At f/8.0, the lens primarily blurs in the radial direction, which is a common occurrence. However, with the prime lens, something interesting happens: the type of astigmatism reverses when comparing the lens at f/1.4 versus at f/8.0. In the MTF charts for the Canon zoom versus prime lens from before, both lenses begin to exhibit pronounced astigmatism at the very edges of the image. However, this is usually an unacceptable trade-off with architectural photography. A wide angle lens with significant barrel distortion can therefore achieve a better MTF since objects at the periphery are stretched much less than they would be otherwise. Therefore, as the angle of view becomes wider, subjects near the periphery become progressively more stretched/distorted in directions leading away from the center of the image. Technical Note: With wide angle lenses, M lines are much more likely to have a lower MTF than S lines, partly because these try to preserve a rectilinear image projection. This quality-reducing artifact is called an " astigmatism," as illustrated below: Whenever the dashed and solid lines begin to diverge, this means that the amount of blur is not equal in all directions. However, things become more interesting progressively farther from the center. At this point you're probably wondering: why show the MTF for both sagittal ("S") and meridional ("M") line pairs? Wouldn't these be the same? Yes, at the image's direct center they're always identical. On the other hand, the prime lens loses quite a bit of contrast when going from f/8.0 to f/1.4, but this is probably because f/1.4-f/8.0 is a much bigger change than f/2.8-f/8.0. The zoom lens barely loses any contrast when used wide open compared to at f/8.0. In the above example, both lenses have similar contrast at f/8.0, although the prime lens is a little better here. Bold lines are often a priority since high values can mean that your images will have a more three dimensional look, similar to what happens when performing local contrast enhancement. Bold lines describe the amount of "pop" or small-scale contrast, whereas thin lines describe finer details or resolution.
#Lens diffraction in photography full
The example below shows how these lines might be measured and shown on an MTF chart for a full frame 35mm camera:īold vs. These lines can either be parallel to the direction leading away from the center (sagittal) or perpendicular to this direction (meridional). The MTF is usually measured along a line leading out from the center of the image and into a far corner, for a fixed line frequency (usually 10-30 LP/mm). What often matters more is knowing how the MTF changes depending on the distance from the center of your image. Knowing just the (i) maximum resolution and (ii) MTF at perhaps two different line frequencies is usually more than enough information. However, the above MTF versus frequency chart is not normally how lenses are compared. A high-end lens with an MTF-50 of 50 LP/mm will appear far sharper than a lower quality lens with an MTF-50 of 20 LP/mm, for example (presuming that these are used on the same camera and at the same aperture more on this later).
The highest line frequency that a lens can reproduce without losing more than 50% of the MTF ("MTF-50") is an important number, because it correlates well with our perception of sharpness. Alternatively, sometimes this frequency is instead expressed in terms of line widths (LW), where two LW's equals one LP.
This frequency is therefore usually expressed in terms of "LP/mm" - the number of line pairs (LP) that are concentrated into a millimeter (mm).
Line pairs are often described in terms of their frequency: the number of lines which fit within a given unit length. No real-world lens is limited only by diffraction, although high-end camera lenses can get much closer to this limit than lower quality lenses. The blue line above represents the MTF curve for a perfect "diffraction limited" lens. Move your mouse over each of the labels to see how high and low quality lenses often differ. The line pair illustration below the graph does not apply to the perfect lens. Low Quality Camera Lens (far from the diffraction limit)Ĭomparison between an ideal diffraction-limited lens (blue line) and real-world camera lenses. Increasing Line Pair Frequency → Very High Quality Camera Lens (close to the diffraction limit)
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