - infinity. A fast lens should be stopped down not to increase DoF, but rather to reduce distortions.


Website (external link) || Instagram (external link) || Facebook (external link) ||Blog (external link) ||
Canon 6D || Canon 16-35/2.8 | Canon 35/1.4 | Canon 50/1.4 | Canon 70-200/2.8 | Sigma 85/1.4

  
  LOG IN TO REPLY
the ­ jimmy
Goldmember
Avatar
1,425 posts
Likes: 30
Joined Dec 2009
Location: west coast of Florida
     
Feb 09, 2012 17:56 |  #769

VisualUniverse wrote in post #13858051 (external link)
I recommend reading this article by Jerry Lodriguss on using lenses for astrophotography...he talks about the effects:
http://www.astropix.co​m/HTML/I_ASTROP/LENSES​.HTM (external link)

He does mention vignetting and optical aberrations, but I didn't see anything about DOF.




  
  LOG IN TO REPLY
tkerr
Goldmember
Avatar
3,042 posts
Likes: 2
Joined Mar 2010
Location: Hubert, North Carolina, USA.
     
Feb 09, 2012 17:57 |  #770

VisualUniverse wrote in post #13858051 (external link)
I recommend reading this article by Jerry Lodriguss on using lenses for astrophotography...he talks about the effects:
http://www.astropix.co​m/HTML/I_ASTROP/LENSES​.HTM (external link)

I already have his astrophotography pages bookmarked, and I have read much of Jerry's stuff long ago.

You must be referring to this which has absolutely nothing to do with Depth Of Field (DOF).
Quote:
All lenses will also exhibit "vignetting", or more properly, "geometric light falloff." This is an uneven circular field illumination when used wide open, which continues to improve as the lens is stopped down down 3 to 4 stops. Optical aberrations such as astigmatism and coma are generally much improved by the time the lens is stopped down two stops.

Camera lenses, however, can be used for shooting the stars! In most cases they must be stopped down a stop or two from wide open to improve coma, astigmatism and chromatic aberrations enough to produce usable stars. Even lenses that work very well wide open, such as the Nikon 180mm f/2.8 ED and Canon 200mm f/2.8, will get sharper if they are stopped down one or two stops.


No mention of nor does it have anything to do with the DOF. It does however have to do with, among other things, something that is called Petzval Field Curvature. The inability for a lens to bring a distant object to focus evenly throughout the entire FOV all the way to the edge on a flat image plane.

FuzzSummit wrote in post #13858094 (external link)
The issue is not stopping down to reduce vignetting, CA, and other distortions (as noted by both Tim and Jerry) -- that goes without saying. The issue is your assertion that depth of field is important in getting sharp stars. As far as I know, whether a star is 500 light years or 5,000 light years, it's all the same to the camera - infinity. A fast lens should be stopped down not to increase DoF, but rather to reduce distortions.

Exactly! ^^^^^^^^^^


Tim Kerr
Money Talks, But all I hear mine saying is, Goodbye!
F1, try it you'll like it.

  
  LOG IN TO REPLY
VisualUniverse
Senior Member
455 posts
Gallery: 32 photos
Likes: 29
Joined Oct 2008
Location: Dallas suburbia, TX
     
Feb 09, 2012 18:01 as a reply to  @ the jimmy's post |  #771

Allow me to clarify: if you care only about the center of the FOV (e.g. a planetary nebula) at a very short exposure where the lens is at ambient temperature and the temperature will not change over time, then concern about DOF is unnecessary. But if you want the entire FOV to contain tight stars and account for focus shift from temp changes, then yes...DOF is a concern.

TKerr was initially attacking my assessment about wide-open lens shooting. Sorry for distracting with the DOF reference.


Website: VisualUniverse.org (external link)
Gear List - Canon and Nikon addict
flickr (external link)

  
  LOG IN TO REPLY
tkerr
Goldmember
Avatar
3,042 posts
Likes: 2
Joined Mar 2010
Location: Hubert, North Carolina, USA.
     
Feb 09, 2012 18:42 |  #772

VisualUniverse wrote in post #13858196 (external link)
Allow me to clarify: if you care only about the center of the FOV (e.g. a planetary nebula) at a very short exposure where the lens is at ambient temperature and the temperature will not change over time, then concern about DOF is unnecessary. But if you want the entire FOV to contain tight stars and account for focus shift from temp changes, then yes...DOF is a concern.

TKerr was initially attacking my assessment about wide-open lens shooting. Sorry for distracting with the DOF reference.

No I was initially asking about your statement that "it is Normal Practice" To stop down a bit from wide open. Which could be a misleading statement for someone just getting started if not clarified.

It might be a practice that some people do stop down, but it is not a practice that everyone does. therefore it is not Normal Practice to stop down the aperture.

And then you replied:

VisualUniverse wrote in post #13858196 (external link)
Yes, but when you open up the lens all the way, star distortion occurs (spherical distortion), especially toward the edge of the field of view, along with chromatic aberration on brighter stars. By giving a little bit of depth of field, these issues are minimized. So for f/1.4 lens, I do 2.0 to 2.5. For f/2.8 lens, I'll do f/3.2 to f/3.5.

I replied that DOF isn't the issue, and I will still argue that it isn't. Because it is not.. DOF has nothing to do with those distortions or aberrations.

There is no DOF at those distances against the dark night sky..
DOF is a non-issue for astroimaging.


What you seem to be concerned with however is an "Even Field" or Flat Field as well as other issues that cause aberrations such as CA, Coma and Astigmatism. Focal ratio is only one, which by increasing can help decrease the effects of some of those problems that are inherent to the lens design and build.

Focus shift is another issue all together. Unless you have a rapid and drastic temperature change, as long as you allow your camera lens to acclimate properly, the amount of focus shift on a small camera lens due to the expansion and contraction of the glass elements is very insignificant if there is any. And the amount of time for a small camera lens to acclimate is very short.
Those things are more of an issue with larger telescopes that are more prone to temperature variances. Otherwise focus shift is a mechanical issue which should be a non issues once set.
Mechanical Focus shift is also a problem using a camera on a telescope when the focuser isn't or doesn't lock down tight enough. When that happens it will gradually start slipping out of focus as the telescope increases its angle while tracking.

Otherwise as long as you have your focus set and locked in you should be fine.
Still Nothing to do with DOF. There is no circle of confusion, nor area of acceptable sharpness.
At those distances, "provided you have a flat field", all the stars are sharp and in focus or they are all out of focus. And they will stay that way throughout your total exposure, or the night if you're taking multiple exposures for stacking.

Even by stopping down, when using a short fast lens you probably won't totally reduce the distortions all the way out to the edge of the field. But you can significantly reduce the effects and more so the amount of CA which will help make your stars appear much sharper and less bloated.


Tim Kerr
Money Talks, But all I hear mine saying is, Goodbye!
F1, try it you'll like it.

  
  LOG IN TO REPLY
luigis
Goldmember
Avatar
1,399 posts
Likes: 3
Joined Jun 2008
Location: Buenos Aires, Argentina
     
Feb 09, 2012 18:48 |  #773

Stopping down IS a normal practice to avoid comma.
DOF is not an issue and never is.
Vignetting is easy to correct with software.
Chromatic aberration is also something you can correct.

Comma is the big problem and even the best lenses exhibit comma wide open. There are a few exceptions of course but since the majority exhibit the problem then we can say that stopping down is a fairly common practice.


www.luisargerich.com (external link)
Landscape Photography & Astrophotography
Follow me on Twitter (external link)
My Awesome Gear List

  
  LOG IN TO REPLY
tkerr
Goldmember
Avatar
3,042 posts
Likes: 2
Joined Mar 2010
Location: Hubert, North Carolina, USA.
     
Feb 09, 2012 18:58 |  #774

luigis wrote in post #13858480 (external link)
Stopping down IS a normal practice to avoid comma.
DOF is not an issue and never is.
Vignetting is easy to correct with software.
Chromatic aberration is also something you can correct.

Comma is the big problem and even the best lenses exhibit comma wide open. There are a few exceptions of course but since the majority exhibit the problem then we can say that stopping down is a fairly common practice.

It would be nice if you could buy accessories for camera lens like you can for a telescope. Such as a Field Flattener and Coma Corrector.
You should be able to by a filter to help with the CA such as an anti-fringing or minus violet filter. Some companies that manufacture them for telescopes Eyepieces and CCD Cameras also make them for camera lenses.

As for astigmatisms, that is another problem that usually indicates a more serious problem with your lens. I.e bad lens grinding or imperfect glass element.

Another way to correct vignetting is to shoot and use Flat Frames. A subject that could be discussed elsewhere on it's own.
http://www.cyanogen.co​m …eld_Frame_Calib​ration.htm (external link)


Tim Kerr
Money Talks, But all I hear mine saying is, Goodbye!
F1, try it you'll like it.

  
  LOG IN TO REPLY
luigis
Goldmember
Avatar
1,399 posts
Likes: 3
Joined Jun 2008
Location: Buenos Aires, Argentina
     
Feb 09, 2012 19:30 |  #775

tkerr wrote in post #13858538 (external link)
It would be nice if you could buy accessories for camera lens like you can for a telescope. Such as a Field Flattener and Coma Corrector.
You should be able to by a filter to help with the CA such as an anti-fringing or minus violet filter. Some companies that manufacture them for telescopes Eyepieces and CCD Cameras also make them for camera lenses.

As for astigmatisms, that is another problem that usually indicates a more serious problem with your lens. I.e bad lens grinding or imperfect glass element.

Another way to correct vignetting is to shoot and use Flat Frames. A subject that could be discussed elsewhere on it's own.
http://www.cyanogen.co​m …eld_Frame_Calib​ration.htm (external link)

Flat frames are probably not needed if you have a good RAW processing software. I think they were the solution for FITs files from CCD cameras were there wasn't a tool to automatically remove vignetting.

What do you think?


www.luisargerich.com (external link)
Landscape Photography & Astrophotography
Follow me on Twitter (external link)
My Awesome Gear List

  
  LOG IN TO REPLY
VisualUniverse
Senior Member
455 posts
Gallery: 32 photos
Likes: 29
Joined Oct 2008
Location: Dallas suburbia, TX
     
Feb 09, 2012 19:38 as a reply to  @ luigis's post |  #776

This is comical.


Website: VisualUniverse.org (external link)
Gear List - Canon and Nikon addict
flickr (external link)

  
  LOG IN TO REPLY
tkerr
Goldmember
Avatar
3,042 posts
Likes: 2
Joined Mar 2010
Location: Hubert, North Carolina, USA.
     
Feb 09, 2012 19:39 |  #777

luigis wrote in post #13858730 (external link)
Flat frames are probably not needed if you have a good RAW processing software. I think they were the solution for FITs files from CCD cameras were there wasn't a tool to automatically remove vignetting.

What do you think?

For vignetting alone you shouldn't need them if you have something like Photoshop or Lightroom. But Flats are also used to help remove some noise and other inconsistencies in the image caused by anything in the optical path that shouldn't be there, such as dust spots. However, If the dust isn't very bad, that too can be removed quite easily in Photoshop.


Tim Kerr
Money Talks, But all I hear mine saying is, Goodbye!
F1, try it you'll like it.

  
  LOG IN TO REPLY
S.n.a.f.u.
Goldmember
Avatar
2,769 posts
Joined Jul 2009
Location: by a river in southern colorado
     
Feb 09, 2012 19:43 |  #778

Damn i wish i could get good into here and not wee wee measuring a contest.is the sigma 8-16mm a good choice for wide angle sky shoots? Or would the 17-50 be a better option?


I'm Russ. Gear List
Life is too short to drink bad wine
-ching chai
Time is a great teacher. Unfortunately it kills all pupils
-Louis Hector Berlioz

  
  LOG IN TO REPLY
tkerr
Goldmember
Avatar
3,042 posts
Likes: 2
Joined Mar 2010
Location: Hubert, North Carolina, USA.
     
Feb 09, 2012 19:47 |  #779

VisualUniverse wrote in post #13858783 (external link)
This is comical.

Glad you are enjoying it.
FWIW, When you refer to Depth Of Field, are you sure you are not actually thinking of Depth Of Focus, as in the symmetry of focus throughout the image plane?

S.n.a.f.u. wrote in post #13858817 (external link)
Damn i wish i could get good into here and not wee wee measuring a contest.is the sigma 8-16mm a good choice for wide angle sky shoots? Or would the 17-50 be a better option?

That depends on what you are after. If you want a real wide field of view to cover more area of the sky then the 8-16mm would be a good choice. But then at those focal lengths, especially the shorter, you will start running into the problems that have been discussed above such as field distortions, coma and vignetting.

However, the wider you go the more tolerant it is to the movement of the stars across the sky. IOW you will be able to get a longer exposure which will collect more details in the night sky.
Typically a good starting figure for shutter speed before trailing will be noticeable is to divide 500 by the lens focal length. Or if you're using a Full Frame Camera try 600 by the focal length. It's not perfect but it will get you close.
For Example with the 8-16mm Sigma @ 8mm on a camera with an APS-C crop sensor you can go as long as 62 seconds
With the other lens at 17mm you can only go as long as about 30 seconds.


Tim Kerr
Money Talks, But all I hear mine saying is, Goodbye!
F1, try it you'll like it.

  
  LOG IN TO REPLY
victorelessar
Member
Avatar
187 posts
Joined Jan 2012
Location: MaceiĆ³, Brazil
     
Feb 10, 2012 06:29 |  #780

guys what is the cheapest atrotrack (im not sure if atrotrack is a brand or the actual object. but i meant the tool rly)?
i say that because here in brazil i cant find anywhere that sells it. and to import would be very expensive for me, so im afraid id have to lose a bit on quality in order to have one.
i see here some equatorial mounts. perhaps it would be cheaper to buy one of those and later try to find the motor for it?


Canon EOS 1100D | Canon EF-S 18-55mm | Helios 50mm 1:2 | sigma DL 70-300mm

  
  LOG IN TO REPLY
sponsored links
(this ad will go away when you log in as a registered member)

839,022 views & 822 likes for this thread
You don't need a telescope
FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial 
AAA
x 1600
y 1600

Jump to forum...   •  Rules   •  Index   •  New posts   •  RTAT   •  'Best of'   •  Gallery   •  Gear   •  Reviews   •  Member list   •  Polls   •  Image rules   •  Search   •  Password reset

Not a member yet?
Register to forums
Registered members may log in to forums and access all the features: full search, image upload, follow forums, own gear list and ratings, likes, more forums, private messaging, thread follow, notifications, own gallery, all settings, view hosted photos, own reviews, see more and do more... and all is free. Don't be a stranger - register now and start posting!


COOKIES DISCLAIMER: This website uses cookies to improve your user experience. By using this site, you agree to our use of cookies and to our privacy policy.
Privacy policy and cookie usage info.


POWERED BY AMASS forum software 2.1forum software
version 2.1 /
code and design
by Pekka Saarinen ©
for photography-on-the.net

Latest registered member is EndaGeorge
744 guests, 297 members online
Simultaneous users record so far is 15144, that happened on Nov 22, 2018

Photography-on-the.net Digital Photography Forums is the website for photographers and all who love great photos, camera and post processing techniques, gear talk, discussion and sharing. Professionals, hobbyists, newbies and those who don't even own a camera -- all are welcome regardless of skill, favourite brand, gear, gender or age. Registering and usage is free.