Lens quality:resoluation and contrast

Lens quality:resoluation ,contrast,MTF

Everyone is likely to be familiar with the concept of image resolution, but unfortunately, too much emphasis is often placed on this single metric. Resolution only describes how much detail a lens is capable of capturing — and not necessarily the quality of the detail that is captured. Other factors therefore often contribute much more to our perception of the quality and sharpness of a digital image.

To understand this, let's take a look at what happens to an image when it passes through a camera lens and is recorded at the camera's sensor. To make things simple, we'll use images composed of alternating black and white lines ("line pairs"). Beyond the resolution of your lens, these lines are of course no longer distinguishable:

ISO 12233 chart

Modulation Transfer Function(MTF)

Modulation Transfer Function or "MTF" is a measurement of the optical performance potential of a lens. MTF charts can give you a better understanding of the optical quality of lenses, and can be useful references when researching, comparing and purchasing a lens.

The y-axis (vertical axis) of a MTF chart plots the transmission of light through the lens with a maximum value of "1.0" which would indicate 100% transmittance of the light, although 100% transmittance of light is not possible because glass is not 100% transparent.

The x-axis (horizontal axis) shows the distance from the center of the image towards its edges. So, the "0" in the lower left corner represents the center of the lens and the numbers along the lower axis represent the distance out towards the edge of the lens in millimeters.

How to read a MTF chart

There are two groups of data plotted on a Nikon MTF chart: Sagital and Meridonial lines.

"Sagital lines" (the solid lines) represent the contrast measurements of pairs of lines that run parallel to a central diagonal line that passes through the middle of the lens from the bottom left hand corner to the top right hand corner.

"Meridonial lines" (the dotted lines) represent line pairs also positioned along an imaginary line from the center of a lens to the edge but these line pairs are perpendicular to the diagonal line.

There are two groups of test lines for each Sagital and Meridonial value: one group or line pairs at 10 lines per millimeter and a second group at 30 lines per millimeter. The lower line pairs (10 lines/mm) will generally be plotted higher on the graph than the more challenging fine resolution 30 lines/mm.

The red 10 line/mm (10 lines per millimeter) indicates the lens' ability to reproduce low spatial frequency or low resolution. This line indicates the lens' contrast values and the higher and straighter this line is the better; because the higher the line appears the greater the amount of contrast the lens can reproduce. 
The blue 30 line/mm (30 lines per millimeter) indicates the lens' ability to to reproduce higher spatial frequency or higher resolution; this line relates to the resolving power of the lens and again the higher the line the better.

The line starts on the left of the chart which represents the center of the lens. As the line moves to the right it indicates the edge of the lens, so you can see how the contrast and sharpness of the lens decreases from the center to the edge of the image.

http://www.quickmtf.com/