What Is The MTF
As mentioned in a previous article, the sharpness of an image is a subjective parameter, so it is impossible to quantify the performance of the lens by simply looking at the details of an image. For this reason, manufacturers have discovered objectives of objective methods to measure the performance of your lenses in the laboratory, especially through computer simulations.
These systems are then converged within that which is currently a standard measurement for all the lenses, called the MTF http://smilesmultimedia.com, or Modulation Transfer Function.
Since no lens is perfect to transmit light, the MTF is very useful to quantify the loss of both the contrast of the resolution.
Most MTF charts that you see around are made with specialized software that measures and simulates the performance of the lens, displaying the results. One such software, for those interested, is Imatest.
One of the advantages of the MTF is that they are able to provide many useful information on one graph:
Astigmatism and lateral chromatic aberration
The shift of focus
These parameters allow us to judge the quality of a lens in an objective manner, and to compare different products from the same manufacturer in quickly and accurately (we will see later why you can not use MTF charts to compare products of different brands).
In all this, I must also mention that there are some parameters that are not provided dall’MTF, namely:
Longitudinal chromatic aberration
The color reproduction
Susceptibility to flare
In practice, the MTF is great to evaluate the performance of some slow but do not provide the full picture of a view (for decision makers). Moreover, always for decision of different manufacturers, the MTF graph of a lens focal length variable will be always produced only for the two extremes: in the case of a 70-200mm lens, you will find MTF charts only for the focal lengths of 70 mm and 200 mm.
How it is measured the MTF
As you know, the performance of a lens varies greatly between its center and the periphery. Most lenses are optimized for maximum PERFORM at the center, which costs the sharpness as that we move away from the center. There are some objectives of the particular optical design can not lose too much sharpness while abandoning the center, but we are talking about rare examples from the substantial cost. Staying in a professional expensive, unfortunately, we will always have the falls of appreciable sharpness.
Considered therefore as varied sharpness estrange from the center, the MTF data are extrapolated to several points along the curve of the lens in order to cover the differences between them. Below is the drawing of the set of points where the MTF is calculated.