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Fantasy MTF
Many manufactures publish what are properly termed fantasy MTF charts.
Here are some of the sneaky approaches used (and generally not documented):
- Computing (not measuring) MTF. Real lenses never are quite as good as the design and often not symmetric across the field.
- Computed MTF is another fantasy in itself: there are the tolerances of surface shapes, glass parameter variations (which batch?), and lens element placement all of which mean that MTF measured in a real lens never lives up to its computed theoretical performance.
- Calculation of ray aberrations might be done monochromatically or with very narrow bandwidth: every lens looks considerably better if (for example) only a narrow range of green light is used.
- Calculation often ignores diffraction, which truly makes the MTF chart pure fantasy defying the laws of physics, even assuming a perfectly assembled lens, because in a real lens diffraction is unavoidable.
- While a claim might be made of “white light”, computations might be made in ways that effectively make it a monochromatic figure.
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- Depth of field and how to bypass depth of field limitations via focus stacking.
- Optical aberrations: what they are, what they look like, and what to do about them.
- MTF, field curvature, focus shift: insight into the limitations of lab tests and why imaging performance is far more complex than it appears.
- Optical aberrations: what they are, what they look like, and what to do about them.
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Intrigued? See Focusing Zeiss DSLR Lenses For Peak Performance, PART ONE: The Challenges, or (one topic of many) field curvature.
(computed, possibly with narrow bandwidth light)