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I have a camera lens specification that shows the following table about the MTF.

I have a few questions about how to read this chart:

  • What does the On Axis, 30% Field, 60% Field, etc... mean?
  • What are the (S) and (T)?
  • How does one typically use this data to determine whether the lens should fit their application?

enter image description here

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What does the On Axis, 30% Field, 60% Field, etc... mean?

'On Axis' means the exact center of the field of view where the optical center axis of the lens intersects the test chart.

'30% Field' means a measurement was made at 30% of the distance from the center of the field of view to the corners of the FoV. For a full frame camera the distance from center to extreme corner on the sensor is 21.633mm, so 30% field on the surface of the camera's sensor would be somewhere on a circle with a center at the center of the lens' optical axis and having a radius of 6.49mm. That is, 30% Field is measured at a point on the test chart that would fall on a point 6.49mm from the center of the sensor.

'60% Field' is similar. It means the distance from the center to the point tested is 60% of the distance from center to extreme corner. For a full frame camera this would be 12.98mm from the center of the imaging sensor.

What are the (S) and (T)?

(S) is Sagittal. That means lines drawn from the center of the field of view towards the edges. Sagittal lines look like spokes on a wheel.
(T) is Tangential. Tangential lines are lines that are perpendicular to the Sagittal lines.

Lenses often have differing acutance rendering (S) and (T) lines. The difference between the (S) and (T) is often described as a lens' degree of astigmatism.

The numbers on the third column describe resolution as percentage contrast between alternating lines of white and black lines. Perfect resolution would result in 100% contrast. A result of 0% would indicate that line pairs are imaged on the sensor as an even gray color with no distinction between the black lines and the white lines.

The numbers in the fourth column describe the pitch of the line pairs of white and black lines as they would be measured on the surface of the imaging sensor or film.

In your example, we can see that when imaging line pairs at 112 LP/mm, the lens in question has 77% contrast at the center of the field. That means that the difference between the black lines and the white lines is 77% of the difference between pure white and pure black.

At 60% field we see that with 223 LP/mm the lens is capable of 48.8% contrast on line pairs oriented in a Sagittal direction and 43.0% contrast on Tangential line pairs. This means the "spokes" of a wagon wheel imaged with the lens would have slightly more contrast than the "rim" of the wheel if the hub was placed at the center of the field of view, as shown (at a much more severe degree of astigmatism) in the center figure below.

enter image description here

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I always maintained that charts like this are very limited as to their helpfulness. That being said: The camera lens projects a circular image. The center portion is called the “circle of good definition”. For most applications, only the center portion is photographically useful. The typical camera contains a mask that blocks off the substandard outer boundaries of this circle. As an example, a 50mm lens might project an image 100mm in diameter however, the useful portion is 45mm in diameter. That’s OK because the full frame format measures 24mm height by 36mm length. The diagonal of this rectangle measures about 45mm, so we can, after all use this lens.

This circle of good definition is best at the center (the axis) of the lens. As we examine an image from axis to edge, degrading is evident. Test are made, we image test charts consisting of closely spaced parallel lines. We examine images of these parallel lines to determine the ability of optical systems to resolve lines (see spaces between the ruled lines).

The target charts have sets of lines ruled parallel to a line drawn from the center to the edge of the image. These are called “Sagittal” lines. Lines drawn perpendicular to a radius line are called “Tangential” lines (sometimes called “Meridian” lines). Anyway, because these lines, due to their direction may image differently, we divide S/T to get an average.

Let me add, the resolving power of an optical system was well studied by John Strutt, English, Astronomer Royal, 3rd Baron Raylaigh, Nobel Prize Physics. The Rayleigh Criterion for theoretical resolving power is Lines per millimeter = 1392/f-number (for center of visual spectrum).

f/1 = 1392 f/2 = 696 f/2.8 = 497 f/5.6 =249 f/8 = 174 f/11 = 127 f/16 = 87 f/22 =63

For most applications the resolution at f/8 is greater than pictorially useful for most applications. This criterion remains valid today.

  • re: the OP first question: what does 30% field mean? And is 90% field still in the portion of the projected image that falls on the sensor? – MikeW Dec 13 '17 at 23:16
  • @ Mike W ---The camera is imaging a large test chart loaded with targets consisting of ruled lines. These are scattered here and there on the chart. The data is taken from the image of a single point. The point on axis will display the highest resolution. Now pick another point to test. Likely 60% of the distance center to diagonal corner. This off axis point will have lowered resolution because the image forming rays are incident (about to hit) from an oblique angle. This cause the image to land slightly dimmer and misconstrued. – Alan Marcus Dec 14 '17 at 2:22
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    Ok. Might want to work that into your answer, as it was the first question asked :) – MikeW Dec 14 '17 at 4:37

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