Focal length Manipulation

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orelza@gmail.com - 13 Apr 2007 15:06 GMT

Hello,

Am working on my final year project and it involoves mosaicing some
aerial photos taken in 1990 using an aerial photo camera with focal
length 153. I scanned these at 800dpi, with dimensions 7242 x 7263
pixels. The software am using requires a Camera calibration file and
the focal length is expressed in pixels. Any body out there who can
help me figure out this using the provided information? or if this
post not clear or you may require some additional information, please
let me know.

Thank you.

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Michael J Davis - 13 Apr 2007 17:27 GMT

orelza@gmail.com observed

>Hello,
>
[quoted text clipped - 6 lines]
>post not clear or you may require some additional information, please
>let me know.

I think we need to have confirmed the following...

1. focal length 153 - is that mm or inches?

2. Original image size (negative?) ex camera.
I'm tempted to assume that an image 7242 scanned at 800 dpi (dots per
inch) comes from a negative at 7242/800 inches = 9.0525 inches.

But dpi doesn't always equate to pixels (eg is this colour or b&w)

If you can answer my questions, I think you can answer your own
question, too!

Mike

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monopix - 13 Apr 2007 18:10 GMT

> the focal length is expressed in pixels.

I can't imagine how you would express the lens focal length in pixels. Are
you sure this is what the software wants? What software are you using? Maybe
someone is familiar with it.

When you say you are 'mosaicing' the photos, are you trying to just match
them up to create a larger image? In which case, there may be other programs
you can use designed to patch together panoramic images.

Peter

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Rob Morley - 13 Apr 2007 19:14 GMT

> > the focal length is expressed in pixels.
>
> I can't imagine how you would express the lens focal length in pixels.

Say you scan a neg at 2000 DPI, and the neg was shot with a 50mm lens.
50mm is 50 x 2000 / 25.4 = 3937 pixels. If you're scanning prints
rather than negs you also need to allow for the enlargement factor.

> When you say you are 'mosaicing' the photos, are you trying to just match
> them up to create a larger image? In which case, there may be other programs
> you can use designed to patch together panoramic images.

I suspect the software he's using is applying some fairly precise
transformations to the images.

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monopix - 14 Apr 2007 14:52 GMT

>> Say you scan a neg at 2000 DPI, and the neg was shot with a 50mm lens.
> 50mm is 50 x 2000 / 25.4 = 3937 pixels. If you're scanning prints
> rather than negs you also need to allow for the enlargement factor.

OK, maybe I should have said 'why' rather than 'how'. Of course you can
quantify any value in any units you want (what's 50mm in lunar distances
then?) but why would a program ask for a value in a unit that is a) not in
common use and b) would have to be calculated.

It sounds to me as if the OP has misunderstood what's required hence my
suggestion he should tell us what he's using.

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