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Seldom
professor emeritus


Reged: 08/05/12

Loc: N of Cedar City Light Dome
Why do I need to refocus after adding a filter?
      #5566738 - 12/11/12 11:05 PM

Looking at Jupiter, I decided it was too bright, so I screwed a two piece polarizing filter to the inner threads of my ParaCorr. I found everything was out of focus after adding the filter. When doing regular (non astro) photography I never had to re-focus after adding filters to my camera, is there something else going on here?

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DavidC
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Reged: 11/24/05

Loc: Mesa, Arizona
Re: Why do I need to refocus after adding a filter? new [Re: Seldom]
      #5566751 - 12/11/12 11:15 PM

Good question. Moon filters I just screw in before I insert the ep. I've noticed when using an OIII, or LP filter, I need to refocus also. The only reason I can think is B cause the light is cut down, but I don't know why that would need refocusing. I'm sure someone will come up with a logical reason.
David


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panhard
It's All Good
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Reged: 01/20/08

Loc: Markham Ontario Canada
Re: Why do I need to refocus after adding a filter? new [Re: Seldom]
      #5566759 - 12/11/12 11:18 PM

Do you place the filter between the ParraCorr and your eyepiece? If so you would be changing the position of the eyepiece slightly in the focuser.

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GaryJCarter
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Reged: 06/06/09

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Re: Why do I need to refocus after adding a filter? new [Re: panhard]
      #5566796 - 12/11/12 11:44 PM

The reason you see the focus point change is the filter has a refractive index that is different than air, thus, the point of critical focus is shifted by the filter in the light path.

Different filters will have different refractive indexes, and for astrophotography filter sets can be had that are specifically designed and manufactured to be par focal.

Edited by GaryJCarter (12/11/12 11:46 PM)


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Seldom
professor emeritus


Reged: 08/05/12

Loc: N of Cedar City Light Dome
Re: Why do I need to refocus after adding a filter? new [Re: panhard]
      #5566840 - 12/12/12 12:26 AM

Quote:

Do you place the filter between the ParraCorr and your eyepiece? If so you would be changing the position of the eyepiece slightly in the focuser.



The filter is between the secondary mirror and the ParaCorr. No optics have moved, just filters inserted.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Why do I need to refocus after adding a filter? new [Re: GaryJCarter]
      #5567039 - 12/12/12 06:33 AM

Quote:

The reason you see the focus point change is the filter has a refractive index that is different than air, thus, the point of critical focus is shifted by the filter in the light path.

Different filters will have different refractive indexes, and for astrophotography filter sets can be had that are specifically designed and manufactured to be par focal.






Light Traveling Through Flat Glass

Also, though apparently not the case here, sometimes a filter does not allow the eyepiece to seat properly in the diagonal or focuser.

Jon


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JLovell
scholastic sledgehammer


Reged: 01/12/10

Loc: Georgia
Re: Why do I need to refocus after adding a filter? new [Re: Jon Isaacs]
      #5567055 - 12/12/12 06:59 AM

What Gary and Jon said. The filter WILL change the focus point in any optical system. The reason you notice it in the telescope and not in a camera has to do with depth of field. Telescopes have virtually none, while cameras have quite a bit. The depth of field in a camera becomes tiny when using high magnification long focal length lenses (like a telescope) and/or wide open apertures (like a telescope). Try using a 300mm or higher lens (which is pretty high magnification for a camera) and the shutter priority mode on your camera to hold the aperture open as wide as it will go (lower f number), and I'd bet you will have to re-focus just a tad on your camera too.

If you don't have a lens that high, use whatever you have with the aperture wide open, and I'd bet you will still see a difference. I have a 50mm lens with f1.7 maximum aperture. I can lay my keyboard on the table rotated 90* to where it normally is and take a picture of it with my aperture open to 1.7 looking down the length of the keyboard, and I can only read about 10 or so of the keys. The rest are so blurred I can't even tell there is writing on them. I'm quite certain if I tried that trick with a tripod and my camera set to manual focus and change nothing but the addition of a filter, the keys that would be readable would be different. Try it!


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JLovell
scholastic sledgehammer


Reged: 01/12/10

Loc: Georgia
Re: Why do I need to refocus after adding a filter? new [Re: JLovell]
      #5567150 - 12/12/12 08:49 AM

Now that I think about it a little more, the long focal length lens on a camera may not show much difference because of where the filter is placed. Under high magnification, the light entering the objective of an optical system that will be collected and focused are almost parallel, and would be perpendicular to the surface of any filter placed in front of the objective. If you had a lens with a filter placed BEHIND any of the lenses in the system, as in a telescope, it would make a difference. I'm still betting that a short FL lens like my 50, opened all the way up, would still show it though. I'll have to try it when I get home from work.

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Seldom
professor emeritus


Reged: 08/05/12

Loc: N of Cedar City Light Dome
Re: Why do I need to refocus after adding a filter? new [Re: JLovell]
      #5567368 - 12/12/12 11:23 AM

Thanks, Gary, Jon, and JLovell. What better way to learn optics than standing in the dark!

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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Why do I need to refocus after adding a filter? new [Re: Seldom]
      #5567604 - 12/12/12 01:39 PM

Question: putting the question of why aside, are you able to achieve focus?

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Why do I need to refocus after adding a filter? new [Re: GeneT]
      #5567825 - 12/12/12 03:22 PM

JLovell,
You did get back on track when reasoning out the action of a filter placed ahead of an objective, but your doubt about the case for faster systems is unfounded when used at optical infinity. Parallel incoming light is not deviated, and so the focus is not altered, no matter how short the objective f/ratio.

It's of interest to note that a binocular prism system is in effect a *very* thick, plane parallel hunk of 'filter' glass placed in the convergent light behind the objective. A full-size Porro Type 1 group that has a width of 25mm has an optical path length through glass (or thickness) of 100mm. The focus position is thus moved back by about 35mm.

Prism diagonals do the same thing, moving the focus rearward by an amount equaling about 1/3 the optical path length through glass.


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Seldom
professor emeritus


Reged: 08/05/12

Loc: N of Cedar City Light Dome
Re: Why do I need to refocus after adding a filter? new [Re: GlennLeDrew]
      #5567891 - 12/12/12 04:11 PM

GlennLeDrew: So if I had a filter big enough to cover my primary (like a filter for a camera lens) parallel rays wouldn't change the focal length of the system, but since I'm inserting glass into the system between the secondary and the EP, the rays are no longer parallel, the filter has a refractive effect and the focal point of the EP moves.

GeneT: Could achieve focus, just curious why it changed.


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Why do I need to refocus after adding a filter? new [Re: Seldom]
      #5568079 - 12/12/12 06:44 PM

Correct on the big filter up front. More so in the good ol' days of film photography, when lenses were mostly made to focus at infinity when hitting a hard stop in the rotation mechanism, adding/removing a front-end filter caused no change in focus. But bigger lenses which used smaller drop-in filters at the back end necessarily had to be made with no such hard stop at infinity, in order to accommodate the focus variations not just between filter/no filter configurations, but also for variations in filter thickness as might be found.

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JLovell
scholastic sledgehammer


Reged: 01/12/10

Loc: Georgia
Re: Why do I need to refocus after adding a filter? new [Re: GlennLeDrew]
      #5568785 - 12/13/12 08:05 AM

Quote:

JLovell,
You did get back on track when reasoning out the action of a filter placed ahead of an objective, but your doubt about the case for faster systems is unfounded when used at optical infinity. Parallel incoming light is not deviated, and so the focus is not altered, no matter how short the objective f/ratio.






At infinity, true, but I was trying to think of an example where the focus WOULD change, and I'm thinking that my example of shooting my keyboard from close up would still do it. With a wide angle lens, the light rays aren't parallel. I haven't tried it yet, but will.


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JLovell
scholastic sledgehammer


Reged: 01/12/10

Loc: Georgia
Re: Why do I need to refocus after adding a filter? new [Re: JLovell]
      #5569097 - 12/13/12 11:43 AM

Well, I've tried it now, and it didn't make a noticeable difference. Maybe the depth of field of a camera, even with the aperture locked open, is just too deep to be noticeable.

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