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Focal length manipulation

 
 
orelza@gmail.com
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Posts: n/a
 
      04-13-2007
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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cgiorgio
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Posts: n/a
 
      04-13-2007

<(E-Mail Removed)> schrieb im Newsbeitrag
news:(E-Mail Removed) ups.com...
> 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.
>

Focal length in pixels (F =153 mm), 800 dpi

153 / 25,4 * 800 = 4819 pixels


 
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Paul Allen
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Posts: n/a
 
      04-13-2007
On Fri, 13 Apr 2007 17:01:46 +0200
"cgiorgio" <(E-Mail Removed)> wrote:

>
> <(E-Mail Removed)> schrieb im Newsbeitrag
> news:(E-Mail Removed) ups.com...
> > 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.
> >

> Focal length in pixels (F =153 mm), 800 dpi
>
> 153 / 25,4 * 800 = 4819 pixels


This looks like an opportunity to learn something.

How does it make sense to express focal length in pixels? Is this
something peculiar to aerial photography? Or does the concept have
wider applicability?

Inquiring minds, and all that.

Paul Allen
 
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Pat
Guest
Posts: n/a
 
      04-13-2007
On Apr 13, 11:29 am, Paul Allen <(E-Mail Removed)>
wrote:
> On Fri, 13 Apr 2007 17:01:46 +0200
>
>
>
> "cgiorgio" <(E-Mail Removed)> wrote:
>
> > <(E-Mail Removed)> schrieb im Newsbeitrag
> >news:(E-Mail Removed) oups.com...
> > > 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.

>
> > Focal length in pixels (F =153 mm), 800 dpi

>
> > 153 / 25,4 * 800 = 4819 pixels

>
> This looks like an opportunity to learn something.
>
> How does it make sense to express focal length in pixels? Is this
> something peculiar to aerial photography? Or does the concept have
> wider applicability?
>
> Inquiring minds, and all that.
>
> Paul Allen


I agree, interesting question. The OP is learning aerial photography
but isn't learning Googling. How interesting is that? So here is
what Google found:

Field of View from EXIF Data

In the images captured by some digital cameras, there is some data
that is useful to determine the field of view for stitching panoramas.

Ideally, this would be the focal length in pixels, but unfortunately
the focal length is given in millimeters. In order to determine the
field of view, it is necessary to know the density of pixels (in
pixels per millimeter) on the virtual imaging sensor associated with
the image. Again, this pixel density is usually given in pixels/inch,
so conversion to pixels/mm is needed as well. The pixel density may be
different for the horizontal and vertical directions.

>From the focal length in pixels, and the number of pixels in each

dimension of the image, it is possible to compute the field of view in
each dimension.

To convert from focal length in mm to focal length in pixels, where
pixel density is given in pixels/inch, use:

focalPixels = focalMM * pixelDensity / 25.4

To compute the field of view from focal length in pixels, use:

fov = 2 * atan( (H - 1) / (2 * focalPixels) )

where H is either width or height, for the field of view in the
respective direction. These equations are encapsulated into the
calculator below.

Field of View from EXIF Data Calculator

E
X
I
F

d
a
t
a FocalLength mm
PixelXDimension pixels
PixelYDimension pixels
FocalPlaneXResolution pixels/unit
FocalPlaneYResolution pixels/unit
FocalPlaneResolutionUnit

c
o
m
p
u
t
e
d Pixel FocalLength in X pixels
Pixel FocalLength in Y pixels
Field of View in X degrees
Field of View in Y degrees

----------------------
So, to the OP, why are we doing your Googling for you. What are they
teaching in college these days?

 
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ASAAR
Guest
Posts: n/a
 
      04-13-2007
On Fri, 13 Apr 2007 08:29:16 -0700, Paul Allen wrote:

>>> 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.

. . .

>> Focal length in pixels (F =153 mm), 800 dpi
>>
>> 153 / 25,4 * 800 = 4819 pixels

>
> This looks like an opportunity to learn something.
>
> How does it make sense to express focal length in pixels? Is this
> something peculiar to aerial photography? Or does the concept have
> wider applicability?
>
> Inquiring minds, and all that.


It may make sense for certain types of aerial photography, such as
when it's used for surveillance and resolving power is of primary
importance. For instance, you may want to be able to read the
numerals on a license plate, and one way to improve your chances
would be to double the focal length of the lens. This would
increase the image size by 2x, allowing 4x the number of pixels to
be used for just the license plate. Or if the lens and the sensor
size remains the same but the pixel width is halved (quadrupling the
sensor's number of pixels), you'd also get 4x the number of pixels
for the license plate. So a one-size-fits-all number that's used to
characterize a lens/sensor system that's based on a function of
focal length and pixel size is probably useful to get a rough
estimate of the resolving power of such a system. It should use,
however, the sensor's pixel size, and not a figure based on an
arbitrary 800 dpi that was used to scan prints after the fact. The
values used in the Camera Calibration File presumably are fixed, and
wouldn't vary depending on how future prints might be scanned.

 
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Don Stauffer in Minnesota
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Posts: n/a
 
      04-14-2007
On Apr 13, 10:29 am, Paul Allen <(E-Mail Removed)>
wrote:
> On Fri, 13 Apr 2007 17:01:46 +0200
>
>
>
> "cgiorgio" <(E-Mail Removed)> wrote:
>
> > <(E-Mail Removed)> schrieb im Newsbeitrag
> >news:(E-Mail Removed) oups.com...
> > > 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.

>
> > Focal length in pixels (F =153 mm), 800 dpi

>
> > 153 / 25,4 * 800 = 4819 pixels

>
> This looks like an opportunity to learn something.
>
> How does it make sense to express focal length in pixels? Is this
> something peculiar to aerial photography? Or does the concept have
> wider applicability?
>
> Inquiring minds, and all that.
>
> Paul Allen


I admit I have not been involved in aerial photography for a long
time, but this community, especially the military folks, were very
conservative, and even specified focal length in INCHES, not even mm
or cm. When I was involved, no one certainly worried about focal
length in pixels.

That being said, I certainly understand the utility of it. One of the
features of an electronic image is that it has no inherent linear
distance measurement. The true pixels per inch MUST be referenced to
the image chip, not any file format or anything like that.

 
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