Posted by
bradscopegems-2 on
Feb 12, 2011; 5:37pm
URL: http://imagej.273.s1.nabble.com/affordable-camera-suggestions-tp3685628p3685633.html
Dear Gabriel,
The 3x is anecdotal and subjective, but I can account
for it by reference to my previous message about the intensity profile of
the image of two point objects separated by the Rayleigh distance. This
curve is shown in practically every textbook dealing with microscope optics.
At the minimum Nyquist sampling frequency, the intensity profile across the
resolved region will consist of just three points: two high points
corresponding to the peaks and a low point in between. Only at about 3x this
frequency will one begin to see something of the shape of the peaks and of
the valley in between. And, of course, a high signal to noise ratio,
requiring many hundreds of photons coming from each resolved point, is
necessary to see the dip at all.
Brad
On 12 February 2011 17:06, Gabriel Landini <
[hidden email]> wrote:
> On Saturday 12 Feb 2011, you wrote:
> > You will not find resolution values in the scant
> > literature supplied by microscope manufacturers,
>
> Hi Brad,
> The Olympus objectives we use came with some blurb where it was stated.
> However, I just checked and strangely they do not provide this information
> in
> their co.uk website. The objectives have their specifications listed but
> not
> resolving power.
>
> > So, if we wish to capture a 20 x 20 mm square area of the intermediate
> > image (a reasonably large fraction of what we see in an eyepiece) we
> need
> > 0.9, 2 and 1.5 megapixels minimum (i.e. to record full detail without any
> > empty magnification.
> > In practice, microscopists usually choose to work with about
> > 3x the linear magnification at the Nyquist minimum, so this means that
> the
> > preferred number of pixels would be nine times this, unless the camera
> was
> > recording a reduced area of the intermediate image. This means that the
> > high pixel numbers of the modern DSLRs are not overkill.
>
> My point, (maybe I did not articulate it well) is that the data being
> stored
> in such large number of pixels would not be adding anything in terms of
> image
> detail and yet it will require larger storage and more processing.
> If this is not taken into consideration one risks processing and reporting
> morphological detail which could not be resolved in the first place. This
> is
> of course obvious to experienced microscopists, but not perhaps to those
> who
> did not think of this in the first place.
>
> Let's use the example of fractal objects imaged with such level of empty
> magnification. Applying the yardstick method, their perimeters measured
> with
> small yardstick sizes will appear smoother than they really are because the
> detail of sizes close to the image pixels cannot be resolved.
>
> I must confess that I wasn't aware of the 3x preference by microscopists.
> Is
> there a reason for this number?
> Regards,
>
> Gabriel
>
--
Dr W. B. Amos FRS
MRC Laboratory of Molecular Biology
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