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True magnification in afocal photography?

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I have often placed a compact digital camera to the eyepiece of my telescopes to capture a 'snap' of the moon or sun.

Something which I have yet to discover is a way to accurately calculate the true magnification of the optical system.

My compact camera has up to 4 x zoom. Max 20mm f/l. Let's assume the telescope has an eyepiece fitted which provides [say] 50x.

How do I calculate the system magnification as I zoom the camera to be rid of vignetting? Is it really as simple as 4 x 50 = 200x?

Is it related to chip size? The camera focal length? The zoom factor as a simple fraction of the camera's max f/l? Or something else entirely?


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Break it down to first principles. You have OTA focal length divided by eyepiece focal length giving the magnification of the telescope. Then your compact will add some magnification factor. For a camera that is the focal length of the lens (not full frame equivalent) divided by the sensor diagonal, aka image circle diameter. Multiply them together. Personally I have never managed to get a compact camera to focus on the eyepiece image, I'm told it has to be set at infinity.

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Thanks. I haven't come across that method.
I have been searching online for an answer without success.

Ixus 117 sensor = 11mm[?]  20/11 = 1.8 amplification factor at full zoom.

That sounds about right! :thumbsup:

Thanks. :)


Sorry I didn't answer your question. The camera usually reverts to Macro for close-up.

Edited by Rusted
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