Re: How many pixels?
- From: Aaron <nghy@xxxxxxxxxxx>
- Date: 21 Apr 2006 04:21:02 -0500
Hi,
I just entered this thread and had a few comments. First Zeiss has
recearched this subject and included a table within their brouchure
for the Axiocam HR that relates the camera resolution in pixels needed
to capture all the information provided by various objectives
(different magnification and NA). You can go to the Zeiss website
(www.zeiss.com) and download the AxioCam brochure in pdf format. you
may be surprised to learn that far more than 2 to 4 megapixels are
needed for some common objectives.. The calculations show that lower
power objectives require greater numbers of pixels than higher power
objectives. And the use of a reducing lens to concentrat the output
of the microscope onto the CCD increases that demand.
Since this result seemed counter-intuitive, as an excercise I computed
the NA per unit magnification of various objectives that I own. I
discovered that although the absolute NA of the objectives increases
with magnification the opposite is true for the NA per unit of
magnification.
Consider two examples of typical objectives
10X 0.25NA 0 .25/10 = .025 NA/1X mag
100X 1.3NA 1.3/100 = 0.013 NA/1X mag
The second point I wish to make is that size of the image output from
the microscope has to be matched to the CCD size to avoid other subtle
problems.. In most situations the image output from the microscope is
too large compared to the CCD size.. So only the center part of the
microscope output falls on the detector. As a practical matter,
this reducing lens is tough to locate and expensive. It becomes the
weak link that prevents the full potential of the microscope and the
camera from being realized. .
While some OEM scope manufacturers and some independent suppliers
(Diagnostic Instruments) provide reducing lenses, obtainig an exact
match with optical qualities equivalent to the microscope itself is
very difficult and almost impossible for microscopes designed with
compensating eyepieces.
If the image falling on the CCD is larger than the CCD itself, the
resulting captured image will represent a subsection of the FOV as
seen in the eyepieces. This process parallels cropping the center of
a digital image and then enlarging the same. Normally this process
leads to degredation of the image details.. The relay/reducing lens
may introduce distortion itself, not provide compensation needed for
the objectives or not match the image out put to the exact size of the
CCD. The qualities of the reducing/relay lens is a very practical
complication to obtaining the highest quality images from the
microscope and camera.
Aaron.
.
On Wed, 19 Apr 2006 17:08:17 GMT, "NoSpam" <NoSpam@xxxxxxxxxxx> wrote:
Hi,.
There have been a few postings recently which made me think about the
maximum
useful number of pixels in a microphotograph. I will present some numbers
and my
conclusion is that about 2 to 4 Megapixels in a digital camera will do
justice to
the capabilities of any light microscope.
It is possible that I have overlooked something and that my conclusions are
incorrect.
Please let me know what you think.
The smallest separation observable with a light microscope is given by
wavelength di-
vided by the product of 2 times the NA. For a wavelength of 0.5 microns and
a NA of
about 1.0 the minimum resolvable distance would therefore be 0.25 microns.
Objectives
with a NA of about 1.0 magnify about 100 times.A resolution element of 0.25
microns
in the object plane will therefore appear with a 25 micron linear size at
the exit pupil
of the objective. A typical value for the diameter of this pupil is 20mm or
about
20 000 microns.There will therefore be 20,000microns/25microns or 800
resolution
alements along a given diameter in this pupil. If we square this number we
get the
approximate number of resolved pixels in the exit pupil as 0.64 Megapixels.
These are
all the resolved picture elements which a camera has to record. Further
magnification
or reduction in size of the exit pupil will not change this fundamental
requirement for
taking a picture at 100x linear magnification with a NA of 1.0.
If instead of a 100x objective with a NA of 1.0 we use a 10x objective with
a NA
of 0.25, then the linear size of a resolved object element in the exit pupil
decreases
by a factor of [(100/10)x(0.25/1.0)] or by 2.5. The total number of pixel in
the exit
pupil will then be 2.5x2.5=6.25 as large as the above number of 0.64
Megapixels
and about 4 Megapixels will be required.
If these consideration reflect reality then cameras with more than 4
Megapixels
will not be needed for microphotography. For high magnification half that
number
may be all that is needed.
I am interested in your comments.
G.R.
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