How do i increase the backfocus of lens?


ArchRival

New Member
Sep 17, 2006
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#1
Specifically Canon lenses.
When focused at infinity, i need the focal point to be greater than 44mm, preferably about 60+mm.
Is there any adaptors or extenders i can use?
Thanks in advance.
 

Octarine

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Staff member
Jan 3, 2008
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Pasir Ris
#2
1) Ask in Canon forum
2) What exactly are you doing there? Usually people want to get rid of back focus.
Are you trying something like macro? Then have a look at macro rings.
 

ArchRival

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Sep 17, 2006
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#3
Trying to put a filter between the camera and lens, but the filter holder will push the camera back, such that the rig cannot reach focus at infinity, so i need a solution to increase backfocus distance.
 

Edwin Francis

Senior Member
Mar 24, 2006
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#4
OK, your question is really vague, but let me see if I understand the gist of it:
- you want to place a filter between the back element of your lens and your camera (that sounds like a strange thing to do, but whatever), but to do that, you are dismounting your lens and moving it away from your camera
- when you do that, you furthest point of focus is too close, and you need to extend that point of focus further away from your setup

If that is correct, what you have done is similar to putting an extension tube between lens and camera. To increase the focus distance, you can:
- move your lens closer to the camera. I have no idea what your setup is like, so don't know if this is possible
- add a negative diopter / diverging lens to your setup (opposite of a macro filter). This will degrade the optical performance somewhat, how much depends on the quality of the lens. If your quality level doesn't need to be high, the only commonly-available diverging lens I can think of are spectacle lenses for short-sightedness. No toric lenses (for astigmatism) or progressive lenses (for old folks like me) cause those are intentionally full of distortion. Or perhaps a high-quality convex mirror will do.

It might help if you are clearer about what you are trying to do and what you want to achieve.
 

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Octarine

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Jan 3, 2008
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Pasir Ris
#5
Trying to put a filter between the camera and lens, but the filter holder will push the camera back, such that the rig cannot reach focus at infinity, so i need a solution to increase backfocus distance.
What kind of filter would that be? If it is a wide angle / ultra wide angle lens and your filter is ND then have a look at gel filters. They will be mounted at the rear element. Do check mirror clearance if your camera is DSLR (again: what is your setup?)
 

ArchRival

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Sep 17, 2006
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#6
My setup is is as follows:

Lens : Canon 70-200F4L, although will be looking at the 400F5.6L prime if this setup works out.
Camera : TEC monochrome CCD with 17mm flange-to-sensor distance, APS-C sized sensor, T-threaded mount.
Filters : Hydrogen alpha, light pollution suppression filters loaded in manual filter wheel. Thickness of filter wheel ~22mm.

Additional adaptor attaching the filter wheel to the lens is ~11mm.

So distance from lens' mounting flange to sensor = 17+22+11 = ~50mm, which exceeds the 44mm required for Canon system.
Hence i need to somehow extend the backfocus distance of the lens.
 

Edwin Francis

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Mar 24, 2006
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#7
well then it looks like your options are adding an optical element (negative diopter/diverging lens) or tweaking your lens' internal elements (only if you know what you're doing, and generally only for cheaper lenses, which yours are not).
I'd say about a -2.7 lens would do, but it might change your distance again, so you'd have to recalculate.

BTW, your system is essentially myopic ;)
 

Octarine

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Jan 3, 2008
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Pasir Ris
#8
Why does the filter need to be between lens and body? Can it be mounted in front of the lens?
The T mount is a 42mm screw mount, right? Then you could add stepping rings to gain the few mm from 17+22 up to 44mm. Does it need 44mm for infinity focus or would 39mm already suffice?
 

ArchRival

New Member
Sep 17, 2006
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#9
well then it looks like your options are adding an optical element (negative diopter/diverging lens) or tweaking your lens' internal elements (only if you know what you're doing, and generally only for cheaper lenses, which yours are not).
I'd say about a -2.7 lens would do, but it might change your distance again, so you'd have to recalculate.

BTW, your system is essentially myopic ;)
Are there any good diopter lenses? I need good quality optics. This rig is for astrophotography, and stars at edge of the image must still be pinpoint, or at least round.
 

ArchRival

New Member
Sep 17, 2006
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#10
Why does the filter need to be between lens and body? Can it be mounted in front of the lens?
The T mount is a 42mm screw mount, right? Then you could add stepping rings to gain the few mm from 17+22 up to 44mm. Does it need 44mm for infinity focus or would 39mm already suffice?
Unfortunately yes, H-alpha filters are expensive, and there don't seem to be any in the regular lens thread sizes (67mm, 77mm, etc...).
Also, the light coming through a H-alpha filter is very little, not enough for the camera to focus. So focusing must be done through another clear filter with same glass thickness before the H-alpha filter is swapped in, hence the filter wheel.

Problem here is with everything assembled, flange-to-sensor distance is about 50mm. Lesser than 44mm is okay, maybe 45mm is still okay, but definitely not 50+mm.
 

shierwin

Senior Member
Dec 29, 2008
3,461
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East Coast
#11
The Canon Registration is 44mm
Your assembled setup is 50mm
What you need will be 6mm extension tube which together with the canon 44mm flange distance will give 50mm
I'm not sure with the setup using canon 70-200mm you can get the setup to focus to infinity
A T-mount lens + adapters etc can give you focus to infinity instead

Any lens, with registration distance less than 44mm mounted, on the canon, will NOT focus to infinity; instead it only give a MACRO effect
 

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Edwin Francis

Senior Member
Mar 24, 2006
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#12
Are there any good diopter lenses? I need good quality optics. This rig is for astrophotography, and stars at edge of the image must still be pinpoint, or at least round.
Sorry, can't help you with that. My experience (long, long ago) was with Milspec gear -- different channels!

Your optical requirements are pretty demanding. Are you in touch with the astrophotography community? They might be able to help more.
 

ArchRival

New Member
Sep 17, 2006
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#13
Thanks for the input.
After much consideration, i've decided to abandon the idea of using telephoto lenses for astrophotography.
Camera lenses are cheaper, easier to maintain and more versatile than astrographs, but their focus is limited, there are too many lens elements, the field may not be entirely flat, and the glass may not pass H-alpha efficiently.

Conclusion, it's better to put money down for a good, small, astrograph instead.
 

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