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No infinity focus

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#1 Hook

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Posted 26 May 2019 - 09:28 AM

Hi.

 

Can someone explain the meaning of no infinity focus to me.

I did search the net but everything was infinity focus. It is concerning a canon 200mm manual prime focus lens. 

Thanks


Edited by Hook, 26 May 2019 - 09:30 AM.


#2 TOMDEY

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Posted 26 May 2019 - 11:02 AM

Well, in the astronomical imaging context, infinity means only one thing: that your camera system (all the optics and the sensor) put the stars conjugate to the detector. This is the case when changing focus, in either direction, will only make things worse.

 

We Optical Metrology guys associate that unique condition with the Zernike Wavefront Power Coefficient (for the -1+2p2 term) being identically-zero.

 

For more-expansive applications (portrait, microscopy, still life, landscape...) it gets far more involved --- things like ~hyperfocal range~ entering into the conversation and art of rendering the most pertinent or pleasing image, including foreground and background being selectively defocused from the main subject of interest.

 

Your lens, a stock Cannon ... would be marketed almost exclusively for the later audience. If you are using it for astronomy, you would want the former, simpler optimum. Most of us here do that by focusing on stars until their images are sharpest. Most imaging software will help you achieve that.

 

NOTE: The do that, your lens will best need the capability of going to either side of true astronomical infinity. Some cameras don't allow for that.    Tom

Attached Thumbnails

  • 126 First 36 Zernike Terms showing power term.jpg
  • 127 200 Zernike Best Focus.jpg


#3 TOMDEY

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Posted 26 May 2019 - 11:07 AM

Ummm... I just noticed that the table (from Jim Wyant) and the colorized pyramid (from elsewhere) differ in notation subscripts. No big deal... the optical literature is very inconsistent, which is fine. Anyway, it is the quadratic power term.    Tom



#4 Hook

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Posted 26 May 2019 - 12:55 PM

Would this solve the problem

 

https://www.ebay.ie/...A9_a4JWM_3T7aB4



#5 ccs_hello

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Posted 26 May 2019 - 02:55 PM

Crappy optics used in those cheap unit.

 

Canon "old" FD lens (flange-back distance: 42mm) when used in

EF-mount Canon EOS cameras (flange-back distance: 44mm) will not reach infinity focus.



#6 Hook

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Posted 26 May 2019 - 05:24 PM

Crappy optics used in those cheap unit.

 

Canon "old" FD lens (flange-back distance: 42mm) when used in

EF-mount Canon EOS cameras (flange-back distance: 44mm) will not reach infinity focus.

Fair do's. Thanks.



#7 the Elf

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Posted 27 May 2019 - 01:43 PM

Some old lenses can be disassembled easily and a bumper that stops the focus ring can be removed. You might find a video on youtube. If you mount it to an astro camera and not a DSLR you will not run into that sort of trouble. If you use a Canon DSLR I can recommend the 200mm Ashai Pentax Super Multi Coated Takumar plus an m42 adaptor. I do have infinity focus and it makes nice imges. Got mine for Eur 50 on ebay.

Example: http://www.elf-of-lo...rtSoul2019.html (klick for full screen)


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