#1  
Old 28-09-2009, 11:14 AM
TrevorW
Registered User

TrevorW is offline
 
Join Date: Aug 2006
Posts: 6,995
Widefield Imaging

Using a Canon 50mm f1.8 lens

with manual focus the AV was set at 13(1/3) to acheive focus

is this normal ??

FWHM values in DSLR Focus where 4.5

No filter
Reply With Quote
  #2  
Old 28-09-2009, 11:17 AM
[1ponders]'s Avatar
[1ponders] (Paul)
Retired, damn no pension

[1ponders] is offline
 
Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
manually set your stops in DSLR Focus to what f stop you are going to image at. The faster the f ratio the more critical the focus. What is focused at f13 may not necessarily be in the CFZ (Critical Focus Zone) at f2.5.
Reply With Quote
  #3  
Old 28-09-2009, 11:19 AM
[1ponders]'s Avatar
[1ponders] (Paul)
Retired, damn no pension

[1ponders] is offline
 
Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
eg CFZ for a 50mm f1.8 at f/1.8 is 7.13 microns. at f13 it is 372 microns.

See poorly drawn example
Attached Thumbnails
Click for full-size image (Critical Focus Zone.JPG)
10.3 KB7 views
Reply With Quote
  #4  
Old 28-09-2009, 11:55 AM
gbeal
Registered User

gbeal is offline
 
Join Date: Oct 2004
Location: New Zealand
Posts: 4,301
Sorry Trevor, I am lost here as to what you were doing.
The camera should be in complete manual mode, correct? If so, set the aperture to max, (f1.8), and focus, with DSLR Focus. Then stop it down a stop or so depending on the aberrations that you suffer, or can't suffer.
Gary
Reply With Quote
  #5  
Old 28-09-2009, 11:56 AM
TrevorW
Registered User

TrevorW is offline
 
Join Date: Aug 2006
Posts: 6,995
The stars would not come into focus using DSLR the higher I set the AV value the better the focus appeared

Images taken at a lower f value appeared washed out or over exposed
Reply With Quote
  #6  
Old 28-09-2009, 12:06 PM
[1ponders]'s Avatar
[1ponders] (Paul)
Retired, damn no pension

[1ponders] is offline
 
Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
Quote:
Originally Posted by TrevorW View Post
The stars would not come into focus using DSLR the higher I set the AV value the better the focus appeared

Images taken at a lower f value appeared washed out or over exposed
My settings in DSLR Focus are ISO800, Tv 4sec, Av 2.5 or 2.8 (working from memory here I drop down a couple of stops to improve the stars at the edges and corners in the final image), Quality medium fine (or fine medium which ever way round it is.) When focusing I'll sometimes put a piece of blu tak against the body of the lens and the focus ring to apply a bit of resistance to focusing other wise its too easy to overshoot backwards and forwards. Don't forget that at f/1.8 you only have 7 thousands of a millimeter of focal zone to work with. It is very very challenging to nail that focus at f/1.8
Reply With Quote
  #7  
Old 28-09-2009, 12:17 PM
TrevorW
Registered User

TrevorW is offline
 
Join Date: Aug 2006
Posts: 6,995
That would probably explain it

the lower the f value the more critical the focus range

it was my first try at wide field with the lens so it's a bit of a learning curve
Reply With Quote
  #8  
Old 28-09-2009, 12:22 PM
[1ponders]'s Avatar
[1ponders] (Paul)
Retired, damn no pension

[1ponders] is offline
 
Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
Definitely persevere with it. Try the blu tak trick, especially if you have a bit of slop in the focus ring. I found that made a big difference in getting as close as I could to focus. After a while you get to know how much you need to turn and its quite easy to nail it.

Btw I don't use the DSLR graphing functions much anymore. I prefer to use the zoom icon set to about 6X and view a fairly faint star until it is all but one singe bright pixel with a few (4) fainter ones at 12, 3, 6, and 9 oclock. Remember to turn the zoom function off before trying to set the capture settings. It tends to lock up the ability to adjust settings until its turned off.
Reply With Quote
  #9  
Old 28-09-2009, 12:50 PM
gbeal
Registered User

gbeal is offline
 
Join Date: Oct 2004
Location: New Zealand
Posts: 4,301
Not sure about this "Av" and "Tv", to me it sound like the automation? What happened to "M"?
Gary
Reply With Quote
  #10  
Old 28-09-2009, 12:59 PM
TrevorW
Registered User

TrevorW is offline
 
Join Date: Aug 2006
Posts: 6,995
Thanks
Reply With Quote
  #11  
Old 28-09-2009, 01:12 PM
[1ponders]'s Avatar
[1ponders] (Paul)
Retired, damn no pension

[1ponders] is offline
 
Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
Av and Tv are the exposure and aperture settings that are set in DSLR Focus for the Focusing Run. Once the focusing run is finished Tv is set to Bulb and the Av (f/stop) is set at the original focusing setting (2.5?) or slower (2.5+) (but not faster 2.5-)

You have to be in M to be able to set these in the first place.
Reply With Quote
  #12  
Old 28-09-2009, 01:29 PM
gbeal
Registered User

gbeal is offline
 
Join Date: Oct 2004
Location: New Zealand
Posts: 4,301
OK Paul, sounds like you know what you are doing, back in the old days, "Av" and "Tv" were "Aperture Variable" and "Shutter Variable", denoting aperture priority and shutter priority. Not manual.
How things have changed, or is it me, LOL.
Gary
Reply With Quote
  #13  
Old 28-09-2009, 01:36 PM
[1ponders]'s Avatar
[1ponders] (Paul)
Retired, damn no pension

[1ponders] is offline
 
Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
Well not really Gary. They still relate to the same things but under Av setting on the camera as you know you change the f/ratio, under Tv you change exposure settings. Under M you can play with them all. Its just that in DSLR Focus you have them at your fingertips with the click of a mouse so no having to push buttons or spin dials on the camera. With DSLR Focus I have one group of settings for focusing and then when I'm ready to capture I switch to capture mode and change Tv to bulb and the Quality from fine medium jpeg to Raw or Raw+jpeg all from withing the program. I can then set the program for the number of exposures, interval, delays etc and let it do the hard yards of controlling the procedure.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 07:43 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
Celestron Australia
Advertisement
NexDome Observatories
Advertisement
Meade Australia
Advertisement
SkyWatcher Australia
Advertisement
Lunatico Astronomical
Advertisement
OzScopes Authorised Dealer
Advertisement
Bintel
Advertisement
Astronomy and Electronics Centre
Advertisement