RESOLUTION AND MAGNIFICATION CALCULATION
EXAMPLES USING A SONY ICX 625 SENSOR
Pixel Size = 3,45 x 3,45 μm
Number of Pixels (H x V) = 2448 x 2050
Desired FOV (Horizontal) = 100 mm
Limiting Sensor Resolution:
image space resolution in lp/mm =
imaging resource guide imaging lenses filters microscopy cameras illumination targets
Section 2.2: Object Space Resolution
In order to determine the absolute minimum resolvable spot that can
be seen on the object, the ratio of the eld of view to the sensor size
needs to be calculated. This is also known as the Primary Magni cation
(PMAG) of the system.
PMAG = sensor size (mm)
fi eld of view (mm)
The ratio associated with system PMAG allows for the scaling of the
imaging space resolution which tells us the resolution of the object.
object space resolution in (lp/mm) = image space resolution (lp/mm) x PMAG
Generally when developing an application, a system’s resolution requirement
is not given in lp/mm, but rather in microns (μm) or fractions
of an inch.
There are two ways to make this conversion:
object space resolution (μm) = 1000 μm/mm
2 x object space resolution in (lp/mm)
object space resolution (μm) = pixel size (μm)
PMAG of the system
While one can quickly jump to the limiting resolution on the object by using
the last formula, it is very helpful to determine the imaging space resolution
and PMAG to simplify lens selection. It is also important to keep in
mind that there are many additional factors involved, and this limitation is
often much lower than what can be easily calculated using the equations.
Learn more about lenses and contrast limitations in Sections 2.3 and 3.
The ability to resolve detail is directly related to both a lens’s
ability to reproduce contrast and the number of pixels utilized.
The images below are of the same test target, but taken by two
di erent lenses using the same number of pixels on the sensor.
Both images are cropped from the center of the sensor. Each
lens’s ability to reproduce contrast is the determining factor in
the performance of the system.
Lens 1 yields a contrast level of 22,6%, while Lens 2 produces
a contrast level of 12,7%. This is a 78% di erence in performance
between the two lenses, even though the images look
somewhat equivalent to the human eye.
It must also be understood that a lens will not necessarily produce
the same contrast at the same frequency across the entire
FOV. Additionally, contrast levels will change as a lens’s f/# is
adjusted. More detail on this can be found in Sections 2.4 and 2.5.
2 x pixel size (μm)
image space resolution in lp/mm =
2 x 3,45 μm
horizontal sensor dimension in mm =
(3,45 μm) x (2448)
= 8,45 mm
vertical sensor dimension in mm =
(3,45 μm) x (2050)
= 7,07 mm
PMAG: PMAG =
object space resolution = 145 lp/mm x 0,0845 = 12,25 lp/mm 41 μm
Section 2.3: Contrast
Contrast describes how well black can be distinguished from white at
a given resolution on an object. For an image to appear well de ned,
the black details need to appear black and the white details must appear
white (See Figure 2.2). The more the black and white information
trend into the intermediate greys, the lower the contrast at that
frequency. The greater the di erence in intensity between a light and
dark line, the better the contrast. While this may appear obvious, it is
The contrast at a given frequency can be calculated in Equation 2.8,
where Imax is the maximum intensity (usually in pixel greyscale values,
if a camera is being used) and Imin is the minimum intensity:
% Contrast =
The lens, sensor, and illumination all play key roles in determining
the resulting image contrast. Each one can detract from the overall
contrast of a system at a given resolution if not applied correctly and
in concert with one another.
(Continued on page 14)
Square Wave Contrast
Figure 2.2: Understanding contrast. Transition from black to white is
high contrast while intermediate greys indicate lower contrast.