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Afocal TECC & Vignetting

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#1 conandrum

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Posted 08 January 2011 - 03:20 AM

Hi people,

A few days ago I completed an XL calculator for Afocal Photography called 'Afocal TECC'.

Description:
Afocal Photography's T.elescope - E.yepiece - C.amcorder C.alculator

An advanced calculator to help you match
a Telescope, an eyepiece and a camera or camcorder together
for the sole purpose of spotting very specific targets between specific altitudes
and capturing these on film at the best resolution possible.

You can download it here:
http://www.ufonv.com....php?f=19&t=936

I want to make this as functional as possible. My latest worry is vignetting. I want to program this into the logic.

Let me give you an example. My camcorder is a canon hf10.
f=4.8-57.6 mm, F/1.8-3.0, 12x optical zoom
35 mm equivalent: Movies: 42.9-514.8 mm
Still images (LW): 42.9-514.8 mm
Still images (L, M, S): 39.3-471.6 mm

After sticking a 5mm diaphragm in front of the lens I could see one form of vignetting as per wikipedia.
I assume that this is what I would see if I had my camcorder looking into an eyepiece with a 5mm exitpupil right?
Zooming into the diaphragm, caused the hole to expand and near the end of my optical zoom, it disappeared completely.
How can I estimate what magnitude of vignetting will be present for different cameras at different zoom lens focal lengths if I know the exit pupil of an eyepiece?

I have the following equations:
FOV of camera (degrees)=(2*180*ATAN( (sensor width) /(2* (camera focal length))))/PI()
FOV at a distance from the camera (m)=(2* (distance from camera) *TAN( (FOV of camera) *PI()/360))
I tried using these to calculate what the camera is able to see just outside the objective lens, but I am not sure what distance I should use. If I use 0, the value is 0.

Is there an answer to these questions?

Thanks

Telis

PS. If you like the calculator and have comments, bugs, suggestions please post them at the forum thread for Afocal TECC so that we keep this one on subject.

#2 Ernest_SPB

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Posted 08 January 2011 - 05:26 AM

After sticking a 5mm diaphragm in front of the lens I could see one form of vignetting as per wikipedia.
I assume that this is what I would see if I had my camcorder looking into an eyepiece with a 5mm exitpupil right?

It is correct only for bad coordination of exit pupil of telescope (EP) and enter pupil of your camcorder. Normally they shall be coordinated better - in the best case their positions shall be the same. Enter pupil of a camcorder is placed some millimeters behind front lens and its position is changed by zooming. So exit pupil of EP shall be set somewhere after frontal surface of camcorder lens. Exact position should be find interactively changing distance between EP and lens of camcorder and watching luminosity of camcorder LCD corners.

The phenomena is hardly ever modeled by physical diaphragm before camcorder lens.

#3 conandrum

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Posted 08 January 2011 - 06:10 AM

I guess you are talking about overcoming vignetting by moving the camera towards or away from the eyepiece.

Is there no way to calculate what this distance should be?
This has to do with eyerelief I suppose?

Are you saying that my calculations in the spreadsheet will be correct ONLY after I fix my camcorder at the correct distance that will eliminate vignetting, using the method you suggested? i.e. Any eyepiece can be used (even the smallest ones, with any camera/camcorder) without fear of vignetting?

Thanks

#4 conandrum

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Posted 08 January 2011 - 06:25 PM

Post deleted by csa/montana

#5 Ernest_SPB

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Posted 09 January 2011 - 05:11 AM

I guess you are talking about overcoming vignetting by moving the camera towards or away from the eyepiece.

Yes. Vignetting strongly depends of EP-camcorder lens distance (S). And the best position is when exit pupil of EP coincides with enter pupil of camcorder.

Is there no way to calculate what this distance should be?

Distance between the pupils shall be equal to null. Distance from camcorder lens frontal surface to its enter pupil (Sp) is attribute of the lens design and depends of zoom factor.

This has to do with eyerelief I suppose?

EP eyerelief (Sp') is only one element of the distance: S = Sp' - Sp.

#6 conandrum

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Posted 09 January 2011 - 07:17 AM

Thanks Ernest,
I imagined something along those lines.

Q1. If you only knew your camcorders focal lengths (from min to max zoom) and your eyepiece focal length, exit pupil and eye relief, would you be able to say whether vignetting would be a problem or not?
Q2. If you had to choose from a variety of eyepieces and the only info you had was your camcorders focal length (from min to max zoom) and the eyepiece focal length, exit pupil and eye relief, how would you sort them from best to worst as far as vignetting is concerned?
Q3. If additional information was available, what info would you need in order to be able to discern whether vignetting would be a problem?
Q4. If you know of a source of information on the subject of afocal photography & vignetting, please let me know.

Thanks in advance
Telis






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