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’sYou 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).