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Everyone of us, at one time or another, have found ourselves debating over one lens for another. Which Lens to get is a difficult question to answer as there is a plethora of lenses out there. How does one compare a lens to another? Fortunately there is something called MTF Chart which can be used to compare lenses.
MTF stands for Modulation Transfer Function. In a very generic lay man's language, modulation means the process of modifying the signal. Ideally a lens should transmit every bit of light it receives but no lens is ideal and the light passing through the lens is affected by the lens. We say that the lens modulated the light. Modulation Transfer Function is a measure of the modulation by the lens and the Chart is the pictorial representation of the same.
The quality of the lens is known by its resolving power and the resolving power of the lens is dependent on its ability to transmit the contrast. Confusing? Let me try to explain. Let us take a 1mm wide strip of white paper and draw 50 parallel black lines on that paper. Automatically the white space between the black lines look like white lines so in essence we got 50 black and 50 white lines for a total of 100 lines, the thickness of each line is 1/100 = 0.01 mm. Now will the lens be able to capture this as distinct lines or will it result into a hazy mixture? For the lens to resolve these into distinct black and white lines, it needs to transmit the contrast between the black and white. So the quality of the lens now depends on the amount of contrast the lens modulates. The less the modulation, the better the contrast is transmitted and the better the picture quality is.
Now let us proceed towards interpreting the MTF Chart.MTF Charts look like these:
Legend
Solid : Meridonial; Dotted : Sagittal
Red : 100 L/mm; Blue : 30 L/mm |
Legend
Solid : Meridonial; Dotted : Sagittal
Thin : 100 L/mm; Thick : 30 L/mm
Blue : f/8; Black : f/2.8 |
The Y-Axis (the vertical Axis) is from 0 to 1 which is a shorthand for 0% to 100% and represents the contrast transmitting capability. A value of 0.8 on the Y-Axis means the lens is able to transmit 80% of the contrast.
The X-Axis (the horizontal Axis) is in mm and represents the distance from the center. On a 35mm frame, the maximum distance from center would be approx 21mm. A value of 15mm on the X-Axis means that it is talking about points which are 15mm away from center.
Only knowing this and looking at the chart we can see that the lens is better at the center than towards the edges.
Different manufacturers present their MTF Charts differently. The legends would help you in your attempt to read the chart. In essence, every chart will have two pair of lines. A pair of lines consists of one solid and one dotted line. More on this later. One pair of lines represents the results for tests done at high resolution (like 100 lines per mm) and the other pair for tests done at low resolution (like 30 lines per mm). The high resolution test results relates to the resolving power of the lens and the low resolution test results relates to the contrast transmitting ability of the lens.
Each pair has one solid and one dotted line. One of them will correspond to Meridonial lines and the other will correspond to Sagittal lines. Generally, solid lines are Meridonial and dotted ones are Sagittal.
To summarize, we are talking about 4 lines in all
Line1 : Sagittal lines for resolution
Line2 : Meridonial lines for resolution
Line3 : Sagittal lines for contrast
Line4 : Meridonial lines contrast
With this understanding of the chart the interpretation is simple.
- The higher up the chart the pair of lines for low resolution are the better is the contrast reproducing ability of the lens
- The higher up the chart the pair of lines for high resolution are, the better is the resolution of the lens
- The less the lines fall with distance from center, the better the image quality is towards the edge.
- The closer the solid and the dotted lines are the more pleasing the out of focus areas would be.
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