Re: DPI and PPI
- From: "J. Clarke" <jclarke.usenet@xxxxxxxxxxxxxxxx>
- Date: Tue, 08 Aug 2006 10:38:20 -0400
Dave Martindale wrote:
Dave Martindale wrote:
The actual PPI of the sensor is very useful if you want to do any
calculations involving actual image distances. For example, if you
have mounted the camera on a telescope, and see two features a certain
number of pixels apart in the image, you can determine their true
angular separation in the sky if you know the sensor PPI and the
telescope objective FL. Or if the camera is mounted on a microscope
and you know the microscope objective magnification, you can measure
distances in the subject.
"J. Clarke" <jclarke.usenet@xxxxxxxxxxxxxxxx> writes:
Now how many photographers actually _do_ this in the real world? Not
astronomers or surveyors or the like, but photographers for whom the image
is the end product and not a means of performing a measurement?
So if I do something useful with an image, not merely pictorial, I'm no
longer a photographer? Photographers take photographs, period.
There's no requirement that the purpose must be pictorial to qualify as
a photographer. I'm neither an astronomer nor a surveyor nor a
machinist, but I do use photography to measure things.
In any case, that was one example. Here's a more pictorial one: With a
DSLR, you have a sensor of a particular size, and you often have several
different lenses with a range of focal lengths in order to be able to
make the subject of a particular photo approximately the desired size
on the sensor. In other words, you change lenses to get the field of
view that you want.
In astrophotography, each telescope has just one fixed focal length,
which determines its image scale. If you have multiple telescopes, you
may have multiple focal lengths available. You may also have multiple
cameras. Knowing the PPI or pixel pitch of the camera sensors helps you
choose which telescope and camera to use when capturing an object of a
particular (angular) size.
The focal length of lenses for photography is virtually always marked
on the lenses, because the information helps you to choose the lens you
want for a particular shot without trying all the ones in your bag.
Similarly, sensor PPI or pixel pitch helps choose the right camera to
use in some situations where the lens FL is fixed - why would you ignore
that info if it is available?
You clearly have special needs that are outside the mainstream of
photography.
--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)
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