Modulation Transfer Function
Kurt Rose, Nadya Spice, Stefano Prezioso
What are MTF’s?
• MTF’s characterize the sharpness of an
imaging system. (Lens, image sensor, film,
etc.)
• Modulation Transfer Functions are
measurements made in the frequency
domain.
• They’re also known as a spatial frequency
response.
What do they do?
• MTF’s are used to measure how accurately the
lens can reproduce detail from an object to an
image. Basically, they can tell when the
cameras resolution falls off.
What do they look like?
The values in image A are black (0) and white (255). If we plot these points, you
will get a graph that looks like image C. Now if you photograph image A, the
lens will blur the bars in a horizontal direction shown in image B. If you plot the
points in image B, you will get a graph that looks like image D.
http://photo.net/learn/optics/mtf/blur2.gif
Why does it blur?
• Light travels in generally straight lines.
However, if begins to “diffract” when it travels
through a small hole such as an aperture.
Large Aperture
Small Aperture
Diffraction Pattern
The reason why it diffracts, even though it’s going through a straight line is because the
some of the light has to travel farther to get to the sensor.
To find MTF’s
You need to photograph a sine test target.
Like this super awesome one in the 3rd year lab!
Using software like ImageJ, record the mean
pixel value for every black and white band.
Similar to the TTF plots we found.
Next measure the max + min of the sin wave and pixel
distance in between.
M=(Max – Min)/(Max + Min)
Find the physical size of the pixels by using this formula.
(Pixels x Sensor Size)/(Image Size)
This should give you the pixel size in mm or similar units
Plot 1/Physical Size [mm]. This will give you the spatial
frequency of the cycle.
This is what a MTF should look like.
This is the MTF that we came up with
http://www.normankoren.com/Tutorials/MTF.html
http://photo.net/learn/optics/mtf/
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
Thank you, third year students! (Specifically Meghan).