When shopping for a new lens, the MTF information can be very helpful in determining features in a lens that may be important to you. In this weeks blog post I will help explain how to read a MTF chart and what it means. So what does MTF stand for? Modulation Transfer Function. Oh.. ok... What does modulation transfer function mean? Ideally a lens would perfectly transmit all the light through the glass of the lens. We all know the world isn't perfect and either are lenses. A lens contains several elements and therefore losses will occur within the lens. When these losses are measured, in terms of contrast, it is called the modulation of contrast. Basically the chart measures how much contrast is lost as the light travels through the lens to the camera's sensor.
Many of us looked at a MTF chart, seen below, with great confusion. We now know the chart measures loss of contrast. First let's tackle the colors. Normally a MTF chart will contain two colors. One black and one colored. In this case it's blue. The black is referring to the lens at it's widest aperture. The blue line refers to an aperture of f8. Almost all lenses will have a higher rating at f8 than wide open so the colors on the chart should most often have a better rating.
What is a good rating? 1 is the best. Anything around .8 is really good, .6 is ok, and anything under that is suspect in terms of image quality. The numbers on the bottom of the cart is referring to millimeters. Zero is the dead center of the lens. As the numbers increase, it represents the millimeters away from the center of the lens. So in this chart the outside edges of the lens, right around 20mm, falls off in quality.
We now know how to read the chart, what the numbers mean, and what the colors represent, but why are there so many lines? Each line represents something different, but to keep it simple we are normally concerned with the "thick lines". This measures the actual contrast/sharpness of the lens. The thin lines represents image resolution, while the dotted lines represents the out of focus areas of the lens. Again the thick lines are most important.
Zoom lenses should have at least two MTF charts. One for the wide part of the zoom and one for the telephoto part of the lens. Lets look at an example of an awesome MTF chart. As you can see, this is the MTF chart for the Canon 500mm f4 IS II. The MTF chart without the extenders attached, is nearly perfect. This indicates an excellent lens wide open as well as stopped down to f8.
Each lens should have a MTF chart attached to it on the manufactures website. With that being said don't compare different manufacturers. For instance comparing a Canon lens MTF with a Tamron lens MTF. Manufacturers have different testing procedures and standards. My suggestion would be to pick out a manufacturer you want to buy from, then compare the MTF's to narrow down your lens selection within that specific manufacturer.
I hope this article helps in understanding and reading MTF's. I encourage you to use them, especially when deciding on your next lens purchase.