How to read CANON's MTF chart

Started Dec 12, 2004 | Discussions thread
Doug Kerr Forum Pro • Posts: 20,899
Re: How to read CANON's MTF chart
1

Hi, Eden,

Eden Yip wrote:

in the explanatory notes it said the difference between solid line
and dotted lines.

but didn't telll the meaning of different color and thickness of
the lines.

as compared the MTF of SIGMA i know one solid line for the
constrast (10mm pattern) and the other one for sharpness (30mm
pattern). but in a CANON MTF chart there are FOUR solid lines,
blue, black, fine blue, fine black, etc.

1. On the Canon MTF charts, the distinction between thick and thin lines is the spatial frequency to which the lines apply, the thick lines being for 10 cycles/mm and the thin ones for 30 cycles/mm.

Comment: The MTF at a low spatial frequency is indicative of the dilution of contrast (by such phenomena as internal scattering of light, or "lens flare". I ideally, that property would me measured at zero spatial frequency, but we cannot measure MTF at zero frequency, so we use the MTF at a low spatial frequency.

The "sharpness" of the lens is indicated by the way the MTF "holds up" as the spatial frequency increases (that is, for "finer and finer detail"). To really understand this, we would need to see a plot of MTF vs. spatial frequency. The normal MTF chart published by Canon (and most lens manufacturers) plots MTF against distance from the center of the frame, with spatial frequency as a parameter (reflected by different sets of lines), and for only one "higher" spatial frequency.

2. The distinction between the black and blue lines is the lens aperture to which the lines apply, the black ones at the maximum aperture of the lens and the blue ones at f/8.0

Comment: It is of course common that the MTF performance of the lens, at any given spatial frequency, will decline with increasing aperture (at least in the aperture range where diffraction effects are not controlling).

3. The distinction between the solid and dotted lines is the orientation of the path across the image to which they apply, or looking at it another way, the orientation of the pattern of test lines to which they apply, the solid lines to the meridional case and the dotted lines to the sagittal lines.

Explanation: The distinction only applies when we are not at the center of the frame. (This is why the meridional and sagittal MTF curves coincide on the MTF chart at zero distance from the center on the Canon-style MTF chart.)

The meridional test pattern comprises lines that are perpendicular to a radius from the center of the frame. If we had a test pattern of concentric circles, its lines would be meridional. Thus the meridional MTF would be taken for "travel" along a path across those lines, which would be a path along a radius from the center of the frame.

The sagittal test pattern comprises lines that are along a radius from the center of the frame. If we had a test pattern of radial lines (a "sunburst" pattern), its lines would be sagittal. Thus the sagittal MTF would be taken for "travel" along a path across those lines, a path at right angles to a radius, a path along a circle centered on the center of the frame.

Comment: The distinction between meridional and sagittal MTF reveals the presence of the aberration called "astigmatism".

Best regards,

Doug

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